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    NEW PROGRESS IN PALEOEARTHQUAKE STUDIES OF THE JIANGSU SEGMENT OF THE ANQIU-JUXIAN FAULT IN THE TANLU FAULT ZONE
    ZHANG Hao, LI Li-mei, JIANG Xin, ZHANG Dong, XU Han-gang
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 880-895.   DOI: 10.3969/j.issn.0253-4967.2023.04.005
    Abstract1256)   HTML23)    PDF(pc) (17177KB)(188)       Save

    Paleoseismology is a discipline that studies prehistoric earthquakes or earthquakes that occurred before instrumental records using geological and geomorphological methods, mainly by trench excavation and Quaternary chronology. It focuses on the time and intensity distribution of large earthquakes, to reveal the recurrence characteristics of large earthquakes and provide basic data for evaluating the probability of future earthquakes. The Tanlu fault zone is the most active fault zone in eastern China. The Jiangsu section of the Tanlu fault zone is mainly composed of five branch faults, which are strongly active in the Pleistocene. Among them, the Anqiu-Juxian Fault continued to be active until the Holocene, which is the seismogenic fault of the Tancheng 8½ earthquake in 1668. The Xinyi-Sihong section is likely to generate strong earthquakes in the future in the south-central section of the Tanlu fault zone.

    The total length of the Jiangsu section of the Anqiu-Juxian Fault is about 170km, with an overall strike of 5°~15°, extending southwards from the north Maling Mountain to the Chonggang Mountain. The geomorphic features are distributed from north to south by the alternation of the bedrock mountain and the sedimentary basin. The Anqiu-Juxian Fault shows a single exposed fault on one side of the bedrock mountain, extending to the basin into two branches in the east and west, of which the east branch is the active late Pleistocene Fault and the west branch is the Holocene active fault. The Jiangsu section of the Anqiu-Juxian Fault is dominated by dextral strike-slip and has both dip and thrust components.

    Lots of research have been done on the ancient earthquakes of the Anqiu-Juxian Fault. The trenches are mostly located in Maling Mountain, Zhangshan Mountain and Chonggang Mountain, which are in the state of uplift and denudation. The Holocene is very thin, and the dating method is mostly optical luminescence. The identification of ancient earthquake events is less since the Holocene, with the accuracy of ancient earthquake time not high and the ancient earthquake sequence not complete. According to the topographic and geomorphological characteristics of the Jiangsu section of the Anqiu-Juxian Fault, three trenches were excavated along the Anqiu-Juxian Fault, of which two were in exposed areas and one in a buried area. Three trenches completely revealed the Holocene sedimentary strata in the Jiangsu section of the Anqiu-Juxian Fault, in which MLTC2 revealed the early Holocene strata, MLTC1 revealed the middle Holocene strata, and HSTC revealed the late Holocene strata. The determination of the age of earthquake events is one of the most uncertain factors in the study of paleoearthquakes and is the main indicator of the recurrence period of paleoearthquakes. At present, most of the paleoearthquake events studied have occurred since the late Pleistocene, and the accuracy of 14C dating is the highest. A total of 13 14C samples were collected from the trenches. Combined with the paleoearthquake events and time revealed by previous trenches, it is concluded that there have been three paleoearthquake events in the Jiangsu section of the Anqiu-Juxian Fault since the Holocene, with theelapsed time of ~3000aBP, ~6000aBP and ~11000aBP, and the coseismic vertical offset are all nearly 1m.

    The 1668 Tancheng M8½ earthquake showed signs of surface ruptures in the exposed area of the Xinyi section of the Anqiu-Juxian Fault, accompanied by a large amount of sandblasting and water gushing in the buried area. Dense fissures and sand veins are observed in the late Holocene strata overlying the fault, indicating the impact of the 1668 Tancheng earthquake. More representative chronological data are needed as to whether the 1668 Tancheng earthquake ruptured Suqian City.

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    GEOLOGICAL DISASTERS AND SURFACE RUPTURES OF JANUARY 23, 2024 MS7.1 WUSHI EARTHQUAKE, XINJIANG, CHINA
    ZHANG Bo-xuan, QIAN Li, LI Tao, CHEN Jie, XU Jian-hong, YAO Yuan, FANG Li-hua, XIE Chao, CHEN Jian-bo, LIU Guan-shen, HU Zong-kai, YANG Wen-xin, ZHANG Jun-long, PANG Wei
    SEISMOLOGY AND GEOLOGY    2024, 46 (1): 220-234.   DOI: 10.3969/j.issn.0253-4967.2024.01.013
    Abstract855)   HTML36)    PDF(pc) (14676KB)(645)       Save

    The MS7.1 earthquake in Wushi, Xinjiang on January 23, 2024, represents the largest earthquake in the Tianshan seismic belt since the 1992 Suusamyr MS7.3 earthquake in Kyrgyzstan. Preliminary precise aftershock localization and initial field investigations indicate an NE-trending aftershock zone with a length of 62km that is concentrated at the mountain-basin transition area. This event produced geological hazards, including slope instability, rockfalls, rolling stones, and ground fissures, primarily within a 30-kilometer radius around the epicenter. The epicenter, located approximately 7 kilometers north of the precise positioning in this study, witnessed a rapid decrease in geological hazards such as collapses, with no discernible fresh activity observed on the steep fault scarp along the mountainfront. Consequently, it is inferred that the causative fault for this main shock may be an NW-dipping reverse fault, with potential rupture not reaching the surface.

    Moreover, a surface rupture zone with a general trend of N60°E, extending approximately 2 kilometers, and displaying a maximum vertical offset of 1m, was identified on the western side of the micro-epicenter at the Qialemati River. This rupture zone predominantly follows the pre-existing fault scarp on higher geomorphic surfaces, indicating that it is not new. Its characteristics are mainly controlled by a southeast-dipping reverse fault, opposite in dip to the causative fault of the main shock. The scale of this 2-kilometer-long surface rupture zone is notably smaller than the aftershock zone of the Wushi MS7.1 earthquake. Further investigation is warranted to elucidate whether or not the MS5.7 aftershock and the relationship between the SE-dipping reverse fault responsible for the surface rupture and the NW-dipping causative fault of the main shock produced it.

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    MORPHOLOGIC DATING OF SCARP MORPHOLOGY BASED ON DIFFUSION EQUATION: A REVIEW
    XU Jian-hong, CHEN Jie, WEI Zhan-yu, LI Tao
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 811-832.   DOI: 10.3969/j.issn.0253-4967.2023.04.001
    Abstract257)   HTML31)    PDF(pc) (5260KB)(170)       Save

    A scarp is a common step-like landform in nature, which consists of a gently sloping plane connected to the upper and lower geomorphic surfaces of differing elevations. Common scarps include fault scarps, terrace scarps, lake shoreline scarps, shoreline scarps, volcanic ash cinder cones, etc. Scarps are often used as strain markers because of their linear characteristics and are favored in the study of active tectonics. However, it is difficult to directly constrain their ages. Instead, they are usually constrained by the ages of the upper and lower geomorphic surfaces. The scarp developed in loose deposits is controlled by a long process of low-energy degradation after a short collapse. This process can be modeled by the diffusion equation because the process can be considered as a slope process under the transport-limited condition. Under this condition, the slope can provide enough loose material for transport, that is, the material transport capacity is less than the material supply capacity. If process assumptions are sufficiently valid and rate constraints can be calibrated independently, the true age of scarps can be obtained. This method is called morphologic dating. This method has been included in many textbooks published overseas, but there have very little research on this method in China. Both linear and nonlinear models have been developed to describe scarp degradation. Linear diffusion models assume that the diffusion coefficient is a constant, whereas nonlinear transport models generally define the diffusion coefficient as a nonlinear function related to the topographic gradient. Compared to the linear transport models, nonlinear transport models can better explain the phenomenon of rapidly increasing deposition flux as the gradient approaches a critical value. In this paper, we review the study history of scarp degradation analysis and the concept model of scarp degradation. We focus on the establishment of the nonlinear model, the role of the different parameters in profile evolution, determining the best-fit age using a full-scarp nonlinear modeling procedure, and so on. Furthermore, we introduce the model of the nonlinear age chart, including the effect of far-field slope on morphologic dating of scarp-like landforms and two examples of the application of the chart, which shows that this method can correctly evaluate the ages of single-event scarps. Finally, we discuss the extension of the concept and method of the scarp degradation model, the applicability of the model, and repeated fault scarp morphological analysis. For nonlinear diffusion models, in addition to n equal to 2, two parameters (critical gradient (Sc) and diffusion constant (k)) need to be constrained. The critical gradient can be obtained from the young scarps in the study area, which roughly represents the initial state of scarp evolution, typically 0.6 to 0.7(30° to 35°). The diffusion constant needs to be characterized by a known age scarp. The slopes of the upper and lower geomorphic surfaces have an obvious influence on the morphology of a degraded scarp. These discussions indicate that both linear and nonlinear models can be used for the degradation analysis of single-event scarps, but a nonlinear diffusion model is recommended for young single-event scarps. The constant slip rate nonlinear model can be used to simulate the evolution history of<10ka high-slip rate active fault scarp. The multiple-event scarp model requires careful evaluation of the fault location and the amount of displacement per event. There are several assumptions in the scarp topography diffusion modeling, which require practice to verify its reliability. With advances in surveying technology, it is now possible to rapidly obtain high-resolution terrain data over broad areas from which numerous topographic profiles can be efficiently extracted. This provides a broad application prospect for scarp degradation analysis and morphologic dating.

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    FINE CHARACTERISTICS OF EARTHQUAKE SURFACE RUPTURE ZONE BASED ON HIGH-RESOLUTION REMOTE SENSING IMAGE: A CASE STUDY OF LITANG FAULT
    YOU Zi-cheng, BI Hai-yun, ZHENG Wen-jun, PENG Hui, LIANG Shu-min, DUAN Lei, QIN Yi-gen
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1057-1073.   DOI: 10.3969/j.issn.0253-4967.2023.05.002
    Abstract252)   HTML36)    PDF(pc) (10517KB)(267)       Save

    Strong earthquakes(magnitude>6.5)typically cause coseismic surface ruptures of several kilometers or even hundreds of kilometers long on the surface. Coseismic surface rupture is the most intuitive geomorphic representation of an earthquake on the surface, and its geometry and distribution characteristics provide important information about the fault activity. Field investigation is the most basic means for research on coseismic surface fractures, but for areas that are hard to access or have harsh climatic environments, field investigation is often greatly limited. In recent years, the increasing abundance of high-resolution remote sensing images and the rapid development of photogrammetry methods can help us quickly obtain high-resolution topographic and geomorphic data of the study area, to better identify the fine geometry of the earthquake surface rupture zone and measure the offsets of geomorphic markers along the fault. The Litang Fault is a sinistral strike-slip fault located within the Sichuan-Yunnan rhombic block on the eastern edge of the Qinghai-Tibetan plateau. Several historical earthquake events have occurred on this fault, such as the 1890 and 1948 earthquakes, and clear seismic surface ruptures still exist along the fault so far. Previous studies have conducted a series of works on the coseismic surface rupture of this fault, but most of these works were based on field investigations or relatively low-resolution remote sensing images, and there is still a lack of fine research on the coseismic surface rupture of the fault. In this paper, the coseismic surface rupture of the 1890 earthquake which occurred on the Litang Fault was selected as the study object. To obtain high-resolution topographic data of this fault, the WorldView satellite stereo images were used to generate a 0.5-m-resolution orthophoto and a 1-m-resolution Digital Elevation Model(DEM)of the Litang fault based on the photogrammetry method. With the high-resolution topographic data, the fine geometry of the 1890 earthquake surface rupture zone was mapped in detail. The mapping results show that the total length of the surface rupture is about 27km, with an overall strike of N40°W. The rupture is mainly characterized by sinistral strike-slip motion, with a certain degree of dip-slip component in local areas. Except for the interval of approximately 6km with no surface rupture at the Wuliang River floodplain in the Litang Basin, the surface ruptures are relatively continuous at other locations. In addition, various rupture styles have been identified along the fault, including en echelon tension cracks, mole tracks, sag ponds, fault scarps, and displaced gullies. Furthermore, the sinistral offsets of 90 groups of linear geomorphic markers such as gullies and ridges were measured along the fault, which range from 1m to 82.4m. We further estimated the Cumulative Offset Probability Distribution(COPD)of the offsets located on the terrace I of the Wuliang River, which are all in the range of 0-9m. The COPD plot displays four distinct peaks at 1.3m, 2.4m, 4.3m, and6.1m, respectively. Previous studies have reported that the terrace I of Wuliang River formed at about(4 620±40)a BP. Thus, it can be indicated that the Litang fault may have experienced at least four strong earthquake events since(4 620±40)a BP, and the smallest peak of 1.3m may represent the coseismic displacement of the most recent 1890 earthquake. The rupture length of the latest 1890 earthquake was about 27km, and the coseismic sinistral offset was about 1.3m, yielding an estimated moment magnitude of MW6.8-7.1. The coseismic offset of the other three earthquakes was about 1.8m, 1.9m, and 1.1m from old to new, respectively, yielding a magnitude estimate of MW7.3, MW7.3, and MW7.0, with a size comparable to the 1890 earthquake. The research results fully demonstrate the potential of high-resolution remote sensing images in the study of fine characteristics of earthquake surface rupture.

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    STUDY ON HETEROGENEITY OF THE STRESS FIELD IN THE YANGBI EARTHQUAKE FAULT ZONE
    WAN Yong-ge, WANG Yu-ru, JIN Zhi-tong
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 1025-1040.   DOI: 10.3969/j.issn.0253-4967.2023.04.013
    Abstract226)   HTML18)    PDF(pc) (7335KB)(109)       Save

    The fine crustal stress field plays an important role in geodynamics. The 2021 Yangbi earthquake sequence occurred in an area with densely deployed seismic stations. Before the mainshock, there occurred multiple 3-4 magnitude earthquakes. The mainshock was followed by strong aftershocks, MS5.0 and MS5.2, occurring 7 and 36 minutes later respectively. The earthquake sequence is a typical example of a “foreshock-mainshock-aftershock” earthquake sequence. The abundant seismic data of the 2021 Yangbi earthquake sequence provide many seismic focal mechanisms for the fine stress field analysis in the study region.

    To study the relationship of the stress field, fault structure, and earthquake dynamics in the Yangbi earthquake source region, the central focal mechanism solution algorithm is selected for the earthquakes with several focal mechanisms to ensure the accuracy of the focal mechanism data, and 93 precisely determined focal mechanism data are determined. The overall stress field in the source region is determined as a compressive stress axis of nearly NS direction and extensional stress axis of nearly EW direction. Then, to reveal the heterogeneity of the stress field in the source region, according to the location of the earthquake sequence, the focal mechanism solutions are divided into 6 regions by using the moving window strategy and obtain the stress field in each sub-region. To verify the inversion results are not caused by the selection of a specific partition mode, we used two different partition methods to discuss the stress field inversion experiments: 1)change the number of sub-regions from 6 to 8, the number of focal mechanisms in each subregion is still 23, and moving the 15 focal mechanisms in each iteration; 2)the number of the sub-region is still 6, change the number of focal mechanism to 28 in each subregion. It can be found that although the different partition strategies are changed, the characteristics of the obtained stress field will not change. Finally, the earthquake dynamics revealed in the heterogeneous stress area are analyzed.

    The results show that the compressive stress axis changed from NNW-SSE direction in the northwest of the Yangbi earthquake focal area to NNE-SSW direction in the southwest region, with the rotation angle of 23°; And the stress shape factor in the northwest part of the rupture zone is always larger than that in the southeast region. Combined with the geodynamics studies of crustal motion map, tomography from seismic data, hydrographic net distribution, and topography of the study region, it is speculated that the change of the stress field in the northwest and the southeast is caused by the combined action of the blocked southward movement of the material in the northern part of the fracture area and the NNE extension in the shallow part of the study area due to the low angle NNE subduction of the Indo-Burma arc. The horse-tail-like fault distribution in the southeast of the Yangbi earthquake fault zone and the mountain and river alignment around the Yangbi earthquake are consistent with the predicted stress deflection and stress shape factor change. These studies are of significance for understanding the characteristics of fault activity and earthquake dynamics in study regions.

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    RELOCATION OF THE 2022 MS6.0 MAERKANG EARTHQUAKE SWARM IN SICHUAN PROVINCE AND ITS SEISMIC FAULT ANALYSIS
    XU Ying-cai, GUO Xiang-yun
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 1006-1024.   DOI: 10.3969/j.issn.0253-4967.2023.04.012
    Abstract205)   HTML27)    PDF(pc) (13832KB)(177)       Save

    The 2022 MS6.0 Maerkang earthquake swarm in Sichuan Province is the first rare strong swarm activity with high frequency, concentrated spatial and temporal distribution, strong explosive and strong magnitude in Maerkang area in the eastern segment of Bayan Har block in China seismic network records. It is also another significantly strong earthquake event in Bayan Har block after the MS7.4 Maduo earthquake on May 22, 2021. The MS6.0 Maerkang earthquake on June 10, 2022 not only broke the 33-year record without MS≥6.0 earthquakes within 100km of the epicenter, but also broke the historical record without MS≥6.0 earthquakes within 50km of the epicenter. The earthquake swarm is mainly located in the nearly “T” shaped conjugate fault structure area composed of the NW strike Maerkang fault and NE strike Longriba fault in the Bayan Har block. This area is a relatively rare region for moderate and strong earthquakes in the history. Therefore, it is of great significance to analyze and discuss the possible seismogenic faults of the Maerkang strong earthquake sequence for the study of seismogenic structures and the risk of strong earthquakes in the weak seismic region of Bayan Har block.

    The earthquake swarm was relocated by double-difference method, and focal mechanisms and centriod depths of MS≥3.6 earthquakes were calculated by using gCAP inversion method. Then the relationship between the stress system in the Malkang area and these earthquake focal mechanisms was analyzed, and fault plane was fitted by using relocation results. Maerkang earthquake swarm is mainly distributed along NW direction, and the initial rupture depth is 9.8km on average. Depth profiles show that earthquakes are mainly concentrated at depth between 0km to 15km. The most earthquakes of early-stage occurred in 48 hours. The mid-stage and late-stage earthquakes are located less than 15km in depth and move to the northwest of the epicenters. Initial rupture depth of the largest MS6.0 earthquake is 12.5km, which is almost at the bottom of the dense area. The focal mechanism of MS6.0 earthquake is 150° in strike, 79° in dip, and 7° in rake on nodal plane Ⅰ, and 59° in strike, 83° in dip, and 169° in rake on nodal plane Ⅱ, with the centroid depth of 9km. Other focal mechanisms of MS≥3.6 earthquake are strike-slip types. Dips of nodal plane of focal mechanism range from 71° to 86°, and there exist different dip directions for one strike of every nodal plane. All azimuths of P axis are in NWW direction, and the plunges are nearly horizontal. The focal mechanisms of MS≥3.6 earthquakes show that the tectonic environment is very favorable for NE or NW strike faults to generate the strike-slip movement. Centriod depths range from 5 to 9km, which are lower than the average depth of 9.8km of relocation, indicating that these earthquakes mainly ruptured from deep to shallow. The relative shear stress of the NW nodal plane are significantly greater than that of the NE nodal plane, and the normal stress of the NW nodal plane was smaller than that of the NE nodal plane, indicating more possibility of strike-slip dislocation on the NW nodal plane. The fault plane fitting results reveal that there are obviously two nearly parallel and nearly NW strike earthquake belts in the epicenter area. Fitted fault plane parameters of the belt in the north branch show the strike 333°, the dip 88°, the slide -22°, and the belt in the south branch show the strike 331°, dip 88°, and slide -23°. It is indicated that the fault properties of these two earthquake belts are basically the same, revealing that most of earthquake activities of the swarm may be controlled by at least two parallel structures near the Maerkang fault with the NW strike, dip 88° and left-lateral strike-slip. Combined with the existing regional geological structure, it is inferred that the Maerkang earthquake swarm may be induced by the NW and NE strike conjugate faults, and the NW strike faults control most of the earthquake activities.

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    PRECISE LOCATION AND SEISMOGENIC STRUCTURE OF THE 2022 LUSHAN MS6.1 EARTHQUAKE
    FU Ying, HU Bin, ZHAO Min, LONG Feng
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 987-1005.   DOI: 10.3969/j.issn.0253-4967.2023.04.011
    Abstract205)   HTML20)    PDF(pc) (7950KB)(137)       Save

    On June 1, 2022, a MS6.1 earthquake occurred in Lushan, Sichuan Province, western China, which is approximately 10km from the Lushan MS7.0 event on April 20, 2013. To understand if the earthquake has the same seismogenic structure as the Lushan MS7.0, we relocated the event in the Lushan area using the multi-stage locating method based on the seismic phase arrival data of the Sichuan Seismic Network from April 20, 2013, to July 1, 2022. A total of 6992 ML≥1.0 earthquakes were acquired, with a relative locating error of 0.5km and 0.7km in the horizontal and vertical directions, respectively, with a travel time residual(RMS)of 0.18s. The results show that the MS6.1 event is located at 102.943°E, 30.382°N with an initial-rupture focal depth of 15.6km, lying on the NW side of the 2013 Lushan MS7.0 event. The sub-surface rupture length of the long and short axis is 10 and 8km, measured from the dense aftershock area in NE-SW and NW-SE directions, respectively. The NE-SW profile in the Lushan area shows that the depth of Lushan MS7.0 earthquake in 2013 was about 15km, similar to that of Lushan MS6.1 and MS4.5 on June 1, 2022. The MS6.1 earthquake sequence, located at the NE end of the long axis, shows no evidence to break through the rupture termination point of the Lushan MS7.0 earthquake and enters the Dayi seismic gap, which is bounded by the 2008 Wenchuan MS8.0 and 2013 Lushan MS7.0 aftershock regions. The short-axis profile shows that the MS6.1 earthquake sequence occurred on a new back-thrust fault in the pre-existing seismogenic structure of the 2013 Lushan MS7.0. The new structure dips SE and ruptures in a slight arc protruding into the NW, parallel to the northern segment of the seismogenic structure of the 2013 Lushan MS7.0 earthquake with a horizontal distance of about 5km. The new and old structures connect at the detachment base to the main segment of the 2013 Lushan MS7.0 earthquake.

    We also inverted the focal mechanism of the Lushan MS6.1 earthquake using the CAP(Cut and Paste)method. The result indicates that the centroid depth of the MW5.7 main event is 14km which is very close to the initial-ruptured depth of 15km calculated by the phase arrival times. The best double couple parameters are 221°/40°/105° for nodal plane Ⅰ and 22°/52°/78° for nodal plane Ⅱ. The parameters are in order of the strike, dip, and rake angles. Combined with the realization of the NE-striking, SE-dipping seismogenic structure characteristics determined by the accurate locating of the earthquake sequence, it can be quickly confirmed that the nodal plane Ⅱ is the fault plane.

    Based on the accurate locating results, focal mechanism solutions, and geodynamic background of the focal area, it is inferred that the seismogenic structure of the Lushan MS6.1 earthquake is induced by the thrust dislocation of a NE-SW trending and SE inclining thrust fault in the southern section of Longmenshan fault zone. Finally, we discussed the relationship between MS7.0 and MS6.1 in the Lushan area. The two could be considered a unique sequence: the mainshock and the maximum aftershock, respectively, regarding spatial relationship and tectonic correlation. However, the time interval of these two earthquakes significantly overextends the statistical relationship between the principal earthquake and the maximum aftershock. Furthermore, considering the effects of the Coulomb stress change produced by the earthquakes repeated at the end of the Dayi gap, Lushan earthquake further enhanced the stress level in the Dayi seismic gap located in its northern segment.

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    RELIABILITY EVALUATION OF QUARTZ ESR DATING METHOD ON EARLY PLEISTOCENE SEDIMENT: A CASE STUDY OF JINGYUAN SECTION
    WEI Chuan-yi, YIN Gong-ming, WANG Xu-long, WANG Duo, JI Hao, LIU Chun-ru, LI Xin-xiu
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1041-1056.   DOI: 10.3969/j.issn.0253-4967.2023.05.001
    Abstract202)   HTML13)    PDF(pc) (4654KB)(170)       Save

    As the most recent period of the geological record, the Quaternary climate change, tectonics and river drainage evolution have been well recorded by Quaternary sediment. Establishing the timing of these geological changes, and of their effects on the earth's environment, is a key element in Quaternary research. Because of dating range limit of quartz OSL dating and 14C dating, lacking of tephra for K-Ar dating, and strict restrictions for 26Al/10Be cosmogenic nuclide dating, the samples older than 200ka were critical but difficult in Quaternary dating, while electron spin resonance(ESR)dating method could provide absolute age for late Pliocene and Pleistocene samples. Previous studies show that quartz Al center and Ti-Li center are the most suitable signals for sediment ESR dating, and have been successfully applied into middle-late Pleistocene sediment dating. However, the application of those two centers ESR chronology into early Pleistocene or pre-Quaternary sediment remains confusion.

    In this study, early Pleistocene Jingyuan gravel layer sediment deposited at Yellow river were collected for ESR dating. The results of comprehensive comparative analysis of high resolution magneto-stratigraphy and deep-sea oxygen isotope curve of loess-paleosol sequences and high credible 26Al/10Be cosmogenic nuclide dating age make the Jingyuan gravel layer as the ideal material to evaluate the dating range, especially lower dating range, of the quartz Ti-Li center and Al center, respectively. The results show that:

    (1)The quartz Ti-Li center and Al center signal intensity of Jingyuan gravel layer was not saturated within 11 000Gy and 130 00Gy additional gamma ray dose, respectively; combined with the long thermal lifetimes of the quartz Ti-Li center(8×106a)and Al center(7.4×109a), guarantee the ESR dating range for million years.

    (2)The single saturation exponential function and “EXP+LIN” functions could provide more accuracy fitting result of equivalent dose of quartz Ti-Li center and Al center, respectively, and the fitting goodness is greater than 0.98.

    (3)The average ESR dating results of quartz Ti-Li center and Al center of Jingyuan gravel layer is~(1.67±0.15)Ma and~(1.65±0.69)Ma, respectively, which is consistent with the previously well-known age within the error range.

    To better understand the lower dating limit of the quartz ESR dating method, based on the previous analysis of the ESR signal thermal stabilities, we discuss the maximum saturation of the ESR signals and ESR signals' sensitivity. Combined with the fitting goodness evaluation of various fitting functions, we propose that the quartz Ti-Li center and Al center ESR dating method could provide reliable chronological constrains on the sand lens of early Pleistocene gravel layer. The results of our study not only provide a solid theorical foundation for the application of quartz ESR dating method for late Pliocene and early Pleistocene fluvial sediments, but also demonstrate a typical practice example of the ESR method on dating late Cenozoic sediments.

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    OPPORTUNITIES BROUGHT BY 3D GEOSCIENCE MODELING FOR EARTHQUAKE NUMERICAL FORECASTING
    YAO Qi, LU Ren-qi, SU Peng, WANG Hui, ZHU Ya-ling, WANG Li-wei
    SEISMOLOGY AND GEOLOGY    2024, 46 (1): 1-18.   DOI: 10.3969/j.issn.0253-4967.2024.01.001
    Abstract188)   HTML24)    PDF(pc) (1599KB)(175)       Save

    Earthquake prediction and forecasting need to transform from the traditional empirical, qualitative, and semi-quantitative to quantitative. The improvement also calls for multi-disciplinary, highly integrated physical and mechanical simulations rather than only a single discipline. The global development of observation technology and the construction of observation networks have already built a data foundation for earthquake numerical prediction and forecasting to a certain extent. However, the biggest constraint is the difficulty of synthesizing a large amount of observation data and quickly establishing complex numerical models with geological significance for numerical calculation. It is a vital issue restricting experimental research and industry development of earthquake numerical prediction and forecasting. Based on a brief introduction of the concept, development, and research status of earthquake numerical prediction and forecasting, this paper analyzes the difficulties in numerical modeling, which essentially come from the disciplinary differences between active tectonics, structural geology, solid earth, seismology, and numerical simulations. The development of 3D Geoscience Modeling and its application in the earthquake industry can establish a large-scale complex earthquake tectonic model close to the real world with geological significance. It provides a significant opportunity and technical means for developing earthquake numerical prediction and forecasting by solving the problems in numerical modeling. 3D geological modeling has built a bridge for multi-disciplinary geological applications. It can multi-disciplinary data fusion, establish a 3D geological model with geological significance and characteristics in line with geomechanical characteristics, and integrate data, geological model, up to building a numerical model, which advances the efficiency of modeling and simulation. Therefore, the rapid development of 3D geological modeling provides an opportunity to solve the modeling difficulties mentioned above in earthquake numerical prediction. Then, we briefly describe the development of 3D geological modeling technology, its application in the seismic industry, and the construction and application of 3D standard fault models domestically and overseas. Here, we introduced the development and essential contents of the Community Fault Model of Southern California in the United States for the Uniform California earthquake rupture forecast, the New Zealand Community Fault Model from the Institute of Geological and Nuclear Sciences Limited, and the Community Fault Model in Sichuan and Yunnan region in China.

    The prospective future of 3D geological modeling and its potential application in earthquake numerical prediction and forecasting makes it a common concern of researchers in earthquake science. The five future modeling trends are the joint modeling of multi-source and multi-precision heterogeneous data, the integrated modeling of the geological model-attribute model-numerical model, flat fault structure modeling, 3D fault structure modeling, data-model-calculation iteration, and mutual driving. Finally, the paper describes the difficulties of applying the 3D geological modeling technique in earthquake numerical prediction and forecasting, including the industry construction, public approval of the 3D Community Fault Model, and the variations of numerical modeling and applications. 3D geological modeling technology can provide more realistic numerical and geometric models for earthquake numerical prediction, forecasting, and related numerical computing fields, reduce construction periods, create fast iterations, and solve modeling difficulties.

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    ACTIVITY PERIOD, FORMATION MECHANISM AND TECTONIC SIGNIFICANCE OF THE KALAYU’ERGUN FAULT IN THE WESTERN KUQA DEPRESSION
    SHEN Bai, ZHANG Zhi-liang, REN Zhi-kun, LIU Jin-rui
    SEISMOLOGY AND GEOLOGY    2023, 45 (6): 1247-1264.   DOI: 10.3969/j.issn.0253-4967.2023.06.001
    Abstract182)   HTML34)    PDF(pc) (5216KB)(146)       Save

    As the NW-trending dextral strike-slip fault on the northern margin of the Tarim Basin, the Kalayu’ergun Fault defines the western boundary between the western Kuqa Depression and Wensu Bulge. It holds immense importance to understand the deformation occurring within the Kuqa Depression. However, there is still ongoing debate regarding the length, activity time and formation mechanism of the Kalayu’ergun Fault. In this study, a comprehensive investigation was conducted, incorporating sub-surface geophysical data, high-resolution remote sensing satellite images, and the findings of previous researchers. The results demonstrate that the Kalayu’ergun Fault cuts off the Awate anticline in the north, and to the south, it extends near the southern flank of the North Kalayu’ergun anticline but does not reach the Middle Kalayu’ergun anticline. The total extension of the fault is estimated to be approximately 40km. And the minimum of the fault strike-slip distance is estimated by the sum of the tectonic shortening of the North Kalayu’ergun anticline and the shortening absorbed by the strata on the northern flank of the Awate anticline through drag, which amounts to about 4.1-4.3km. Additionally, the Kalayu’ergun Fault has been active since its formation in the early Pliocene, but its activity intensity has been weakened obviously. The activity of the Kalayu’ergun Fault corresponds to the deformation time of the North Kalayu’ergun anticline, which is consistent with the deformation time determined using the same structural sedimentary constraints. This indicates that the North Kalayu’ergun anticline was formed under the combined action of near north-south compressional and horizontal shear stresses. The development of this transverse fault is synchronous with the overthrust structures on both sides and is developed in synchrony with the strong uplift of the southern Tian Shan orogenic belt since the late Cenozoic. The formation of the Kalayu’ergun Fault can be affected not only by the differences in the basement nature on both sides but also closely related to the difference in the thickness of the gypsum salt layer. The former resulted in variations in horizontal shortening on both sides of the fault, leading to the tearing of the Cenozoic sedimentary cover. The latter, which under the action of the extrusion stress, influenced the generation and evolution of salt-overlying beds, and then influenced the formation of the fault. In addition, the existence of prior salt structures, also known as salt diapirs, may have also played an important role in the formation of the fault. As the boundary fault in the western part of the Kuqa Depression, the Kalayu’ergun Fault is responsible for accommodating crustal shortening on both sides and even in the whole eastern and western parts of the Kuqa Depression. As a result, the shortening of the Kuqa Depression gradually decreased from east to west. Furthermore, the Kalayu’ergun Fault also had significant impacts on geomorphology, as it controls and modifies the landscape in the southern Tian Shan foreland basin. In the meanwhile, the Kalayu’ergun Fault creates favorable conditions for the transportation and accumulation of oil and gas resources.

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    FOUR-COMPONENT BOREHOLE STRAINMETER TRANSFER FUNCTIONS AND ROCK PROPERTIES ESTIMATION: A CASE OF JIANGNING STATION, JIANGSU, CHINA
    YANG Xiao-lin, YANG Jin-ling, SU Li-na, FENG Jing, WANG Jun
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1170-1186.   DOI: 10.3969/j.issn.0253-4967.2023.05.008
    Abstract181)   HTML29)    PDF(pc) (5264KB)(94)       Save

    To date, more than 60 four-component borehole strainmeters have been deployed in China to provide more data on geodynamics and earthquake precursors. In practice, the strain signals recorded by the four-component borehole strainmeters are greatly disturbed by the effect of barometric pressure at different frequencies. Therefore, precisely deciphering the frequency-dependent strain response to barometric pressure changes in the frequency domain is very significant for continuous high-resolution borehole strainmeter measurements. However, no much related research has been conducted in this day. For these more than 60 four-component borehole strainmeters being operated in Mainland China, the study of properties(e.g., Young's modulus and Poisson's ratio)of borehole surrounding rock are seldom probed via rock mechanical testing has not been given enough attention, compared that with in the USA and Japan, consequently, which makes it difficult in the understanding of the driving mechanism of atmospheric effects.

    Beginning at 2006, a YRY-4 type borehole strainmeter was installed approximately at 50m depth in the Jiangning area to precisely monitor the earthquake precursors and tectonic movements in Jiangsu Province. For daily observation, the atmospheric effect is quite obvious in different frequency bands. Therefore, we applied transfer function to uncover features, such as barometric pressure coefficient and phase shift, of frequency dependence observed at Jiangning station in Jiangsu Province in high-(>8cpd), intermediate-(0.5~8cpd), and low-frequency(0.1~0.5cpd)bands. Furthermore, the double bush mechanical model was adopted to estimate the parameters of elastic modulus and Poisson's ratio for the borehole surrounding rock.

    The results show that: 1)The values of coherence in the frequency band(0.1~30cpd)are higher than in others. 2)The barometric pressure responses were very stable and valid in the low-frequency band, and remained stable in the high-frequency band(8~30cpd), but significantly fluctuated in the intermediate-frequency band. 3)If the impacts of the diurnal and semidiurnal tidal waves are neglected, the spectra of barometric pressure response coefficients for the four borehole sensors and areal strain were quasi-linear and stable. 4)In the high-frequency band, the spectra of phase responses for borehole strains behaved exponentially, strongly depending on frequency, with the average phase delay of about 24.2°. 5)The estimated elastic modulus and Poisson's ratio were 33.9GPa and 0.27 according to the averaged barometric-pressure response coefficients of areal strain, respectively, in which the estimated parameters showed good agreement with the results obtained via rock mechanical testing.

    These above findings will be useful for separating the nonlinear barometric responses from the four-component borehole strainmeter records, as well as for estimating the mechanical parameters of the borehole surrounding rock. In the future, more and more rock cores taken from boreholes may disappear with time. So exploring the potential values of transfer function is an important work in the field of borehole strain study. In the near future, we will reconstruct the rock mechanical parameters of borehole surrounding rock for other unstudied sites in mainland China.

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    APPLICATION OF SMALL UNMANNED AERIAL VEHICLE(sUAV)IN THE SELECTION OF SUITABLE SITES IN PALEO-SEISMIC STUDY OF BEDROCK FAULT SURFACES
    ZOU Jun-jie, HE Hong-lin, ZHOU Yong-sheng, WEI Zhan-yu, SHI Feng, GENG Shuang, SU Peng, SUN Wen
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 833-846.   DOI: 10.3969/j.issn.0253-4967.2023.04.002
    Abstract179)   HTML21)    PDF(pc) (6000KB)(161)       Save

    Bedrock normal fault scarps, as classical topographic features and geomorphological markers along mountain range fronts, form in consolidated bedrock due to faulting in extensional settings. They generally preserve more complete records of paleo-earthquakes than fault scarps in unconsolidated sediments. With the development of technologies such as fault surface morphology measurement and terrestrial cosmogenic nuclide dating, bedrock fault planes have become a nice object for paleo-earthquake study in bedrock areas. The reconstruction of paleo-seismic history from a bedrock fault scarp in terms of the times, co-seismic slips and ages by a combination of quantitative morphological analysis, TCNs dating and other physical/chemical index has been proven feasible by several previous studies.

    However, this success heavily relies on a suitable site selection along the bedrock fault scarp because erosional processes can exhume the bedrock fault surface, and the sedimentary processes can bury the bedrock fault surface. Namely, non-tectonic factors such as gully erosion, sediment burial, and anthropogenic activity make bedrock fault planes difficult to record and preserve paleo-seismic information.

    Therefore, to successfully extract paleo-seismic information from the bedrock area, it is necessary to select suitable study points along the bedrock fault scarp in advance. Traditional survey and mapping methods are time-consuming and labor-intensive, and it is difficult to understand bedrock fault scarps. The resolution of satellite images cannot obtain the fine structure of bedrock fault scarps. Small unmanned aerial vehicle(sUAV), combined with Structure-from-Motion(SfM)photogrammetry has emerged over the last decade. It is used as an established workflow in acquiring topographic data by filling the spatial gap between traditional ground-based surveys and satellite remote sensing images. As a low-altitude photogrammetry technology, it can quickly obtain high-precision three-dimensional surface structures of bedrock fault scarps.

    In this paper, taking the Majiayao bedrock fault scarp at the northern foot of Liulengshan in Shanxi Rift as an example, the high-precision and three-dimensional topographic data of the bedrock fault was obtained by using sUAV combined with SfM photogrammetry technology. The high-resolution and high-precision images of tectonic landforms can be obtained conveniently and efficiently by sUAV survey. The sUAV-obtained photos can be further processed by the SfM photogrammetry for generating a digital 3D structure of the bedrock fault scarp with true or shaded color.

    The non-tectonic factors such as rock collapse, sediment burial, and gully erosion along the bedrock fault scarp are identified by interpreting the 3D model of the bedrock fault scarp. The profile shape characteristics of the erosion, burial and tectonic fault scarps are summarized through fine geomorphological interpretation and fault profile analysis. For the erosion profile, the hanging wall slope is down-concave, showing that the fault surface below the ground surface has been partially exposed. For the bury profile, the hanging wall slope shows an obvious concave-up shape, indicating that the lower part of the bedrock fault surface has been partially buried by the colluvium. For the tectonic profile, the hanging wall slope shows a smooth and stable slope, showing the exhumation of bedrock fault scarp is controlled purely by tectonics. Finally, the study sites suitable for paleo-earthquake study on bedrock fault surfaces were selected, showing the important role of sUAV aerial survey technology in the selection of paleo-earthquake study sites in bedrock areas.

    This study illustrates that based on the high-precision three-dimensional surface structure of the bedrock fault plane from sUAV aerial survey, the existence of non-tectonic factors such as gully erosion, sedimentary burial and bedrock collapse can be clearly identified. These non-tectonic sites can be excluded when selecting suitable sites for paleo-earthquake study indoors. The shape analysis of bedrock fault scarp is also helpful to determine whether the bedrock fault surface is modified by surface process and suitable for paleo-seismic study. The sUAV aerial survey can play an important role in paleoseismic research in the bedrock area, which can accurately select the study points suitable for further paleo-seismic work in the bedrock area.

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    DISCUSSION ON SEISMOGENIC STRUCTURE OF THE JUNE 2022 MS6.1 EARTHQUAKE AND ITS RELATIONSHIP WITH THE APRIL 2013 MS7.0 EARTHQUAKE IN LUSHAN, SICHUAN PROVINCE
    CHEN Han-lin, WANG Qin-cai, ZHANG Jin-chuan, LIU Rui-feng
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1233-1246.   DOI: 10.3969/j.issn.0253-4967.2023.05.012
    Abstract177)   HTML32)    PDF(pc) (7565KB)(153)       Save

    In this paper, we relocated earthquakes occurred from April 2013 to July 2022 in Lushan seismic zone, inversed focal mechanism solution of the Lushan MS6.1 earthquake on June 1, 2022 and discussed the seismogenic structure of the Lushan MS6.1 earthquake and its relationship with the MS7.0 earthquake in April 2013.

    The results of the focal mechanism solution show that the Lushan MS6.1 earthquake in 2022 is a thrust earthquake. The strike, dip and azimuth of nodal plane Ⅰ are 228°, 46° and 104° and for nodal plane Ⅱ are 28°, 46° and 76° respectively. The results of earthquake relocation show that the focal depth of the Lushan MS6.1 earthquake sequence is shallow in the north and deep in the south, the fault length is about 10km. The focal depth is mainly concentrated between 10km to 19km. The fault dip is southeast with an angle of 60°. The initial rupture point of the main shock of the Lushan MS6.1 earthquake is at a depth of 20km, located at the deepest part of the fault. The fault ruptured from deep to shallow. The Lushan MS7.0 earthquake occurred on April 2013 strikes northeast and dips northwestward, but there exists a reverse fault in the aftershock sequence that has the same direction of strike but the opposite direction of dip. This reverse fault is consistent with the strike and dip of the MS6.1 earthquake occurred in June 2022. It appears as two parallel faults in the profile. In addition to the reverse fault on the west side, the embryonic of another reverse fault seems to appear on the east side of the middle of earthquake sequence. These faults are about 10km away from the surface. The distribution of earthquakes in two northwest-oriented depth profiles shows that the dip angles of the main shock and the reverse fault of the MS7.0 earthquake is different at different locations, and these faults are not simple straight planar sections. From one year after occurrence of the MS7.0 earthquake to occurrence of the MS6.1 earthquake, the seismic activity on the main fault decreased but the seismic activity on the reverse fault on the west side of the MS7.0 earthquake sequence was more active during this period, most of the seismic activity occurred near the reverse fault that is parallel to the MS6.1 earthquake fault.

    By analyzing the seismogenic structure and seismic activity characteristics of the Lushan seismic zone, we concluded the Lushan MS6.1 earthquake on June 1, 2022 is caused by a blind thrust fault with strike towards northeast and dip towards southeast, located 10km away from the surface. It has the opposite directions of strike and dip of the Longmenshan Fault. The epicenters of the Lushan MS7.0 earthquake in April 2013 and the MS6.1 earthquake in June 2022 are located near the surface exposure traces of the Shuangshi-Dachuan Fault and the Xiaoguanzi Fault, respectively. However, according to the analysis of the relocation aftershock depth in profile, the aftershock extension to the surface does not coincide with the surface exposure positions of the Shuangshi-Dachuan Fault and the Xiaoguanzi Fault. Therefore, the seismogenic faults of these two earthquakes are not the Shuangshi-Dachuan Fault and the Xiaoguanzi Fault, but two blind reverse faults. The Shuangshi-Dachuan Fault near the MS6.1 earthquake sequence and the main shock fault of the 2013 MS7.0 earthquake are thrust faults dipping northwest, while the Lushan MS6.1 seismogenic fault has opposite direction of dip. The seismogenic fault of the Lushan MS6.1 earthquake and the main thrust fault of the 2013 MS7.0 earthquake, which strikes northeast and dips northwest with the reverse thrust fault of the hanging wall, which strikes northeast and dips southeast, together form a double layer Y-shaped structure. These faults are all blind thrust faults and belong to the Qianshan-Shanqian Fault system in the southern segment of the Longmenshan fault zone. The seismogenic structure in the Lushan seismic zone is a complex fault system composed of one main northeast strike fault with dipping northwest, and three faults dipping southeast.

    From one year after occurrence of the Lushan MS7.0 earthquake to the occurrence of the Lushan MS6.1 earthquake, most of earthquakes in the Lushan seismic zone occurred near a reverse fault which is parallel to the Lushan MS6.1 earthquake seismogenic fault. These earthquakes are located in the area where the coulomb stress change caused by the MS7.0 earthquake acts as loading effect. The Lushan MS6.1 earthquake sequence is mainly distributed in the area where the coulomb stress change plays an unloading role caused by the Lushan MS7.0 earthquake. The research results showed that the coulomb rupture stress caused by the Lushan MS7.0 earthquake on the seismic nodal plane of the MS6.1 earthquake has a restraining effect on the MS6.1 Lushan earthquake.

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    SELF-NOISE MODELS OF SEISMOMETERS BASED ON PROBABILITY DENSITY FUNCTION REPRESENTATION
    WANG Kai-ming, YU Da-xin, ZHAO Li-jun, LI Wen-yi, YE Qing-dong
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1187-1199.   DOI: 10.3969/j.issn.0253-4967.2023.05.009
    Abstract174)   HTML9)    PDF(pc) (4725KB)(97)       Save

    The self-noise levels of different seismometer models directly affect the quality of seismic observations and further limit earth science research based on seismic records. We conducted a test of the self-noise level of seven types of seismometers at the Malingshan seismic station, in which the instrument types included short-period, broadband, very-broadband and ultra-broadband. Three seismometers of each type were set up, and the observation period was from November 22, 2018, to March 26, 2019. In this paper, based on continuous seismic waveforms from seven models of seismometers, the self-noise power spectral density(PSD)of the seismometers was calculated by using the three sensor correlation analysis method, and the probability density distribution of the self-noise PSD of the seven models of seismometers was obtained by using the PDF representation. Based on the mode values of the PDF distribution, the self-noise models of the three channels(UD, EW and NS)of the seven models of seismometers are given respectively.

    For the ultra-broadband seismometer CMG-3T-360, in the microseism band(0.1Hz to 1Hz), the self-noise of the horizontal components(EW and NS)is higher than that of the vertical components(UD)and is consistent with the trend of the seismic noise, which may be attributed to misalignment of the horizontal direction between seismometers. In the low frequency band(<0.03Hz), the self-noise level of the horizontal component is higher than that of the vertical component, and small changes in the barometric pressure may lead to higher incoherent noise in the horizontal direction of the sensor at long periods. Compared with the vertical direction, the horizontal direction of the seismometer is more susceptible to air disturbances. At a frequency of 0.005Hz, the instrument self-noise of the horizontal component is close to the seismic background noise, and the instrument self-noise of the horizontal component is the main source of noise recording. Installing a heat and wind shield can effectively reduce the seismometer self-noise in the low frequency band. When using the CMG-3T-360 to observe long-period seismic signals, a shield with both thermal insulation and windproof function is required.

    The self-noise level of the short-period seismometer JS-S02 is lower than that of TDV-33S and lower than that of the New Low Noise Model(NLNM)between 0.15Hz and 7Hz. In the UD and EW channels, the self-noise level of TDV-33S is lower than the NLNM model between 0.17Hz and 0.5Hz. The higher instrument self-noise further limits the extraction of long-period seismic signals in the digital recordings of short-period seismometers.

    For the broadband seismometer TDV-60B and the very broadband seismometer TDV-120VB, the self-noise levels are basically consistent in the vertical direction and also are higher than those of the broadband seismometer JS-60 and the very broadband seismometer JS-120. In the horizontal direction, the two self-noise levels in the microseism band and the low frequency band show different characteristics, i.e., the self-noise levels of TDV-60B are lower than those of TDV-120VB in the microseism band and higher than those of TDV-120VB in the low frequency band. When the frequency is lower than 0.03Hz, the self-noise levels of TDV-60B and TDV-120VB on the horizontal channels are close to those of JS-60 and JS-120, respectively.

    For JS-60 and JS-120, in the vertical direction, the self-noise levels of both are close to CMG-3T-360 in the microseism band. The self-noise level of JS-120 on the vertical channel is lower than 5dB away from CMG-3T-360 in the low frequency band and lies within the 68%confidence interval of the PSD; in the high frequency band(>2Hz), it is higher than CMG-3T-360 confidence interval of the PSD. In the horizontal direction, the self-noise levels of JS-60 and JS-120 are lower than those of CMG-3T-360 between 0.15Hz and 1Hz and in the microseism band, respectively. The self-noise levels of JS-60 on the horizontal channel show good agreement with those of CMG-3T-360 in the high frequency band. The self-noise of JS-120 on NS channel is higher than CMG-3T-360 confidence interval in the low frequency band. When extracting long-period seismic signals, a seismometer with lower noise level in the low frequency band should be selected as much as possible.

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    INTERSEISMIC SLIP RATES AND SHALLOW CREEP ALONG THE NORTHWESTERN SEGMENT OF THE XIANSHUIHE FAULT FROM INSAR DATA
    CHEN Yi, ZHAO Bin, XIONG Wei, WANG Wei, YU Peng-fei, YU Jian-sheng, WANG Dong-zhen, CHEN Wei, QIAO Xue-jun
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1074-1091.   DOI: 10.3969/j.issn.0253-4967.2023.05.003
    Abstract173)   HTML20)    PDF(pc) (10431KB)(177)       Save

    Located in the eastern boundary of the Qinghai-Tibetan plateau, the Xianshuihe fault zone is one of the most active left-lateral strike-slip faults in Chinese mainland. As the southern boundary of the Bayanhar block, the Xianshuihe Fault accommodates the southeastward transport of material toward southeastern Asia. Earthquakes have occurred frequently along this fault, especially in the northwestern segment. More than 20 earthquakes with MW>6.0 have ruptured since 1700. The most recent MW>7 earthquake was the Luhuo earthquake in 1973, and the most recent MW>6 earthquake was the MW6.6 Luding earthquake in 2022. As one of the most active faults in mainland China, the present slip pattern of the Xianshuihe Fault, especially the shallow creep characteristics along its northwestern segment, has attracted much attention.

    The primary goal of determining slip rates of active faults using geodetic data is to quantify the seismic potential of the faults. Illuminating the long-term slip rate and shallow creep distribution of faults is the basis for calculating the seismic moment rate and evaluating the seismic potential. Due to the backwardness of early measurement methods, the seismic deformation along the Xianshuihe Fault was previously based on geologic, cross-fault short baseline and leveling surveys. With the application of GPS in tectonic geodesy, more and more GPS stations are installed near active faults, which provide accurate constraints on the long-term slip rates of the fault. Subsequently, the appearance of InSAR technology has brought a beneficial supplement to GPS, providing high spatial resolution surface velocity maps, which have been widely used to measure deep and shallow creep along active faults. It is the key to accurately characterize the fault slip behavior and evaluate the seismic potential.

    In this study, 119 Sentinel-1 satellite descent data from December 2014 to December 2021 were processed to obtain the average line-of-sight(LOS)velocity field of the northwestern segment of the Xianshuihe Fault based on the small baseline InSAR method. Then the elastic screw dislocation model was used to fit the fault normal InSAR LOS velocity profiles to estimate the long-term slip rates and shallow creep rates. Combined with the viscoelastic earthquake cycle model, the effects of the earthquake recurrence period, and rheology of the lower crust and upper mantle on slip rate estimation in Luhuo segment are analyzed. The main results are as follows:

    (1)The average InSAR LOS velocity field is in the northwestern segment of the Xianshuihe Fault during 2014—2021 has been obtained with a large range and high spatial resolution. The velocity field results show an obvious velocity gradient across the surface trace of the Xianshuihe Fault, which is consistent with the left-lateral strike-slip characteristics of the Xianshuihe Fault.

    (2)To investigate the slip rate variation along the northwestern segment of the Xianshuihe Fault, we used the two-dimensional elastic screw dislocation model to fit the 14 fault-normal velocity profiles selected along the northwestern segment of the Xianshuihe Fault and estimated the long-term slip rates and shallow creep rates using the Markov Chain Monte Carlo(MCMC)method. The results show that the overall slip rates of the NW segment of the Xianshuihe Fault range from 7.2mm/a to 11.0mm/a, and gradually decrease from west to east. The shallow creep rate ranges from 0.3mm/a to 3.1mm/a. The high creep rate appears mainly at Xialatuo and the segment from Daowu to Songlinkou. The shallow creep rates in other places are close to zero, implying that the fault is completely locked.

    (3)According to historical earthquake records, the recurrence interval of the Luhuo segment is set to be 150 years, 200 years, and 400 years, and the viscosity of the lower crust and upper mantle is set to be 5.0×1018Pa·s, 1.0×1019Pa·s, and 5.0×1019Pa·s. The slip rate of the Luhuo segment is estimated to be (7.91±0.3)~(9.85±0.4)mm/a using the MCMC method, which is slightly lower than the (10.14±0.5)mm/a obtained by the pure elastic model. In addition, when the earthquake recurrence interval is 150 years and the viscosity of the lower crust and upper mantle is 5.0×1019Pa·s, we simulate the fault-normal velocity at 5 years, 20 years, 75 years, and 125 years after the 1973 Luhuo earthquake, and find that in any period of the seismic cycle, the estimation of fault slip rate will be biased to some extent if the viscoelastic contribution of the lower crust and upper mantle is ignored.

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    STUDY ON THE CHARACTERISTICS AND MECHANISM OF LONG-PERIOD GEOELECTRIC FIELD VARIATION: A CASE STUDY FROM DULAN SEISMIC STATION
    XI Ji-lou, ZHAO Jia-liu, GAO Shang-hua, WANG Xiao-lei, LI Guo-you, MENG Fan-bo
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1092-1111.   DOI: 10.3969/j.issn.0253-4967.2023.05.004
    Abstract168)   HTML9)    PDF(pc) (4729KB)(128)       Save

    The geoelectric field is an important geophysical field, and the long-term geoelectric field is also an important component of the Earth's electromagnetic field, which mainly refers to the geoelectric field with a variation period greater than 100,000 seconds. The long-term geoelectric field can dynamically reflect the trend changes of spontaneous electric field, periodic changes of telluric electricity field, and other seasonal disturbances. Furthermore, it can further reveal the basic characteristics and physical mechanisms are related to the sources of the electromagnetic field, through such analysis and research on the changes in the long-term geoelectric field.

    In this paper, a systematic study has been conducted on the basic characteristics and variation mechanism of the long-term geoelectric field, which is mainly based on long-term geoelectric field observation data Dulan seismic station in Qinghai Province from 2015 to 2022. Firstly, based on interference preprocessing and reliability analysis, various methods and approaches such as wavelet analysis, median filtering, convolutional filtering, linear fitting are used to filter out high-frequency disturbances in the geoelectric field and to extract and analyze the components of long-term geoelectric field variation. At the same time, based on the analysis results of the abnormal changes in the electric field at the Dawu seismic station before and after the Madoi MS7.4 earthquake, the possible correlation and main field source mechanism between the abnormal variation of the electric field at the Dulan seismic station and the earthquake has been analyzed and discussed.

    The research results show that, from the observation data of geoelectric field at Dulan seismic station: 1)It has relatively strong long-term stability and variation reliability, with the arithmetic mean value of the daily correlation coefficient not less than 0.98, and that of the daily variation difference not more than 0.2mV/km, in the Same measurement direction; 2)The FFT spectrum analysis results include not only short and medium cycle changes such as 24h, 12h, 8h and 6h, but also long cycle changes such as annual cycle, semi-annual cycle and so on; 3)The daily variation range shows seasonal variation with high in summer and low in winter, and the significant variation includes annual cycle, half-year cycle, 27-day cycle, half-month cycle; 4)The trend variation shows a typical periodic annual changing feature as the sine wave change, and the annual extreme value and change amplitude are basically the same; 5)Before and after the Madoi MS7.4 earthquake, there were obvious abnormal distortions from the annual variation.

    Theoretical and mechanism analysis shows that the periodic changes in the daily variation amplitude of the geoelectric field at the Dulan seismic station mainly come from the periodic changes in spatial field sources, and the trend periodic changes of the geoelectric field mainly come from the long-term tidal effect generated by the gravitational action of the sun. During the process of Earth's orbit and rotation around the sun, the periodic changes in direct solar radiation, as well as in the ionospheric plasma concentration and the underground fluid seepage field caused by the gravitational force of the sun, are the important excitation mechanisms for this long-term periodic change of the geoelectric field.

    According to the comprehensive analysis, it is believed that the variation of the geoelectric field data observed at the Dulan seismic station has excellent objectivity, reliability, and long-term stability, which can reflect the characteristics of long-term geoelectric field changes and abnormal distortion characteristics of the geoelectric field before and after strong earthquakes. And the change mechanism is in good agreement with the theoretical analysis results and has strong research and application value.

    In summary, the research results which carried out on the long-term variation of the geoelectric field, as well as revealed the important mechanisms of the generation and variation of Earth's electromagnetic fields, will have more important enlightening significance and reference value in the multifaceted research work about the geoelectric field observation, such as the site survey, observation system construction, observation data analysis and application et al.

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    COMPARISON OF THE EFFECTS OF EARTHQUAKE-TRIGGERED LANDSLIDE EMERGENCY HAZARD ASSESSMENT MODELS: A CASE STUDY OF THE LUSHAN EARTHQUAKE WITH MW5.8 ON JUNE 1, 2022
    MA Si-yuan, XU Chong, CHEN Xiao-li
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 896-913.   DOI: 10.3969/j.issn.0253-4967.2023.04.006
    Abstract168)   HTML19)    PDF(pc) (10815KB)(125)       Save

    Earthquake-induced landslides, as an important secondary geological disaster, typically occurring during or shortly after an earthquake, have the characteristics of large quantity and scale, wide distribution, complex mechanism, serious casualties and economic losses, and long-duration post-earthquake effect. Rapidly and accurately obtaining the spatial distribution and potential hazard assessment of coseismic landslide following an earthquake is critical for emergency rescue and resettlement planning. Currently, the most commonly-used coseismic landslide hazard assessment methods include the data-driven machine learning methods and the Newmark method based on mechanics mechanism. The 2022 MW5.8 Lushan earthquake provides a valuable window for us to carry out rapid emergence assessment of earthquake-induced landslides with different evaluation models. In this study, a new generation of China's earthquake landslide hazard model(hereinafter referred to as Xu2019 model)and a simplified Newmark model are used to carry out the rapid landslide assessment of Lushan event. The Xu2019 model selects 9 earthquake-induced landslide inventories around China as training samples and uses a total of 13 influencing factors such as elevation, relative elevation, slope angle, and aspect, and etc. to generate a near real-time evaluation model for coseismic landslides based on the LR method. The model can rapidly assess coseismic landslides towards a single earthquake event according to the actual PGA distribution. For Newmark model, the cumulative displacement(Dn)is calculated by the critical acceleration(ac)and PGA maps. For the landslide inventory of this earthquake event, we completed the landslide inventory covering the entire affected area based on high-resolution optical satellite images(Planet)with 3m resolution acquired on 6 July 2022. Based on the coseismic landslide inventory including 2 352 landslides with an area of 5.51km2, the accuracy and applicability of the two models are estimated. The results show that the landslide area calculated based on Xu2019 model is 5.07km2, which is very close to the actual landslide area, and the predicted area calculated based on Newmark model reaches 21.3km2. From the perspective of the spatial distribution of the prediction results, the distribution of the predicted high failure probabilities of the two models is roughly same, with the high probability values mainly located on the left side of the seismogenic fault. However, the difference lies in the low probability predictions of the northwest region of Baoxing county by the Xu2019 model. A zoomed-in view of a specific area comparing the spatial distribution of predicted landslide probabilities with the landslide abundance area shows that most actual landslide are concentrated in the medium to high failure probability areas predicted by the Xu2019 model, with only a few sporadic events occurring in the low probability zone. On the other hand, the Newmark model primarily identifies high instability probability regions in steep slope areas, which correspond closely to the actual landslide and collapse occurrences. However, the predicted hazard level of the northwest region i.e. the landslide highly developed area is obviously low by Xu2019 model, while the prediction result based on Newmark model for the southwest region is obviously overestimated. In terms of the LR model, the prediction results are very close to the actual landslide distribution, and the majority of the landslides are essentially located in areas with a high failure probability, indicating that the model has a relatively high prediction accuracy. The ROC curve is used to assess the model's accuracy. The results suggest that the model based on Xu2019 outperforms the Newmark model, with a prediction accuracy of 0.77, while the prediction accuracy of the Newmark model is 0.74. Overall, both two models have good practicability in the rapid evaluation of cosesimic landslide. However, the Newmark model needs multi parameter input, and these parameters themselves and the way of human acquisition are uncertain, which results in that the model evaluation is greatly affected by subjectivity.

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    THE ANALYSIS OF THE STARTING YEAR AND THE COM-PLETENESS OF SEISMIC RECORDS FOR EARTHQUAKES WITH MAGNITUDES M≥7 IN THE BORDER REGION OF SICHUAN, YUNNAN AND TIBET
    ZHOU Jie-yuan, ZHOU Qing, RAN Hong-liu
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 914-935.   DOI: 10.3969/j.issn.0253-4967.2023.04.007
    Abstract167)   HTML13)    PDF(pc) (8795KB)(122)       Save

    Earthquake catalog is the foundational data for analyzing seismic activity, assessing seismic hazard, and studying earthquake prediction. The majority of historical earthquake records are sourced from historical documents, with a significant portion of these records found in local gazetteers. Compiling historical literature is an essential way in analyzing seismic activity because historical accounts of earthquakes often provide more detailed and accurate information than geological data. Among these sources, analyzing relevant content in local gazetteers, such as the historical development of local governance, military garrison, official records, and descriptions of disasters and auspicious events, plays a crucial role in seismic activity research. This article aims to acquire historical earthquake records by consulting local gazetteers, folk books, and other historical sources containing natural, social, and political records. These records serve as historical foundations for analyzing the completeness of seismic data records.

    The border region between Sichuan, Yunnan, and Tibet is located in the northwest secondary block of the Sichuan-Yunnan block, which is one of the areas with frequent strong earthquakes in China. The Xianshuihe fault zone and the Jinshajiang fault zone are the northeastern and northwestern boundary faults of the Sichuan-Yunnan block, respectively. They are large-scale and highly active fault zones formed due to the eastward escape of the Tibetan plateau caused by the relative movement between the Indian and Eurasian plates. Previous studies on active tectonics have shown that major earthquakes with magnitudes of 8 and above, as well as over 80% of strong earthquakes with magnitudes of 7, mainly occur in the boundary zones of active blocks with intense structural deformation and high stress accumulation. Moreover, the known active faults in the study area, such as the Batang fault and Litang fault, are also major faults that significantly have influence on the occurrence strong earthquakes. The Sichuan-Yunnan-Tibet adjacent region is home to significant infrastructure, including the Sichuan-Tibet railway and hydropower stations. Analyzing the completeness of earthquake data in the border region of Sichuan, Yunnan, and Tibet can contribute to the assessment of fault hazards and the analysis of regional seismic activity trends. This, in turn, can help minimize the damage caused by earthquakes to critical infrastructure and further enhance the safety and security of people’s lives and properties.

    This study reviewed the local gazetteers of 44 counties in the border region between Sichuan, Yunnan, and Tibet, and summarized the establishment and historical evolution of each county. Based on the analysis of the road evolution from Sichuan to Tibet and from Yunnan to Tibet, we examined the significant roles of important transportation hubs and nodes, such as stations, pond flood, and grain platforms, in regarding of recording earthquakes. Combining various historical sources and previous research on the completeness of earthquake data in the region, we conducted a comprehensive analysis to determine the probable starting years for the availability of seismic records of magnitude 7 and above in the Xianshuihe area and the three parallel rivers area. Additionally, based on the data of the length and short axis of isoseismal lines from 88 earthquakes, an elliptical model was used to derive the seismic intensity attenuation relationship for the Sichuan-Yunnan block. By placing the fitted isoseismal lines of magnitude 6 and 7 earthquakes in the study area, we analyzed their impact range, providing a spatial dimension basis for the completeness analysis of seismic data.

    This article provides a comprehensive analysis and demonstration of the complete starting years of seismic data in the border region between Sichuan, Yunnan, and Tibet from both temporal and spatial perspectives. The results indicate that due to the establishment of grain stations and Tangxun along the Sichuan-Tibet road, as well as the appointment of officials, several counties in the Xianshuihe area, including Kangding, Luhuo, Garzê, Litang, and Yajiang, were developed between 1719 and 1736. At the same time, there are relatively abundant historical documents related to earthquakes in the Xianshuihe area. Local chronicles, reports from governors and resident ministers, written records in Tibetan temples, and accounts from lamas have documented earthquake surveys, disaster assessments, and relief efforts. By combining these historical sources with the analysis of intensity attenuation relationships in the Sichuan-Yunnan block, the affected areas of earthquakes with magnitudes 6 and 7 can be determined that the period from 1719 to 1736 marks the starting years with complete M≥7 earthquake data in the Xianshuihe area. The towns of Batang, Mangkang, and Changdu in the three parallel rivers area are also significant nodes and hubs along the road to Tibet. They were established with administrative institutions and granaries between 1719 and 1728, and the road network extensively covered Tangxun in the region. In considering the seismic records and historical sources in the three parallel rivers area, as well as referencing the recording capabilities of granaries, administrative institutions, and Tangxun in the Xianshuihe area, and estimating the potential recorded seismic magnitudes based on the intensity attenuation relationships of the Sichuan-Yunnan block, it can be suggested that the period from 1719 to 1728 is a possible starting point for complete earthquake data with M≥7 in the three parallel rivers area. In areas farther away from the road to Tibet, such as Jiangda, Gongjue, Baiyu, Xinlong, and the northern regions of Batang and Litang, as well as the large contiguous regions of Derong, Xiangcheng, Daocheng, and Jiulong, the eastern boundary is the Xianshuihe fault zone, while the area between the two zones is divided by the northeast-oriented Batang fault. Previous seismic geological investigations have found that within the aforementioned regions, the influence of the Jinshajiang fault zone extends along the Batang-Derong-Benzilan line. In remote areas away from the road and with sparse population, the possibility of individual earthquakes with magnitudes above 7 occurring but being missed cannot be ruled out. However, in other areas not located on active fault zones, it can be considered unlikely to experience earthquakes with magnitudes above 7. Based on the analysis of the data, the starting years of earthquakes with a magnitude of 6 and above should be the same as those of earthquakes with a magnitude of 7 and above. However, according to the analysis of the average occurrence rate of earthquakes per year, there is a significant lack of records for earthquakes of magnitude 6 and above. This may be due to the sparsely populated and vast nature of the Tibetan region during historical times, limited administrative capabilities of officials, and lack of earthquake historical records and documents. Therefore, it is not possible to determine the exact starting year for complete data on earthquakes of magnitude 6, which would be the same as for earthquakes of magnitude 7 and above.

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    CRUSTAL DENSITY STRUCTURE OF THE EASTERN TIBETAN PLATEAU AND ITS GEODYNAMIC IMPLICATIONS
    LI Dan-dan, TANG Xin-gong, XIONG Zhi-tao
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 936-951.   DOI: 10.3969/j.issn.0253-4967.2023.04.008
    Abstract167)   HTML14)    PDF(pc) (6726KB)(116)       Save

    The continuous collision and convergence between the Indian and Eurasian plates have caused strong uplift and deformation within the Tibetan plateau and the surrounding areas. The eastern Tibetan plateau, as an important channel for the eastward and south-eastward expansion of plateau materials, is an critical area for understanding the interaction between the Tibetan plateau and the eastern tectonic blocks and for understanding the eastward escape of plateau deep materials, which is of great significance for studying the uplift and deformation mechanism of the Tibetan plateau. A large number of studies on the eastern Tibetan plateau have provided an important basis for revealing the uplift mechanism of this region. However, its complex geology makes it difficult in understanding the uplift mechanism from the single geophysical interpretation. The gravity field reflects the density properties of the subsurface material, and can be related to the wave velocity properties of the seismic data by certain translation relationships. In addition, gravity data can improve the crustal model of the area not adequately covered by seismic data, which can not only provide the three-dimensional crustal density structure of the area, but also reflect the relationship between the spatial distribution of earthquakes and the crustal structure from a gravity perspective. In this paper, based on the previous research results, we selected field survey gravity data of nine intersecting lines and used the deep seismic reflection as the constraint to invert the density interface depth distribution of each line by using human-computer interaction mode, and then used the kriging interpolation method to obtain the three-dimensional Moho depth and basement depth in the area, and then we obtained the sediment thickness by analyzing the difference between the topography and the basement depth. The inversion results show that the overall trend of Moho depth is deep in the west and shallow in the east, with the deepest depth in the west being 61km and the shallowest in the east being about 40km. There is a large difference between the two sides of the arc belt formed by the Longmenshan-Anninghe-Xiaojinhe fault, with the northwest side of the arc belt basically above 52km, among which the Moho depth is about 58km in the Bayankara block and the northern part of the Chuan-Dian rhombus block, and about 53km in the Chuan-Dian rhombus block and the southern part of the Indo-China block. The Moho depth is about 42km in the Sichuan Basin on the east side of the arc belt, which constitutes a gradient zone of Moho depth around the Tibetan plateau. There also exists a depressional zone of Moho in the Bayankara block, which may be related to the eastward flow of plateau material and the blockage of Sichuan Basin, so that part of the asthenosphere material accumulates and squeezes, thus forming a relatively thicker crust and the sinking of Moho. Part of the eastward overflowing asthenosphere material turns to the south and south-east direction, resulting in the thickness of the crust in the southwest of the Chuan-Dian rhombus block is greater than the east and west sides. At the same time, the late Paleozoic mantle column activity led to the uplift of the lithosphere and the intrusion of high-density material into the lithosphere, which also blocked the southward flow of material from part of the Tibetan plateau. From the inverted sediment thickness, the sediment on the eastern Tibetan plateau is relatively thicker in the center of several tectonic blocks, up to 7km thick, while the sediment at the edges of the blocks is relatively thinner, and even bedrock is exposed in some areas. Combined with the spatial distribution characteristics of earthquakes in this area, the Moho depth and sediment thickness distribution in the eastern Tibetan plateau are strongly correlated with the distribution of earthquakes in this area, which has important reference value for future earthquake prediction.

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    NUMERICAL SIMULATION OF INFLUENCING FACTORS OF SURFACE RUPTURE IN OVERLYING SOIL LAYER OF THRUST FAULT
    GUO Ting-ting, XU Xi-wei, YUAN Ren-mao, YANG Hong-zhi
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1200-1218.   DOI: 10.3969/j.issn.0253-4967.2023.05.010
    Abstract158)   HTML19)    PDF(pc) (10497KB)(154)       Save

    When the strong earthquakes occur, the deformation and rupture of overlying soil caused by the dislocation of focal faults is one of the important reason for the destruction of ground structures. In the process of a strong earthquake or large earthquake, the deformation reaction and failure of the overlying soil of underground concealed faults are very complicated. To study and analyze the characteristics and influencing factors of surface deformation and fracture of the overlying soil layer, in this paper, the influences of fault dip Angle, fault displacement, and overlying soil thickness on surface deformation and fracture of overlying soil are analyzed by the finite element numerical simulation method comprehensively. The results show that: 1)With the increase of fault vertical dislocation of 1m to 4m, the surface equivalent strain gradually increases, the surface rupture is more likely to occur, and the surface rupture width also wider. With the increase of the thickness of the overlying soil layer from 20m to 60m, and the increase of the fault inclination from 30°, 45°, 70° to nearly 90°, the surface equivalent strain is gradually smaller, the surface rupture is more likely to occur, and the surface fracture width becomes smaller, which means that the amount of dislocation required for the same rupture state needs to increase. 2)When the vertical dislocation of the fault is about 3.3%for the thickness of the overlying soil, the surface rupture occurs only as the fault dip angle is 30°, no surface rupture occurs as the dip angle is 45° and 70°. When the vertical dislocation of the fault is about 5% of the thickness of the overlying soil, the surface rupture occurs only as the fault dip angle is 30° and 45°, no surface rupture occurs as the dip angle is 70° and approaching 90°. When the vertical dislocation of the fault is about 6.6% of the thickness of the overlying soil, the surface rupture occurs as the fault dip angle is 30°、 45° and 70°, and surface rupture is expected to occur as the dip angle is approaching 90°. When the vertical dislocation of the fault is about 10% of the thickness of the overlying soil, the surface rupture occurs as the fault dip angle is 30°, 45°, 60°and approaching 90°. 3)When the amount of vertical dislocation and the thickness of the overlying soil are certain, the ratio of surface rupture width between the hanging wall and footwall which is less affected by fault dip ranges from 3︰1 to 3︰2~1︰1 with the increase of fault dip Angle from 30°, 45° to 70°. When the fault inclination Angle is 30°, with a decrease of vertical dislocation of 4m to 1m, or the increase of overlying soil layer thickness of 20m to 60m, the ratio of surface rupture width between hanging wall and footwall is slightly larger from about 3︰1. When the fault inclination Angle is 45°, with the decrease of vertical dislocation of 4m to 1m, or the increase of overlying soil layer thickness of 20m to 60m, the ratio of surface rupture width between hanging wall and footwall is slightly larger from about 2︰1. When the fault inclination Angle is 75°, with the decrease of vertical dislocation of 4m to 1m, or the increase of overlying soil layer thickness of 20m to 60m, the ratio of surface rupture width between hanging wall and footwall is slightly larger from about 3︰2~1︰1. Under the above fault dip conditions, the ratio of surface rupture width between hanging wall and footwall is less affected by the amount of vertical dislocation and the thickness of overlying soil. As the inclination is approaching 90°, the ratio of surface rupture width between the hanging wall and footwall is about 1︰1, which is not affected by the vertical dislocation and the thickness of the overlying soil layer. 4)The deformation and fracture of the overlying soil layer first began with the soil fracture at the interface of the fault bedrock and soil. With the increase in the amount of dislocation, a fracture point appeared on the surface when the fault dip Angle was 30°, 45°, and 70°. However, when the dip Angle of the fault was close to 90°, there were two initial rupture points on the surface. With the increase of vertical dislocation or the decrease of the overlying soil layer thickness, the overlying soil layer through fracture is finally formed.

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    THE LATE QUATERNARY ACTIVITY CHARACTERISTICS AND SLIP RATE OF BATANG FAULT IN SE TIBETAN PLATEAU
    HUANG Wei-liang, ZHANG Jia-le, XIANG Wen, YANG Qian-hao
    SEISMOLOGY AND GEOLOGY    2023, 45 (6): 1265-1285.   DOI: 10.3969/j.issn.0253-4967.2023.06.002
    Abstract154)   HTML30)    PDF(pc) (17828KB)(219)       Save

    The southeastern margin of the Tibetan plateau is one of the most intensely deformed regions in the continental crust. A series of active faults with varying lengths and mechanical properties have segmented the lithosphere into multiple active blocks, with the Sichuan-Yunnan block being one of the most tectonically active regions. Its eastern boundary is characterized by secondary fault zones such as the Xianshuihe-Anninghe-Zemuhe, Xiaojiang, and Daliangshan fault zone, forming a narrow and continuous strike-slip deformation zone with a total length exceeding 1 100km. The western boundary of the Sichuan-Yunnan Block is mainly composed of the Jinsha River and the Red River fault zone, with the Jinsha River fault zone consisting of more than 20 roughly parallel secondary faults, forming a complex fault zone with 30~200km width. Despite recent GNSS network observation revealing the current tectonic deformation rates in this region, there is still a lack of research on the deformation characteristics and rates of individual active faults. This limitation makes it difficult in the assessment and understanding of seismic hazards in the area, restricting the scientific understanding of the current deformation mode in the southeastern margin of the Tibetan plateau.

    The Batang Fault, located within the Jinsha River fault zone at the western boundary of the Sichuan-Yunnan block, is a NE-trending main fault that obliquely cuts across the Jinsha River Fault, dividing later into northern and central segments. Presently, the Batang Fault is characterized by dominant right-lateral strike-slip motion. The deformation characteristics and rates of this fault since the Late Quaternary are crucial for understanding the spatial distribution of strong earthquakes and deformation patterns in the Sichuan-Yunnan block.

    The Batang Fault has a total length of 115km and is a Holocene right-lateral strike-slip active fault. The fault extends along the margins of bedrock mountains on both sides of the Maqu river and Jinsha River valleys, trending NNE or NWW to SEE, with a steep dip. The fault exhibits linear distribution of topographic features such as slopes, ridges, triangular facets, and fault scarps, essentially controlling the boundaries of bedrock mountains. In view from the geomorphology, the Batang Fault appears continuous and straight without distinct segmentation, except for localized small-scale step-like features. The Batang Fault has preserved abundant Late Quaternary activity evidence in two areas, Huangcaoping village and Batang county. This study utilized unmanned aerial vehicle photogrammetry to establish sub-meter digital terrain data for Huangcaoping and Batang site, accurately measuring displaced features such as alluvial fans and gullies affected by faulting. In Huangcaoping site, the fault has cut through multiple mountain-front alluvial fans, causing varying degrees of horizontal displacement in features such as gullies and the margin of the alluvial fans. This provided a scale for quantifying fault displacement. In Huangcaoping, five large-angle gullies intersect with the fault, one of which is a large gully developed in the bedrock mountain area. The gully has a deep incision, a narrow valley, and a rapid downstream turn to the right after exiting the mountain. The left bank of the gully preserves two geomorphic surfaces, Qo(older)and Qi(younger)surface, with the fault cutting across both surfaces, forming linear steep terrain. The measured total right-lateral offset of this gully since exiting the bedrock mountain area is(46±9)m. To constrain the activity rate of the Batang Fault at this location, we used cosmogenic nuclide single clast dating to determine the exposure age of the oldest geomorphic surface, Qo, as(12.5±0.5)ka. Considering that the formation of the river predates the Qo geomorphic surface, the age-constrained slip rate of the fault at this location is considered a maximum value, estimated at(3.6±0.8)mm/a. At Batang county, the Batang Fault has preserved clear faulted topography when cutting through the Moqu alluvial fan. The southern edge of the Moqu alluvial fan has been displaced by the fault, providing a well-preserved geomorphic marker for determining the strike-slip displacement of the fault. The Batang Fault, when intersecting the steep edge of the Moqu River alluvial fan, caused an obvious right-lateral offset, determined by comparing the consistent morphology of the steep edge on both sides of the fault. The right-lateral strike-slip displacement along the southern edge of the alluvial fan is measured at (40±5)m. The cosmogenic nuclide depth profile dating was used to determine the age of the faulted alluvial fan. From a vertical profile excavated along a man-made road on the edge of the alluvial fan, four mixed samples of small pebbles were collected from bottom to top. The calculated exposure ages of the debris flow alluvial fan are (15.2+3.2/-5.4)ka (without consideration of erosion)and (16.4+3.9/-5.6)ka (with consideration of erosion). Combining the fault displacement along the southern edge of the alluvial fan and the cosmogenic nuclide depth profile ages, the slip rate of the Batang Fault at this location is estimated to be of(2.6±0.6)mm/a (without erosion)or(2.4±0.8)mm/a (considering erosion). We believe that the age results with consideration of erosion effects is closer to the true values, thus we take 2.4mm/a as the activity rate of the Batang Fault at this location. The two slip rate values of the Batang Fault obtained in the Huangcaoping and Batang county sites are similar, indicating a right-lateral strike-slip rate of 2~4mm/a since the Late Quaternary. This rate accounts for 50%~80% of the present GPS observation shear deformation across the western boundary of the Sichuan-Yunnan block, indicating that the Batang Fault is a major deformation absorption zone in the Jinsha River fault zone. However, this rate is lower than the predicted~10mm/a using block models. The discrepancy may be due to the different understanding of the deformation mode at the western boundary of the Sichuan-Yunnan Block. In the block model, block sliding mainly relies on the primary boundary fault to regulate, but the long-term and lower geological activity rate of the Batang Fault obtained in this study does not match the assumption of a higher activity rate for the boundary fault in this model. The continuous and diffuse deformation characteristics of crustal deformation in the southeastern margin of the Tibet plateau may corroborate the lower activity rate of the Batang Fault obtained in this study.

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    RESEARCH ON SHALLOW STRUCTURAL CHARACTERISTICS OF THE ZHUMADIAN-HUAIBIN DEPRESSION IN THE SOUTHERN MARGIN OF THE NORTH CHINA BLOCK BY USING REFLECTION SEISMIC DATA
    ZHU Guo-jun, FENG Shao-ying, YUAN Hong-ke, HOU Li-hua, QIN Jing-jing, HAN Jian, WU Quan, ZUO Ying
    SEISMOLOGY AND GEOLOGY    2023, 45 (6): 1419-1431.   DOI: 10.3969/j.issn.0253-4967.2023.06.010
    Abstract152)   HTML9)    PDF(pc) (8547KB)(92)       Save

    The Zhumadian-Huaibin depression, which is located in the southern margin of the North China block, is a NW trending faulted basin between the thrust nappe belt on the northern margin of Qinling Mountains and Xiping-Pingyu uplift and controlled by the NW trending Zhumadian-Xixian Fault and Suyahu Fault. To find out the risk base of earthquake disasters and identify the characteristics of seismotectonic in Zhumadian City, based on the analysis of deep seismic exploration results, we used high-resolution shallow seismic reflection imaging technology to complete a shallow seismic profile, about 22km long, and obtain the fine near-surface structure image and fault characteristics of Zhumadian-Huaibin depression.

    As regards seismic data acquisition, we used an observation system with 3m channel spacing, 15m shot spacing, 180 recording traces and 18 folds. The seismic wave is generated by a M18-612 vibrator, with a scanning frequency band of 20-160Hz and a scanning length of 12s. The data processing adopts the common center point stacking method, with a focus on improving the signal-to-noise ratio. The processing process mainly includes the elimination of waste traces, static correction, pre-stack filtering, predictive deconvolution, velocity analysis and NMO correction, residual static correction, common center point stacking, post-stack denoising, etc. The resulting shallow seismic profile has a high signal-to-noise ratio, clearly reflecting the near-surface structural changes and fault characteristics of the Zhumadian-Huaibin depression. Similar to the characteristics of deep seismic reflection profiles, the Zhumadian-Huaibin depression on the shallow seismic profile also exhibits a fault-controlled fault basin.

    The shallow seismic profile reveals multiple sets of distinct stratigraphic interface reflections, which are characterized by continuous horizontal and dense vertical layering on the profile, with typical sedimentary stratigraphic reflection characteristics. Taking the bottom interface of the Neogene and Paleogene as the boundary, there are three distinct sets of reflection characteristics in the upper, middle, and lower layers, reflecting the sedimentary differences of different tectonic periods. The lateral continuity of the reflection waves in the Neogene and Quaternary strata is good, and the overall performance is a tilted layer with high west and low east, reflecting the overall subsidence of the Southern North China region since the Neogene, and forming relatively stable Neogene and Quaternary systems; The bottom interface of the Neogene and the overlying Paleogene show obvious angular unconformity, reflecting the sedimentary discontinuity between the Neogene and Paleogene formed by the overall uplift and erosion of the Southern North China region in the late Oligocene; The lateral fluctuation of reflected waves in the Paleogene strata reflects that during the Paleogene period, the southern margin of the North China block entered a stage of fault basin development with the Southern North China region, and the the Paleogene strata was controlled by tectonic movements and fault activities; Under the Paleogene bottom interface, at both ends of the profile the reflected wave energy is weak and the continuity of the same phase axis is poor, it is speculated that it is early Paleozoic sedimentary rock and Archean dense metamorphic rock mass.

    The results show that the Zhumadian-Huaibin depression was formed during the development stage of the fault basin in the Southern North China Basin in the Paleogene; The Zhumadian-Xixian Fault, which controls the western boundary of the depression, is composed of four east-dipping normal faults, manifested as a set of fault step belts that fall down layer by layer from west to east, and has not staggered the bottom interface of the Neogene, is speculated to be an active fault in the late Paleogene period; The Suyahu Fault, which controls the eastern boundary of the depression, is composed of three west-dipping normal faults, and has staggered upward to the middle-upper part of the Neogene, and is speculated to be an active fault in the middle-late Neogene period.

    Suyahu Fault has a significant impact on the local changes in the near-surface strata and the trend of modern rivers and lakes, it is recommended to focus on earthquake prevention and disaster reduction work in the Zhumadian City. This study provides a geophysical basis for further understanding the near-surface structural characteristics and basin-controlling fault activity of Zhumadian-Huaibin depression, which has important scientific value and social benefits for earthquake disaster mitigation and urban planning of the Zhumadian City.

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    CHARACTERISTICS AND RELATIONSHIP BETWEEN GRAVITY VARIATION AND CRUSTAL VERTICAL DEFORMATION BEFORE AND AFTER CHANGNING EARTHQUAKE
    WANG Jia-pei, TAN Hong-bo, LI Zhong-ya, LIU Shao-ming, ZHANG Yi, HAO Hong-tao, HU Min-zhang, SHEN Chong-yang
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 952-969.   DOI: 10.3969/j.issn.0253-4967.2023.04.009
    Abstract150)   HTML12)    PDF(pc) (11162KB)(96)       Save

    The MS6.0 Changning earthquake in Sichuan Province caused heavy casualties and property losses. Many scholars have carried out a lot of research on the tectonic background and seismogenic mechanism of the earthquake. However, whether the triggering mechanism of the earthquake is related to fluid injection remains controversial, and further comprehensive analysis should be conducted based on multidisciplinary observation data. The preparation and occurrence of large earthquakes are often closely related to crustal deformation and mass density changes. The comprehensive study of these two methods is conducive to capturing the real crustal dynamics information. To study the process of material migration in the earth's crust before and after the earthquake and its triggering mechanism, the characteristics of gravity changes before and after the earthquake, the crustal vertical deformation data and their relationship were analyzed by using the gravity and GNSS data in Sichuan.

    The characteristics of regional gravity change before and after the earthquake show that the epicenter of the Changning earthquake is located in the gradient zone of positive and negative gravity change anomalies and presents obvious reverse before and after the earthquake. According to the summary of previous earthquake cases, this region may have entered the short- and medium-term stages of earthquake preparation. The wavelet multi-scale decomposition method is used to process the regional gravity change characteristics of two periods before and after the earthquake, and the gravity change characteristics of different depths and scales are obtained. The results show that in the shallow part of the seismic region, the local variation characteristics of the dispersion are obvious, mainly distributed in the boundary of the active fault zone and block. The trend change characteristics are significant in the deep part, and the gravity gradient zone associated with the earthquake is found. In theory, there is an approximate ratio relationship between surface gravity change and crustal vertical deformation, and different ratio coefficients contain different geophysical meanings. The relationship between gravity changes and crustal vertical deformation at four sites around the epicenter is extracted. The results are inconsistent with the approximate law and show different characteristics, including subsurface material migration information at each location.

    Based on the above research results, the process of material migration in the crust before and after the Changning earthquake and its triggering mechanism are comprehensively analyzed and discussed. The results show that the long-term driving force of the Changning earthquake is the result of the deep migration of materials from the Qinghai-Tibetan plateau to the southern boundary of the Sichuan Basin. In the shallow surface of the epicenter and the surrounding area, there may be some cavities or airbags and other Spaces, and the loss and filling phenomenon of gas, liquid, or high-density substances may occur before and after the earthquake. Combined with the previous research results on the seismogenic mechanism of the Changning earthquake, it is indicated that the factors such as salt mining, gas production and wastewater reinjection in this area may be one of the reasons for the triggering of the Changning earthquake.

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    3D STRUCTURAL MODELLING OF THE ANNINGHE-ZEMUHE-XIAOJIANG FAULT ZONE IN THE EASTERN BOUNDARY OF SICHUAN-YUNNAN BLOCK USING MULTI-DATA AND IMPLICIT MODELING METHODS
    WANG Mao-mao, HU Shun-yang, MA Hao-ran, LIANG Bo-yu, ZHANG Jin-yu, LU Ren-qi
    SEISMOLOGY AND GEOLOGY    2024, 46 (1): 19-34.   DOI: 10.3969/j.issn.0253-4967.2024.01.002
    Abstract147)   HTML20)    PDF(pc) (8144KB)(150)       Save

    The Anninghe-Zemuhe-Xiaojiang fault zone is located at the intersection between the Qinghai-Xizang Plateau and the Yangtze block, representing the eastern boundary of the Sichuan-Yunnan block with frequent seismic activities. Its overall kinematic characteristics involve left-lateral strike-slip motion, and the fault structures along its strike are complex, posing significant challenges in accurately characterizing the 3D structural features of deep faults. The main issues include the structural complexity of the fault surfaces, uncertainties in the intersection relationships of fault systems, spatial constraints of blind faults, and the definition of fault surfaces in regions with weak seismic activity. Traditionally, 3D structural modeling for fault geometry heavily relies on high-resolution seismic reflection profiles, 3D seismic data volumes, and borehole data. It defines the geometric shapes of objects with limited nodes in a triangular mesh, and then simulates the topological structure of objects by connecting these nodes. However, obtaining high-resolution seismic reflection data in active tectonic areas like the eastern boundary of the Sichuan-Yunnan block is challenging, and even when available, it is often sparse in space. Alternatively, a large amount of relocated earthquakes and surface fault traces are generally used to create initial three-dimensional models of active faults. However, this approach overlooks the contributions of focal mechanism solutions in constraining the modeling, with more subjectivity in the selection of relocated seismicity, and does not adopt a differentiated weight strategy for various data sources. In this study, a 3D implicit modeling approach, combining deep and shallow geological and geophysical data that are generally available in active tectonic environments, was used to construct a detailed 3D structural model of the Anninghe-Zemuhe-Xiaojiang fault zone at the eastern boundary of the Sichuan-Yunnan block. The modeling process effectively integrated the fault plane constraints provided by focal mechanism solutions with surface fault traces and relocated seismic data, using a multi-iteration process with differentiated weight to increase the accuracy of the fault models. This approach ultimately represented the 3D complex structural features of the eastern boundary of the Sichuan-Yunnan block using multiple data sources. The modeling results show that the Anninghe-Zemuhe fault zone is characterized by a steep strike-slip fault structure with along-strike geometry variations. The Anninghe Fault shows its steepest dip angle in the central segment and gradually becomes gentler to both ends. Meanwhile, the Zemuhe Fault exhibits several asperities that are perpendicular to the direction of fault slip at a depth of 5~15km. By contrast, the north-to-central segment of the Xiaojiang fault zone is more complex. The western branch of the Xiaojiang Fault, which is an east-dipping, left-lateral strike-slip fault, is characterized by a relatively gentle fault plane with an average dip angle of 76° to 78°. The west-dipping segment of the eastern Xiaojiang Fault has a steeper dip with an average angle of 85°. The detailed 3D structural model of active faults constructed through implicit modeling can be used for analyzing fault roughness and fault system studies, which are crucial for understanding the distribution of asperities on fault planes and conducting seismic rupture simulations. Implementing the implicit modeling approach allows for the development of improved fault surface representations that can contribute to Community Fault Models in active tectonic environments, and support fault system modeling, rupture simulations, and regional hazard assessments.

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    THE EMPIRICAL GREEN'S FUNCTION-BASED SIMULATION OF NEAR-FIELD STRONG GROUND MOTIONS OF THE 2021 YANGBI EARTHQUAKE(MS6.4), YUNNAN, CHINA
    WU Hao, IRIKURA Kojiro, LIN Guo-liang
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 864-879.   DOI: 10.3969/j.issn.0253-4967.2023.04.004
    Abstract144)   HTML12)    PDF(pc) (6772KB)(79)       Save

    Both short(0.1s)and long(1~2s) periods ground motions were observed at the near-field strong-motion stations during the mainshock of the 2021 MS6.4 Yangbi earthquake, Yunnan. As there are only two strong-motion stations observed by the National Strong Motion Observation Network System of China in the near-field, the strong ground motion records at seven intensity stations observed by the Local Earthquake Early Warning Network of Yunnan Province are combined to construct the characterized source model with the empirical Green's function method. The broadband strong ground motions in the near-field are synthesized by using the characterized source model. We firstly select out an MW4.3 foreshock as the empirical Green's function event. Then, we calculate the observed source spectral ratios at seven stations assuming that the source spectrum obeys to the ω-2 law. The results show that both the observed source spectral ratios in the frequency of 0~0.2Hz at strong-motion stations and 0~0.5Hz at intensity stations deviate from the ω-2 law assumption. Thus, we determine the size of the sub-fault by fitting the observed source spectral ratio averaged at two strong-motion stations to the theoretical source spectral ratio in the frequency of 0.2~30.0Hz. Taking two strong-motion stations and four intensity stations as the target stations, we adopt the simulated annealing method to determine the optimal parameters required for ground motion synthesis, such as proportional parameters N and C, the position of rupture starting point within the strong motion generation area, rupture velocity, and rise time, when the misfit function between synthesized and observed ground motions reach the minimum at the target stations. We fix the rupture starting point at the hypocenter of the mainshock, and determine N=4, C=3.69, rupture velocity of 2.5km/s, and rise time of 0.56s. In this study, the characterized source model used for the strong ground motion simulation consists of one strong motion generation area which is 5.4km along the strike direction and 5.4km along the dip direction, and with the stress drop of the strong motion generation area of 12.8MPa. In general, the ground motions synthesized by the characterized source model and those optimal parameters are comparable with the observed ground motions at the target stations. Further, we apply those optimal parameters to synthesize ground motions at other three intensity stations which are not taken as the target stations. In consideration of the different instrumental responses in the strong-motion and intensity stations, we impose different frequency band of bandpass filters on both the observed and synthesized ground motions, i.e. the frequency band is 0.2~30.0Hz for the ground motions at the strong-motion stations, while the frequency band is 0.5~30.0Hz for the ground motions at the intensity stations. The ground motions of the mainshock in the 53YBX strong-motion station are characterized by the large amplitude of the response spectrum around 0.1s which obviously exceeds the level of design response spectrum for rare earthquake, and the pulse(~1s)is observed in the east-west direction. The synthesized ground motions in this study are in good agreement with the above characteristics, except that the amplitude of the synthesized ground motions around 1s is smaller than the observed one, which may be caused by the small amplitude of the empirical Green's function. Moreover, the long-period(~2s)ground motions in the 53DLY strong-motion station are reproduced by the synthesized ground motions. On the other hand, the synthesized ground motions in the frequency band of 0.5~30.0Hz agree well with the observed ground motions in the intensity stations with sufficiently large signal-to-noise ratios. Finally, we examine the relationships between two source parameters and the seismic moments. The strong motion generation area and the flat amplitude of the acceleration source spectrum in the short period range are found to have linear relations with the seismic moment, which is consistent with the empirical scaling relationships. In the future work we will continue to examine the applicability of the empirical scaling relationships for prediction of strong motions by analyzing more earthquakes with different magnitudes in China.

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    ANALYSING THE SURFACE DEFORMATION AND PRESENT-DAY MAGMA ACTIVITY OF CHANGBAISHAN-TIANCHI VOLCANO FROM 2015 TO 2022 WITH INSAR TECHNOLOGY
    XIONG Guo-hua, JI Ling-yun, LIU Chuan-jin
    SEISMOLOGY AND GEOLOGY    2023, 45 (6): 1309-1327.   DOI: 10.3969/j.issn.0253-4967.2023.06.004
    Abstract144)   HTML25)    PDF(pc) (10999KB)(113)       Save

    The surface deformation information can effectively reflect the activity status of the magma chamber under the volcano, which is very important for understanding the evolution process of volcanic activity. By capturing deformation anomalies, the volcanic hazard can be assessed, providing insights into the supply, storage, and triggering mechanisms of volcanic magma systems.

    According to statistics, there are 14 active volcanoes in China with potential eruption risks. Among them, Tianchi volcano of Changbaishan is considered the largest and most dangerous active volcano within China’s borders. It is located on the northern edge of the Sino-Korean Plate, situated to the east of the Dunhua-Mishan fault at the outermost edge of the Northeast rift system and to the west of the back-arc basin of the Japan Sea. Multiple groups of faults in the NE-SW and NW-SE directions are widely developed in the region. Since 2002, seismic activity in the Tianchi volcano area has gradually increased, with the annual average earthquake frequency rising from dozens to over a hundred times, reaching its peak in 2003 with over a thousand occurrences. However, seismic activity has gradually decreased after 2006. Nevertheless, between 2020 and 2022, two episodes of seismic swarms occurred beneath the Tianchi volcano, with epicenters exhibiting a dispersing pattern gradually spreading from beneath the volcanic vent. This indicates that the Tianchi volcano still retains the potential for eruption.

    This study investigates the Tianchi volcano as the research area. It utilizes Sentinel-1A/B images from three orbits, namely ascending and descending passes, and employs advanced techniques including Small Baseline Subset(SBAS)InSAR and Stacking InSAR to retrieve Line of Sight(LOS)surface deformation results of the Tianchi volcano from 2015 to 2022. Additionally, InSAR observations are used as surface constraints, and the geometric distribution of the magma reservoir in Tianchi volcano is inverted using the Mogi point source model. By analyzing the inferred volume change rate of the magma reservoir and integrating it with previously published results obtained from geodetic measurements, the mechanisms underlying the variations in the magma reservoir and the temporal sequence of volcanic activity in Tianchi volcano are explored. The primary conclusions are as follows:

    (1)According to the acquired LOS InSAR average deformation rate data from 2015 to 2022, covering the Tianchi volcano, the deformation results from different orbits show good consistency in their distribution. Near the volcano crater, there is an overall trend of deformation, while in areas farther away from the crater, local deformation exists. Over the past seven years of monitoring, there has been a slow subsidence phenomenon near the volcano crater, with a deformation rate of approximately -4mm/a to -2mm/a. By extracting the profile deformation time series from one descending orbit, it is found that the maximum cumulative deformation is about -40mm. The results of the deformation time series indicate that the surface deformation of the Tianchi volcano was relatively small between 2014 and 2017, indicating relatively stable magmatic activity during this period. However, starting in 2018, there has been a certain degree of accelerated deformation, and surface deformation mainly occurs around the volcano crater.

    (2)According to the inversion results of the Mogi model, the shallow magma chamber beneath the Tianchi volcano has an estimated depth of approximately 6km, with a volume change rate of -3.3×105m3/a. The geographical location of the magma chamber is situated slightly below and to the west of the Tianchi volcano crater. The inversion results indicate that during the monitoring period, the magma chamber displayed an overall slow contraction. It is speculated that the deformation activity of the magma chamber may be attributed to magma cooling and crystallization processes.

    (3)According to the inversion of geodetic measurement data on magma chamber volume changes, during the period from 1995 to 1998, the magma chamber of the Tianchi volcano underwent progressive expansion deformation at a sluggish rate. The Tianchi volcano experienced significant surface uplift deformation from 2002 to 2005. During this period, the magma chamber exhibited a rapid expansion deformation with a fast volume change rate. Starting from 2006, the surface deformation rate weakened, and the volume change rate slowed down. From 2009 to 2011, the inversion of leveling observation data indicated a contraction of the magma chamber volume. Throughout the observation period of this study, the magma chamber continued to exhibit a contraction phenomenon. From 1995 to 2022, the Tianchi volcano underwent a process of magma activity, transitioning from a state of quiescence to perturbation and back to quiescence.

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    3D P-WAVE VELOCITY STRUCTURE OF CRUST IN FUJIAN AREA AND ITS TECTONIC IMPLICATIONS
    LI Qiang, WU Jian-ping
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 970-986.   DOI: 10.3969/j.issn.0253-4967.2023.04.010
    Abstract142)   HTML28)    PDF(pc) (6249KB)(80)       Save

    The Fujian area is located tectonically at the southeastern margin of the South China continent, which consists of three sub-blocks, the northwest Fujian block, the southwest Fujian block and the east Fujian block. This region is the forefront of the interaction between the Eurasian plate and the Philippine Sea plate. Geologically, the Fujian area has undergone a complex tectonic evolution process, and the huge intrusive-volcanic rocks formed by multi-stage tectonic changes were widely exposed in this region. Since the inversion of the crustal three-dimensional P-wave velocity structure was important for understanding the tectonic evolution process and the deep seismogenic environment in the region, a lot of research work has been carried out in Fujian area, including seismic body wave tomography, ambient noise surface wave tomography and artificial seismic profiles. Although some important features of the crustal velocity structure in this region had been obtained by natural seismic body wave or ambient noise surface wave imaging, the grid lateral resolution was relatively poor(generally above 0.5° horizontally), which made it difficult to constrain effectively the detailed features of the fault zone velocity structure in this region. For example, the Fu'an-Nanjingfault zone, as an important fault zone in the region, which controlled the magmatic intrusion activities before the Mesozoic, the features of its deep velocity structure have been rarely revealed. Although the resolution of artificial seismic profiles was high, it covered a relatively limited detection range in this region.

    In this paper, 3203 natural local earthquakes were selected using the observation reports of Fujian seismic network from 1999 to 2021 and integrating some data from neighboring provinces, which includes both 76423 absolute arrival time data and 389021 P-wave relative arrival time data from131 seismic stations. The test results of checkboard showed that the northwest Fujian block had poor recovery at all depths due to the limited internal seismic ray coverage, most areas of the southwest Fujian block had good recovery at all depths, and the east Fujian block could been recovered at all depths except for its northern region which had poor recovery at 0km, 25km and 30km depth. Under this resolution condition, the three-dimensional crustal P-wave fine velocity structure in Fujian region was obtained. The arrival time residual conforms to a Gaussian distribution before and after the inversion. The travel time residuals of the seismic phases were mainly distributed in the range of -1.5 to 1.5s before the inversion, and these travel time residuals of the seismic phases were mainly distributed in the range of -0.5 to 0.5s after this inversion. The travel time residuals were reduced significantly and were more concentrated around 0. Using the velocity structure obtained from the inversion and combining with the geological structure and geophysical field characteristics of this region, the tectonic implications which may be related to these features of velocity structure in the region were discussed. The main results are as follows:

    (1)In the near-surface shallow layer, the P-wave low-velocity feature is mainly correlated better with the NW-trending faults, such as the Nanri island fault, Meizhou bay fault, Yong'an-Jinjiang fault and Jiulong river fault. This may be related to the relatively young activity age and more fragmented shallow parts of the NW-trending faults. The lateral variation of velocity is small in the middle and upper crust at 5km and 10km depths relative to other depths, but there is a relatively high velocity zone of P velocity in northeastern Fujian area.

    (2)The P-wave velocity structure shows generally a relatively low velocity feature at 15~25km depth within the southwest Fujian block, especially in the south of the Yong'an-Jinjiang fault zone. Although the range distribution of this low velocity anomaly is relatively large, the magnitude of the anomaly is not large, and the upper crust and the bottom of the lower crust in the southwest Fujian block do not show this anomalous feature. On the other hand, the magnetotelluric sounding of the middle and lower crust of this block shows a high resistivity and the receiver function shows a low Poisson's ratio, this suggests that the low-velocity feature of this block is not caused by partial melt or ductile shear zone, but may be mainly caused by the more quartz-rich composition of the regional crust.

    (3)There exist two P-wave low velocity anomalies in the middle-lower crust of the East Fujian block, which are below the two high thermal anomalous area of the geothermal heat flow in this region. It may suggest that the formation of these two relative low velocity anomalies may be related to the transformation of the coastal area into an extensional environment and the upwelling of deep mantle materials caused by the high-angle retraction of the Paleo-Pacific plate in the late Yanshanian period.

    (4)The P-wave velocity features show that the velocity at the two sides of the Fuan-Nanjing fault zone is different obviously in the middle and lower crustal depths. This may imply the Fu'an-Nanjing fault has a certain control on the distribution of crustal velocity structure in the region, which is consistent with its deep characteristics of cutting the Moho interface which reflected by the Bourg gravity anomaly and aeromagnetic anomaly, which further confirms that it is a major deep fault zone in the region.

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    QUANTITATIVE ASSESSMENT OF SEISMIC DAMAGE IN TYPICAL SCENARIOS BASED ON OBLIQUE AERIAL PHOTOGRAPHY: THE CASE OF THE MAERKANG EARTHQUAKE SWARM, SICHUAN, CHINA
    XIAO Ben-fu, YUAN Xiao-xiang, CHEN Bo, ZHANG Lu-lu, LIANG Yuan-ling, QI Yu-ping, YANG Lu-yao, LIU Yang
    SEISMOLOGY AND GEOLOGY    2023, 45 (4): 847-863.   DOI: 10.3969/j.issn.0253-4967.2023.04.003
    Abstract141)   HTML23)    PDF(pc) (18321KB)(134)       Save

    Post-earthquake disaster information extraction and quantitative evaluation are the foundations of earthquake relief work. The effectiveness of its transmission mode and the accuracy of evaluation results directly affect the efficiency of post-earthquake emergency analysis, emergency response decision-making, and earthquake relief. In recent years, the nationally targeted poverty alleviation, reinforcement of housing facilities in earthquake-prone areas, and the rapid promotion of urbanization have greatly improved the overall seismic protection capability of buildings, and the reference indexes of seismic damage assessment have changed from the previous focus on the damage indexes of the roof of the disaster-bearing body to the comprehensive damage indexes of the roof of the disaster-bearing body, external walls, and subsidiary structures. The traditional two-dimensional damage assessment method based on a quasi-vertical perspective can hardly meet the requirements of an accurate quantitative assessment of earthquake damage at this stage.

    With the rapid development of aerial photography technology, the efficiency and accuracy of earthquake hazard information extraction and quantitative earthquake hazard assessment have been greatly improved. The three-dimensional seismic damage scenario visualization model based on UAV oblique photography technology has the advantages of multi-angle, high accuracy, and rich texture, which can accurately reflect the characteristic differences of seismic disasters of disaster-bearing bodies. It can be used to realize multi-dimensional and high-granularity seismic damage information extraction, thus effectively improving the accuracy of a quantitative assessment of single-time and space-time seismic damage, and can provide scientific and technological support for practical quantitative assessment of seismic damage.

    On June 10, 2022, a M6.0 magnitude earthquake swarm occurred in Maerkang City, Ngawa Tibetan and Qiang Autonomous Prefecture(also known as Aba Prefecture), Sichuan Province, in which the epicenter locations of three earthquakes of M5.8, M6.0 and M5.2 were located in Caodeng Town, Maerkang City, Aba Prefecture, Sichuan Province, with source depths of 10km, 13km and 15km, respectively. After the swarm, the Sichuan earthquake agency initiated a level Ⅱ emergency response, and the earthquake site working group rushed to the earthquake site with UAV equipment to carry out oblique photography of typical buildings, geological hazards, and other earthquake damage.

    In this study, the quantitative evaluation process of typical scenario visualization seismic damage based on oblique photography technology is constructed. Based on the texture, spectrum, shape, position, and combination of UAV remote sensing images, the interpretation signs of scenario visualization seismic damage are established. Taking the Maerkang 6.0 earthquake swarm in Sichuan on June 10, 2022 as an example, the seismic damage information of typical scenario visualization houses in the meizoseismal area is extracted, and the quantitative evaluation of seismic disaster in the meizoseismal area is realized. At the same time, the feasibility and accuracy of the method are verified by the field seismic damage investigation results. The results show that: 1)the typical scenario visualization model constructed based on oblique photography can reflect the earthquake damage information on the top, exterior walls and bottom of buildings, which can intuitively reflect the earthquake damage aftershocks. Compared with the traditional post-earthquake remote sensing images from a quasi-vertical perspective, this model can more effectively extract the basic information and seismic damage information of the building hazard-bearing body. 2)According to the equivalent seismic damage index, the seismic damage degree of 520 houses in 3 types of building structures is quantitatively evaluated. Among these the equivalent seismic damage index of a Tibetan stone-wood structure is 0.60, the equivalent seismic damage index of a brick-concrete structure is 0.44, and the equivalent seismic damage index of a reinforced concrete frame structure is 0.37. The seismic intensity of the study area is determined to be Ⅷ degree(8 degree), which is consistent with the field survey results. The OA values and Kappa coefficients of seismic damage extraction based on visualization and field survey were 92% and 0.87, respectively, while the OA values and Kappa coefficients of seismic damage extraction based on UAV orthophotograph and field survey were 45% and 0.25, respectively, which showed that the seismic damage extraction based on visualization was more accurate than that based on UAV orthophotograph in terms of recognition accuracy and precision. Compared with the quantitative assessment method of seismic damage based on UAV orthophotograph, the quantitative assessment method of earthquake damage based on scenario visualization is more effective in terms of recognition accuracy and precision. The typical scenario-based visualized seismic damage extraction method based on oblique photography technology provides a new idea for high-precision UAV remote sensing data for building damage extraction work, and its extraction results can be used as a reference basis for seismic intensity assessment, earthquake emergency rescue and personnel command and dispatch.

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    RESEARCH ON RAPID ESTIMATION METHOD OF EARTHQUAKE MAGNITUDE BASED ON STRONG-MOTION RECORDS
    MENG Rui, ZHANG Yuan-fan, XIE Xiao-feng, NIE Zhi-xi, WANG Zhen-jie, SHAN Xin-jian
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1219-1232.   DOI: 10.3969/j.issn.0253-4967.2023.05.011
    Abstract137)   HTML15)    PDF(pc) (7538KB)(114)       Save

    Earthquake Early Warning(EEW)is the rapid acquisition of earthquake epicenter, magnitude, and occurrence time after a destructive earthquake has started to issue alerts to the public before the arrival of transverse waves and long-period surface waves. Magnitude estimation plays a significant role in EEW algorithm research, serving as a fundamental component for early warning, post-earthquake disaster assessment, and emergency response. Seismic monitoring methods primarily focus on technologies like High-rate Global Navigation Satellite System (HR-GNSS) and strong-motion instruments. HR-GNSS is capable of capturing high-precision ground deformation signals and offers the advantages of a non-saturation recording range, making it crucial for rapid estimation of earthquake magnitudes during major seismic events. However, due to the low GNSS sampling rate and high instrument noise, observational noise often overshadows the deformation signals obtained during low-magnitude earthquakes. Additionally, the sparse distribution of GNSS stations currently impacts the accuracy and timeliness of magnitude estimation. Strong-motion observation methods, characterized by high sampling rates, low noise, and dense station distribution, are widely applied in magnitude estimation. Prevalent methods for strong-motion magnitude estimation often rely on P-wave arrival time information for timely determination of magnitude, commonly used in earthquake early warning systems. Yet, these methods are susceptible to saturation effects, leading to underestimation of magnitudes for large earthquakes. Moment magnitude estimation methods are closely associated with rupture characteristics of the seismic source and hold clear physical significance. However, determining this magnitude necessitates knowledge of the rupture extent and slip distribution along the fault plane, which are challenging to precisely obtain at the moment of earthquake occurrence. Hence, such methods are generally employed for post-event magnitude calculations.

    Addressing these challenges, this paper proposes a novel method for rapidly estimating earthquake magnitudes using Peak Ground Velocity(PGV)derived from strong motion. First, a comprehensive dataset of strong-motion acceleration records is compiled, covering nearly 20 years and including 5 596 records from 23 global seismic events with magnitudes ranging from 6.0 to 9.0. These records encompass epicentral distances from 1km to 1 000km, with source depths within 60km. A uniform processing approach is applied to standardize the records in terms of time domain orientation, measurement units(converted to cm/s2), and file formats. Data from each station is categorized into three directions: East-West(EW), North-South(NS), and Vertical(UD). Subsequently, the data is converted into the Seismic Analysis Code(SAC)file format, which is specialized for digital seismic waveform data exchange. Ensuring accurate PGV measurements from strong-motion data involves meticulous data preprocessing. This includes removing the mean acceleration from the first 5 seconds before the seismic event for simple bias correction, followed by baseline correction using a high-pass filter with a cutoff frequency of 0.02Hz. The preprocessed strong-motion acceleration records are then integrated to obtain velocity, enabling the measurement of PGV. A robust PGV-based magnitude estimation model, suitable for rapid earthquake magnitude estimation, is constructed using the least-squares regression method.

    Furthermore, the constructed PGV-based magnitude estimation model undergoes comprehensive experimental analysis. Initially, the residuals between observed PGV values from 5596 strong-motion records and PGV values predicted by the regression model are computed to evaluate the precision of the constructed PGV-based magnitude estimation model. The model is validated using four earthquake events not included in its construction: the 2021 Damasi MW6.3 earthquake, the 2012 Nicoya MW7.6 earthquake, the 2008 Wenchuan MW7.9 earthquake, and the 2014 Iquique MW8.2 earthquake. This validation process assesses the reliability of the constructed magnitude estimation model. Finally, the paper conducts a study on rapid magnitude estimation to evaluate the timeliness and accuracy of the PGV-based magnitude estimation model within this context.

    The experimental results indicate that the predicted values of strong-motion PGV are largely consistent with the observed values for 23 seismic events, with a root mean square error of residuals measuring 0.296. For the four seismic events that were not included in the modeling process, the estimated magnitudes based on strong-motion PGV correspond closely to the moment magnitudes reported by the United States Geological Survey(USGS). The absolute deviations for these events are 0.15, 0.14, 0.05, and 0.13 magnitude units, with an average absolute deviation of 0.12 magnitude units. In the investigation of rapid magnitude estimation, the following outcomes were observed: For the Damasi MW6.3 earthquake, an initial magnitude of 5.03 was calculated at 13 seconds, approaching the theoretical magnitude at 63 seconds, and reaching a convergent magnitude of 6.09 at 76 seconds. Regarding the Nicoya MW7.6 earthquake, a preliminary magnitude of 4.57 was computed within 6 seconds, approximating the theoretical magnitude at 30 seconds, and converging to 7.46 at 50 seconds. In the case of the Wenchuan MW7.9 earthquake, a preliminary magnitude of 4.06 was determined within 19 seconds. At 50 seconds, the calculated magnitude approached the theoretical value, and it converged to 7.81 at 84 seconds. For the Iquique MW8.2 earthquake, an initial magnitude of 6.45 was estimated within 2 seconds, nearing the theoretical magnitude at 55 seconds, and achieving a convergent magnitude of 8.04 at 70 seconds. The convergence time for rapid magnitude estimation for all four events was consistently under 90 seconds.

    This experimental findings underscore the applicability of the constructed PGV-based magnitude estimation model for rapid earthquake magnitude estimation. The model's ability to counter saturation effects and prevent magnitude underestimation reinforces its robustness and offers substantial technical support for earthquake early warning systems and post-earthquake emergency response strategies.

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    CRUSTAL FINE VELOCITY STRUCTURE IN THE ERYUAN AREA, YUNNAN FROM DOUBLE-DIFFERENT TOMOGRAPHY
    CAO Ying, QIAN Jia-wei, HUANG Jiang-pei, ZHOU Qing-yun
    SEISMOLOGY AND GEOLOGY    2024, 46 (1): 162-183.   DOI: 10.3969/j.issn.0253-4967.2024.01.010
    Abstract132)   HTML18)    PDF(pc) (14442KB)(119)       Save

    The Eryuan area is located in the central part of the northwest Yunnan region on the southeastern edge of the Qinghai-Xizang Plateau. The geological structure in the area is complex, including the Weixi-Qiaohou-Weishan Fault and Honghe Fault, as well as the Longpan-Qiaohou Fault and Heqing-Eryuan Fault, which intersect in an “X” shape. The geothermal activity in the area is also active and exhibits strong fault structural characteristics. Earthquake activity is also frequent in the Eryuan area. Since 2013, multiple earthquakes with MS≥5.0 have occurred on the west side of the Weixi-Qiaohou-Weishan Fault. Given the complex geological and geothermal background and seismic activity in the Eryuan area, we use the seismic travel time data recorded by the Yunnan Regional Seismic Network and the Northwest Yunnan Dense Array from January 1, 2008, to July 20, 2023, VP/VS model consistency-constrained DD tomography method to obtain the 3D VP, VS, and VP/VS models and relocation results in the Eryuan and its surrounding areas. The research results indicate that: 1)based on the relocation results, the dense small seismic cluster develops at the intersection of the Weixi-Qiaohou-Weishan Fault, Honghe Fault, and Heqing-eryuan Fault deserves special attention. Four earthquakes with MS≥5.0 that have occurred since 2013 are mainly distributed on the west side of the Weixi-Qiaohou-Weishan Fault, with an NNW-SSE trend distribution. The fault system at the western boundary of the Sichuan-Yunnan block is very complex, and its seismic risk deserves our special attention. 2)Based on the tomography results, the widely distributed low-velocity anomalies in the upper crust of the study area may be related to the crustal material migration pathway caused by the escape of the Sichuan-Yunnan diamond block towards SSE. 3)Combining imaging results with geochemical research results for inference, the high VP/VS below 10km depth beneath the small seismic cluster at the intersection of the Weixi-Qiaohou-Weishan Fault, Honghe Fault, and Heqing-Eryuan Fault may correspond to hot spring geothermal fluids. Moreover, the circulation process of hot spring fluids in complex fault systems may have penetrated to a depth of 7~10km below the seismic cluster, accompanied by some small earthquakes. However, there is no imaging evidence of fluid within a depth of 5~7km where some earthquakes are densely distributed. The occurrence of these earthquakes may be related to the distribution of brittle rocks, but it cannot be ruled out that fluid may have a potential infiltration effect on them in the future. Based on the results of velocity structure, they are combined with pre-existing seismology research results. It is found that the Eryuan earthquake sequences on March 3 and April 17, 2013 were mainly located within low VP, low VS, and low VP/VS anomaly. Generally, if there is fluid present, VS decreases faster than VP, resulting in high VP/VS. So low VP/VS indicates that low VS is not caused by fluids, which may be due to lithology. Therefore, the imaging evidence indicates that there is no fluid in the region where the sequence is located, thus inferring that the occurrence of the sequence is not directly related to the fluid. The MS5.1 Yangbi earthquake sequence on March 27, 2017, is spatially close to the Eryuan sequence and also has the same velocity structure characteristics, so it may not be directly related to fluids. The mainshock and some aftershocks of the MS5.1 Yunlong earthquake sequence on May 18, 2016, were mainly located in high VP, high VS, and relatively high VP/VS regions. High VS indicates that it is not the fluid that causes relatively high VP/VS. It is speculated that there may not be fluid in the region where the mainshock is located, and the fluid did not directly participate in the occurrence process of the sequence.

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    DEEP STRUCTURAL CHARACTERISTICS IN THE XINGTAI SEISMIC ZONE REVEALED BY PRECISE RELOCATION OF SMALL EARTHQUAKES AND TOMOGRAPHY
    LIU Kang, YANG Ting, LI Hong-guang, FANG Li-hua, SONG Jian
    SEISMOLOGY AND GEOLOGY    2023, 45 (6): 1328-1348.   DOI: 10.3969/j.issn.0253-4967.2023.06.005
    Abstract129)   HTML24)    PDF(pc) (11306KB)(145)       Save

    From March 8th to 29th, 1966, five earthquakes(M≥6)occurred in the Xingtai area, with the MS6.8 earthquake on March 8th and the MS7.2 earthquake on March 22nd being the most severely damaged. The Xingtai earthquake resulted in over 8 000 deaths and the economic losses up to 1 billion yuan. The Xingtai earthquake has opened the scientific practice of earthquake prediction in China and is a milestone in the development of earthquake science in China.

    Based on previous research results, there is a deep fault beneath the Xingtai earthquake area, which is the energy source of earthquakes, while there is a relatively independent fault system in the shallow part, which is generally recognized by scholars. However, the divergence regarding the seismogenic structure of the Xingtai earthquake mainly focuses on the unclear coupling relationship between the deep and shallow structural systems in the seismic area. The structural relationship between deep seismic faults and the shallow Xinhe Fault system requires new evidence to determine. In addition, previous scholars have proposed the viewpoint of “Newly generated Fault”, which can better explain the rupture characteristics of the Xingtai earthquake, but it still needs to be supported by the inversion results of the seismic rupture process based on the three-dimensional crustal fine structure. There are many small earthquakes in the Xingtai area. Deep structural information can be obtained using small earthquake data. Especially after 2009, the significant improvement in earthquake positioning accuracy in North China has made it possible to obtain new insights into deep structures. By locating small earthquakes, the spatial distribution and motion characteristics of faults are characterized, and seismic travel time tomography reveals the deep crustal velocity structure characteristics of the earthquake area. Combining previous geophysical exploration results, conducting deep and shallow structural analysis is of great significance for studying the spatial distribution, motion characteristics, and coupling relationship between deep and shallow structural systems of the fault system in the study area. The continuous aftershocks after the 1966 MS7.2 earthquake in Xingtai, Hebei Province, have provided favorable conditions for conducting studies on deep tectonic structures in the region.

    In this paper, based on the observation data of the Hebei seismostation from 1991 to 2021, we obtained the precise position results of 9 644 earthquakes in Xingtai and its neighboring area using the double-difference positioning method, and depicted the spatial patterns of deep ruptures. Based on the observation data of the North China Mobile Seismic Array from 2006 to 2008, 38 578 P-wave arrivals were used to obtain high-resolution travel time tomography results in the study area. This study shows that there are strong lateral heterogeneities in the velocity structure of the crust in the study area, with obvious low-velocity anomalies in the upper crust and high-velocity anomalies in the middle and lower crusts between the Xinhe Fault and the Yuanshi Fault, and the Xingtai earthquake is located at the junction of the high- and low-velocity anomalies, which has the medium conditions for accumulating large amounts of strain energy and is prone to rupture and stress release. The general trend of the dense zone of small earthquakes in the Xingtai earthquake area is relatively consistent with that of the eastern boundary of the high- and low-velocity anomalies. It is assumed that the deep and shallow fractures spreading along the eastern boundary of the high- and low-velocity bodies have been connected up and down and that the boundary of the anomalies is also a part where velocity changes are relatively strong and easily lead to seismic rupture; the results of various seismic and geological surveys have revealed that a deep major rupture that cuts through the entire crust exists beneath the Xingtai earthquake zone, with SE tendency and the upper breakpoint located near Dongwang, and the Xingtai earthquake prompted the deep and shallow pre-existing ruptures to connect from top to bottom.

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    STATE OF ART AND PERSPECTIVE ON DATABASE CONSTRUCTION FOR LOW-TEMPERATURE THERMOCHRONOLOGY
    DAI Meng-yao, WANG Ping, LI An-bo, DING Lu, LIU Pin-qin, DAI Jin-gen, ZHANG Hui-ping, LIU Shao-feng
    SEISMOLOGY AND GEOLOGY    2023, 45 (6): 1432-1451.   DOI: 10.3969/j.issn.0253-4967.2023.06.011
    Abstract128)   HTML21)    PDF(pc) (5500KB)(99)       Save

    Low-temperature thermochronology is a key technology for studying neotectonics and landscape evolution. However, it is intrinsically different from the other geochronological methods in the data expression, analysis and interpretation. In recent years, with the widespread adoption of low-temperature thermochronology techniques, the size volume of data has continuously increased, giving rise to many studies on tectonic geomorphic evolution based on big data. However, these data are mostly scattered across literature from different sources, with inconsistent formats and contents, and varying data quality, which to a certain extent hampers innovative research based on big data. There is a need to construct specialized databases to cope with the growing low-temperature thermochronology data and meet the demands of innovative research using big data.

    In this paper, four conventional geochronological databases, including National Geochronological Data Base, Geochron, Petlab, DataView, and recent databases, AusGeochem and Sparrow are reviewed for comparison of their capability in data sources, data volume, data storage structure, completeness of data content, data entry methods, data retrieval methods, coverage areas, database update patterns, and data analysis tools. The conventional geochronological databases, of which the thermochronological data comprise only a small part, are generally stored in databases similar to or outside this subject, such as radioisotope chronology database, geochronology database, petrological mineral and geological analysis databases. They amplify the commonalities between different disciplines, and thus focus only on the presentation of sample units. It is not suitable for “big data” research, because all the data are managed by relational database with strictly structured tables and limited data sources. It was found that conventional geochronological databases design approaches are often suitable for absolute age data. However, low-temperature thermochronology differs from conventional geological dating methods, as its age values only record cooling time. The more geologically significant cooling history comes from numerical simulations based on elevation profiles, track lengths, and the diffusion dynamics models of the(U-Th)/He system. Additionally, the innovation in experimental techniques also imposes new requirements on the construction of thermochronology databases.

    Comparing with the conventional geochronology databases, recent databases focus more on low-temperature thermochronological data and support both the structured and unstructured data with variable data sources, which makes it more comprehensive and professional. These databases own the characteristics of flexibility and expandability, especially for the addition of new dating methods and experimental methods, the storage of big data and the linkage between laboratories and database. Using different types of database platform and associated APIs, both relational and non-relational data can be involved and managed for data query, analysis and visualization. However, the construction of these recent databases is still in the preliminary exploration stage, and ensuring the continuous growth of data remains a challenge. Moreover, establishing a flexible numbering system for sustainable and expandable unique identification of samples and data is also an important task for recent databases. Finally, in addition to raw data, numerous thermal history information is included in published paper related to fission track. These interpretations or inverted results constitute interpretive data, which are crucial for reconstructing cooling history or tectonic uplift. Therefore, how to incorporate such data into the database is also a question that must be considered during database design.

    The key to supporting the database lies in the users who it oriented. Considering the needs of users in professional field for scientific research management, experimental analysis and “big data” innovative research, as well as in view of the problems existing in the current databases, we put forward following suggestions for the future construction of low-temperature thermochronology database.

    Firstly, in order to ensure the activity of specific low-temperature thermochronology database. from a technical perspective, artificial intelligence technologies such as natural language processing or other forms of machine learning algorithms should be utilized to semi-automatically or automatically extract information from paper, assisting users in quickly extracting relevant information and understanding the content of the literature. Platforms like Semantic Scholar, GeoDeepDive, and DeepShovel have implemented interactive features in data mining, wherein data is normalized and automated into the database based on user-specified rules, significantly reducing manpower and time costs in data acquisition, providing great convenience. In terms of ideology, the open-sharing academic ecosystem has given rise to open-sharing platforms such as arXiv for preprints, data repositories like Pangaea, and the Deep-Time Digital Earth integrated online research platforms, drastically shortening the cycle from research and experimentation to publication. This facilitates the incorporation of the latest research data into databases, greatly expanding the data sources. Regarding user volume, academic social networks possess advantages in academic tracking and dissemination, breaking down academic-related hierarchies, promoting academic exchange and cooperation, and attracting more users.

    Secondly, more detailed data storage capabilities and simpler data operation behaviors help improve the expansibility of the database. Most existing geochronological databases use relational databases, which are a strictly structured way of storing data. The most typical data structure presentation form is two-dimensional table, which is very suitable for logical geological data. However, non-relational databases are not tables but databases oriented towards structured and unstructured data storage requirements, which have filled the gaps in relational databases. In practical applications, the advantages of both types of databases can be combined to comprehensively include basic geological information and interpretive information, achieving the effect of New SQL.

    Thirdly, highlight its highlight. Chronological data of sample and the single data that make up the sample chronology are significant, it will be effective in distinguishing low-temperature thermochronology from other similar disciplines if the coding style of sample and single data that are not registered on IGSN can be standardized to highlight the characteristics of subject data.

    Finally, by combining the strengths of both conventional and recent databases, incorporating the concept of open academia, leveraging advanced information mining and transmission technologies, and utilizing a storage approach that combines structured and unstructured data, it can greatly meet the comprehensive needs of users, ranging from laboratories to scientists, and further to data consumers.

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    EVIDENCE FOR THE HOLOCENE ACTIVITY OF THE LEIBO FAULT ZONE
    ZHANG Guo-xia, SUN Hao-yue, LI Wei, SUN Wen
    SEISMOLOGY AND GEOLOGY    2024, 46 (1): 141-161.   DOI: 10.3969/j.issn.0253-4967.2024.01.009
    Abstract127)   HTML27)    PDF(pc) (18061KB)(134)       Save

    The Yingjing-Mabian-Yanjin tectonic zone(YMYTZ)is an important boundary structure between the southeastern margin of the Tibet Plateau and the Sichuan Basin. It consists of several small-scale secondary faults with different strikes and is generally characterized by the intersections of north-northwest oriented longitudinal faults and nearly east-west oriented transverse faults. The YMYTZ is seismically very active in the late Quaternary and hosted several moderate-strong earthquakes, including two M≥7 earthquakes since 1216AD, namely the 1216 Mahu earthquake and the 1974 Daguanbei earthquake. After the Daguanbei earthquake, several M≥6 earthquakes and hundreds of M≥5 earthquakes occurred along the YMYTZ to date, implying it is a newly generated seismotectonic belt. Even so, the activity of each fault is still unclear, bringing out great uncertainty in understanding the current crustal deformation pattern and in evaluating the regional seismic potential. Specifically, although several M≥6 earthquakes have occurred along the Leibo fault zone in the southern segment of the YMYTZ, the late Quaternary activity of the fault zone has not been well determined due to insufficient work as well as subsequent lack of solid evidence. The Leibo fault zone strikes NE-SW and spreads on the southeast flank of the Chenqiangyan-Shanzhagang anticline. It starts at the Huanglang township near the Mahu Lake, cuts through the Jingkou Mountain, Lianhuashi, and Leibo, and extends southwestwards to the vicinity of Lianlajue. The latest investigation shows that the Leibo fault zone consists of four subparallel right-lateral strike-slip faults named F1—F4 from the north to the south, respectively. These fault branches together constitute a 43km-long and 10km-wide structural belt. Previous paleoseismic work along the Leibo fault zone found that the faults ruptured the late Pleistocene sedimentary layers with their upward terminations covered by the undeformed Holocene deposits, implying it was active in the late Pleistocene and has not been active since the Holocene. However, the ground surface traces of the Leibo fault zone are the most obvious among the faults in the YMYTZ, and recent seismologic studies show strong seismic activity for the Leibo fault zone, bringing out a controversy about whether it is active in the Holocene or not.

    To address the late Quaternary activity of the Leibo fault zone, we conducted detailed indoor deformed geomorphic feature interpretation on remote sensing imageries like 2m-resolution GF-2 imagery and high-resolution imageries on Google Earth, and further mapped the fault traces in the field using an unmanned aerial vehicle(UAV)derived digital orthographs and digital surface models(DSM). Based on the geological and geomorphological surveys, two trenches were excavated at Pengjiashan and Luohangou along the northern(F2)and southern(F4)branches of the Leibo fault zone respectively. On the trench walls, surface-rupturing paleoearthquakes were identified for each fault according to criteria for faulting events like cut-and-cover structures, scarps, and colluvial wedges. Subsequently, we collected and dated several radiocarbon samples from the sedimentary layers immediately before and after the rupturing events, and finally carried out stratigraphic sequence calibration using the acquired ages with the OxCal 4.4 program to constrain the timings of the revealed paleoearthquakes.

    According to the identification criteria of paleoseismic events, it was revealed 3 paleoearthquakes in the Pengjiashan trench on the northern fault branch(F2)and another 7 rupturing events in the Luohangou trench along the southern fault branch(F4). Radiocarbon sample dating constrain the ages of the paleoearthquakes along F2 to be 21190—20590BC(EP1), 20550—12120BC(EP2), and after 12090BC(EP3), while the latest two paleoseismic events on F4 occurred 9270—5040BC(EL6)and after 5000BC(EL7). Our paleoseismic studies show that the LFZ has experienced several surface-rupturing earthquakes in the Holocene, verifying it is a Holocene active fault zone. Moreover, the ages of the paleoseismic events revealed on two fault branches do not overlap with each other, suggesting they are different paleoearthquakes so that the fault branches in the Leibo fault zone are independent seismogenic structures. By collecting and analyzing the magnitudes of strike-slipping earthquakes that have generated surface ruptures in western China since 1920, it is believed that the minimum magnitudes of the paleoearthquakes determined on the Leibo fault zone are 6.5. Through the empirical relationships between magnitude and surface rupture length, it is estimated that the LFZ has the capability to produce an earthquake with M≥7.

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    EFFECTS OF STRIKE-SLIP FAULT GEOMETRIC COMPLEXITY ON EARTHQUAKE RUPTURES PROPAGATION: A REVIEW
    WANG Hui, CAO Jian-ling, YAO Qi, WANG Li-wei, ZHU Ya-ling
    SEISMOLOGY AND GEOLOGY    2024, 46 (1): 201-219.   DOI: 10.3969/j.issn.0253-4967.2024.01.012
    Abstract124)   HTML16)    PDF(pc) (1897KB)(137)       Save

    Active strike-slip fault usually hosts major earthquakes. Therefore, studies on fault segmentation, which is controlled by geometric complexity, are very important for assessing the maximum magnitude of potential earthquakes. Based on previous literature, we summarized the behavior of earthquakes on strike-slip faults related to fault geometry, segmentation, and cascading rupture from the aspects of field investigation and numerical modeling.

    Previous field investigations have shown that geometrically complex sections of a strike-slip fault usually play the role of barriers that can separate earthquake rupture segments and effectively stop the propagation of earthquake rupture. Statistical results based on the limited field investigations of the surface rupture illuminated semi-quantitatively the influence of geometrically complex sections on the rupture behavior of the earthquake. Since not all earthquakes can produce surface rupture zones, the case number of surface ruptures are unlikely to meet statistical requirements in the coming years. In addition, knowledge gained from field investigation is mainly statistical results based on fault geometry and kinematics. They have some consistency but vary greatly, indicating the complexity of seismic rupture modes on strike-slip faults. No simple threshold that can be used as a criterion to refine the capability of earthquake rupture propagation on strike-slip faults with complex geometry. Based on the statistical results of field surveys, geologists have applied the factor, that complex geometricity controls earthquake rupture behavior, in seismic risk assessment. Lacking support from dynamic analysis, it is necessary to develop and integrate physics-based dynamic models to help improve earthquake-rate models and probability models.

    Numerical modeling can not only present the earthquake simulation scenario but also provide insights into the fundamental physics of dynamic rupture propagation. The modeling results improve our understanding of how the geometric complexity of the fault influences the dynamic rupture propagation. Different modeling approaches focus on different aspects of this challenging scientific problem, each with unique advantages and disadvantages. 2D modeling is relatively simple. They allow modelers to consider more complex physical processes, variated parameters, and constraints from the field and laboratory observation, etc. They provide a comparative benchmark on rupture dynamics on a strike-slip fault with complex geometry. 3D modeling can provide closer approximations to realistic faults and more direct comparisons to observations. The simulations of one earthquake rupture process may focus on the influence of single/multiple parameters on the rupture process. While the multicycle earthquake simulation can predict spatiotemporal patterns of earthquake ruptures on a strike-slip fault. Both simple 2D modeling and complex 3D modeling have shown that one of the most important factors affecting rupture behavior on strike-slip faults is the fault geometric complexity. In addition, other dynamic factors, such as the initial stress, the properties of the rock medium, and nucleation location, also affect dynamic fracture propagation on strike-slip faults. It indicates that rupture behavior on a certain strike-slip fault has its unique characteristics that are controlled by dynamic factors such as the regional tectonic environment and the properties of the fault itself. The numerical modeling provides a dynamic perspective on the complexity of rupture behaviors on strike-slip faults given by field investigations.

    In the China Seismic Science Experimental Site, 3D dynamic modeling supported by fault detection, dense geophysical observations, and high-performance computation will provide new insights into the rupture behavior in the complex multi fault system. That is helpful in determining the maximum magnitude of a potential earthquake.

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    CHARACTERISTICS OF FOCAL MECHANISM AND STRESS FIELD IN THE EASTERN BOUNDARY OF THE SICHUAN-YUNNAN BLOCK
    GUO Xiang-yun, FANG Li-hua, HAN Li-bo, LI Zhen-yue, LI Chun-lai, SU Shan
    SEISMOLOGY AND GEOLOGY    2024, 46 (2): 371-396.   DOI: 10.3969/j.issn.0253-4967.2024.02.008
    Abstract123)   HTML18)    PDF(pc) (11846KB)(108)       Save

    It is important to study the characteristics of the tectonic stress field studies which could provide a deeper understanding of the internal stress environment of the crust. It can provide useful assistance for exploring the relationship between the tectonic stress field and earthquake development. At the same time, it plays an important role in understanding block interactions, fault movement, tectonic deformation, and revealing the dynamic mechanical processes of the continent. The focal mechanism solutions contain abundant information reflecting the stress field.

    In this paper, using the broadband records from 128 permanent and temporary regional stations from the Chinese National Seismic Network(CNSN)deployed in the Sichuan-Yunnan Province and its adjacent, we determined the focal mechanisms of 3 951 earthquakes by the cut-and-paste(CAP)method and the HASH method. The friction coefficient and stress properties of the main active fault and characteristics of the tectonic stress field in this area are analyzed by using two different methods which are the damped inversion method(STASI)and iterative joint inversion method from focal mechanisms.

    The results of the focal mechanisms show that: there are 2 512 strike-slip earthquakes in the study area, accounting for 63.58% of all earthquakes; there are 818 normal fault type and normal strike-slip type earthquakes, accounting for 20.70% of all earthquakes; there are 621 reverse strike slip and reverse thrust earthquakes, accounting for 15.72% of all earthquakes. The most of earthquakes in the study area are distributed in active fault zones, the strike of the fault plane is consistent with the orientation of active fault zones. It revealed predominantly strike-slip faulting characteristics of earthquakes in the Eastern Boundary of the Sichuan-Yunnan Block, while the reverse thrust of earthquakes is mainly concentrated in the Longmenshan fault zone, as well as the NW trending Mabian-Yanjin Fault and the NE trending of Ludian-Zhaotong and Lianfeng faults which lied on the eastern boundary of the Sichuan-Yunnan block. Overall, the characteristics of the source mechanism are consistent with the regional tectonic background.

    Results of the stress field inversion confirmed main active fault in the Eastern Boundary of the Sichuan-Yunnan Block is under a strike-slip stress regime, maximum and minimum compressional stress axes are nearly horizontal. The maximum compressional axes are primarily oriented in NW-SE and NWW-SEE direction, and they experience a clockwise rotation from north to south. Against the strike-slip background, normal faulting stress regimes and reverse faulting stress can be seen in the regional areas. The most prominent is the Daliangshan fault zone, which has obvious differences from the overall characteristics of the stress field with the eastern boundary of the Sichuan Yunnan Block. The maximum horizontal principal stress in the northern section shows a nearly EW direction, with a strike-slip type stress property, and the NW-SE direction in the southern section, with a thrust type stress property. The distribution characteristics of the stress field are consistent with the fault type of sinistral strike-slip and thrust on the eastern boundary of the Sichuan Yunnan block

    The shape ratio R-value varies significantly, the R-value in the Sanchakou area is relatively high, with obvious extrusion characteristics, the R-values of the Xianshuihe fault zone, Anninghe fault zone and Xiaojiang fault zone are all between 0.25-0.5, showing NE-SW compression and NW-SE tension, and the tensile stress may be much less than the compressive stress(strike-slip type). The R values of the northern segment of the Daliangshan fault zone, the southern segment of the Anninghe fault zone, and Zemuhe fault zone are all between 0.5-1, showing NW-SE compression and NE-SW tension, and the compressive stress is greater than the tensile stress. To sum up, the current stress characteristics of the eastern boundary of the Sichuan Yunnan rhombic block are shear strain and local compression or tension.

    There are different friction coefficients of the main faults in the study area: The Anninghe fault zone is 0.60, the Xianshuihe and Zemuhe fault zones are 0.80, the Xiaojiang fault zone is 0.75 and northern and southern sections of the Daliangshan fault zone are 0.65 and 0.85. The friction coefficients of the Xianshuihe Fault, the southern section of the Daliangshan Fault, and the Zemuhe Fault are above 0.75. The high friction coefficients of these fault zones may be because they are strike-slip faults, and the friction coefficients themselves are relatively high. The southern section of the Xiaojiang fault zone may be related to the development of fault gouges in the fault zone.

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    NUMERICAL SIMULATION OF FAULT ACTIVITY AND STRESS STATE BEFORE MS6.9 MENYUAN EARTHQUAKE
    LI Yuan, YANG Zhou-sheng, PANG Ya-jin, LIANG Hong-bao, LIU Xia
    SEISMOLOGY AND GEOLOGY    2023, 45 (6): 1286-1308.   DOI: 10.3969/j.issn.0253-4967.2023.06.003
    Abstract123)   HTML14)    PDF(pc) (7987KB)(95)       Save

    The Menyuan MS6.9 earthquake occurred on January 8, 2022, which is the third strong MS>6 earthquake on the western part of the Lenglongling fault following two Menyuan MS6.4 earthquakes that took place in 1986 and 2016. In order to explore the fault deformation and stress states of different timescales before the MS6.9 Menyuan earthquake and the dynamic environment of frequent strong earthquakes in the area nearby the epicenter, with GPS velocities of 1991—2015 and 2017—2021 as boundary constraints, a fine three-dimensional viscoelastic finite element model was established. The model included the impacts of tectonic units, the layered structure of the crust-mantle, the inhomogeneity of the medium, the interactions of many different faults, and the shape of the faults. It also refined the key faults in the region and their geometric characteristics. The basic pattern of stress accumulation in the Qilian Mountain tectonic region under the long-term tectonic movement environment, the long-term slip rate and stress accumulation rate of faults and their change characteristics during the five years before the Menyuan MS6.9 earthquake are calculated and analyzed. Combining the results of the source mechanism solution and cross-fault level observation, the following conclusions are obtained:

    (1)According to the simulation results for a longer period of 1991—2015, the stress field in the study area gradually rotates clockwise, with NNE-SSW extrusion and NWW-SEE tension to NE-SW extrusion and NW-SE tension from west to east. The direction of the principal compressive stress is mostly perpendicular to the fault strike. The region near the epicenter of the Menyuan MS6.9 earthquake has been subjected to long-term NE-SW extrusion and NW-SE tensional stress. The maximum shear stress accumulates faster than the surrounding area. The above stress accumulation characteristics overall promote NW-oriented shear and NE-oriented extrusion movement of faults, which contribute to the generation and occurrence of strike-slip and thrust earthquakes on the NWW-oriented Lenglongling Fault.

    (2)The simulation results show that most NWW-orientated faults exhibit a left-lateral strike-slip and thrust nature. In contrast, NNW-orientated faults display a right-lateral strike-slip and extrusion nature. The fault’s stress nature corresponds with its movement nature. Spatially, the overall trend of fault movement in the study area is that the extrusion rate gradually decreases from west to east, and the slip rate gradually increases from west to east. This indicates that the Qilianshan tectonic belt plays a significant role in transforming and adjusting the tectonic deformation of the northeastern margin of the Qinghai-Tibetan plateau.

    (3)The fault movement and its stress distribution show significant segmentation, indicating the crucial role of fault geometry in fault movement. The western segment of the Lenglongling Fault has a geometric inflection pattern, causing stress accumulation variability and uncoordinated movement between different segments. Compared to the surrounding fault segments, this fault segment has a higher rate of stress accumulation yet experiences hindered movement in space which causes a lower slip rate. fault zones that exhibit motion deficits and rapid energy accumulation are more susceptible to earthquakes.

    (4)Compared to the period between 1991 and 2015, the simulation outcomes obtained during 2017—2021 demonstrated noticeable differences and irregularities in the distribution of motion and stress increment fields along the fault, which were segmental in nature. Within~5 years before the Menyuan MS6.9 earthquake, the strike-slip rate at the western segment of the Lenglongling fault is further reduced, the accumulation rate of shear stress was significantly increased; the extrusion rate was significantly weakened, and the rate of positive stress accumulation was slowed down. These recent changes in fault motion and stress are conducive to promoting left-lateral slip-strike earthquakes on this fault segment.

    (5)From a hydrostatic perspective, the above studies demonstrate that the epicenter region had accumulated high stress for a long time before the earthquake, and as the earthquake approached, the positive stress on the seismic fault surface increased slowly, and the friction increased synchronously, leading to the weakening and deficit of movement on the local fault segment.

    In conclusion, the western segment of the Lenglongling fault has a strong stress background and favorable conditions for the occurrence of strong earthquakes, and the risk of strong earthquakes is still predicted to exist in the future.

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    STUDY ON THE REGIONAL GRAVITY GRADIENT FIELD MODELING IN IRREGULAR AREA WITH CONSIDERATION OF SURFACE FLUCTUATION: A CASE STUDY OF YUNNAN PROVINCE
    LIU Jin-zhao, LIANG Xing-hui, YE Zhou-run, CHEN Zhao-hui, HU Min-zhang, HAN Yu-fei, WANG Qing-hua, LIU Dong, HAO Hong-tao, ZHANG Shuang-xi, CHEN Ming
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1129-1146.   DOI: 10.3969/j.issn.0253-4967.2023.05.006
    Abstract120)   HTML16)    PDF(pc) (9247KB)(104)       Save

    The gravity gradient full tensor can sense the slight changes of the gravity vector in different directions, and because of the large number of components(6 components), it can reflect more information on different sides of the same field source than the gravity vector. On the other hand, the ground gravity survey has accumulated a lot of basic gravity vector data, and using the mathematical relationship between the gravity vector and the full tensor of gravity gradient to build a full tensor gravity gradient field model is helpful to the depth mining of existing gravity data information.

    Based on the mathematical relation and covariance function relation of disturbance potential, gravity anomaly, gravity disturbance and gravity gradient disturbance, along with the Least Square Configuration(LSC)algorithm, in this paper, the numerical formulas have been derived in detail for modeling the full tensor of regional gravity gradient disturbance field from gravity disturbance or gravity anomaly which distributed on undulating surface with non-grid pattern. The full tensor of gravity gradient derived from the grid distributed gravity anomaly data in West Arnhem Land, Australia by using the spectral domain(two-dimensional fast Fourier transform)method were taken as the “reference value”, Then, the full tensor of gravity gradient, in the same area with irregular regional gravity data distribution, derived form the LSC algorithm based on derived formulas were taken as the “evaluated value”.

    By comparing the differences between the “reference value” and the “evaluated value” of the gravity gradient, we found that: 1)The “evaluated value” obtained by the LSC method is consistent in spatial pattern of variation with the “reference value” derived from the spectral domain method. The vertical component of the gravity gradient disturbance describes the boundary characteristics of the field source more precisely than the vertical component of the gravity anomaly, and other gravity gradient disturbance components provide the information representation of the same field source, which provides inspiration for the in-depth interpretation of the field source. 2)Each difference component ΔδΓxxfft-lsc,ΔδΓxyfft-lsc,ΔδΓxzfft-lsc,ΔδΓyyfft-lsc,ΔδΓyzfft-lsc  and ΔΓzzfft-lsc  between the “evaluated value” and the “reference value” of the gravity gradient has systematic deviation, which is 5.54E, 5.30E, 1.85E, 6.55E, 2.09E and 9.67E respectively. It is much lower than the difference between measured gravity gradient and that from constructed model in previous studies. The results show that the accuracy of the two methods is within the allowable range, but the accuracy of the modeling method based on the Least Square Configuration needs to be further verified by the measured data.

    Finally, based on the measured surface differential gravity anomaly values in regional area of Yunnan province, the annul gravity gradient field variation model for this region, about 20km in half wavelength, is firstly constructed and presented by using the LSC with the derived formulas in this paper. The different gravity gradient component models show more abundant interannual scale signal variation characteristics, and demonstrate more local signal characteristics from different sensitive directions in Yunnan region during this time period. In addition, in the vicinity of Zhaotong city in northeast Yunnan province, because there is no gravity points available, both the annual difference results of gravity value and the annual change model of gravity gradient show uniform signal characteristics, indicating that the modeling method in this paper does not introduce additional false “anomalies”. In southern Yunnan province, where gravity points are relatively sparse, both the annual difference results of gravity values and the annual change models of gravity gradients are dominated by long-band signals, and no additional signals will be added due to the increase of expansion order of the LSC algorithm, which is also consistent with our intuitive cognition. These provide support for further research on the relationship between crustal material migration, hydrological changes and earthquakes in Yunnan region.

    The procedure and method proposed in this paper can improve the efficiency of using measured gravity data(mainly gravity anomaly and gravity disturbance). Moreover, it can provide basic data for better understanding and interpretation of gravity data, gravity gradient data and their relationship with different field sources in geophysics and geology.

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    ANALYSIS FOR SYNTHETIC ADJUSTMENT OF ABSOLUTE AND RELATIVE GRAVITY OF JINZHAI GRAVITY BASELINE IN DABIE MOUNTAIN
    XU Ru-gang, ZHANG Xin-lin, LIANG Xiao, SUN Hong-bo, CHU Fei, HUANG Xian-liang, TAN Hong-bo, WANG Jian
    SEISMOLOGY AND GEOLOGY    2023, 45 (5): 1147-1169.   DOI: 10.3969/j.issn.0253-4967.2023.05.007
    Abstract118)   HTML13)    PDF(pc) (5036KB)(69)       Save

    In the data process of the regional gravity monitor network, the absolute gravity reference value is the control datum of the gravity monitor network and is usually used for calibrating the scale of the relative gravimeter. Due to the coupling influence of the gravimeter scale and the control datum, it is difficult to quantitatively analyze their influence of on the adjustment processing of the regional gravity monitor network data.

    Based on the hybrid gravity observation data of the Jinzhai gravity baseline field in Dabie Mountain, the paper decouples the influence of the relative gravimeter scale and gravimetry control datum. According to analyzing the average point value, the point value, section difference, and the accuracy, we obtain the influence of the gravimeter scale and control datum on data processing of the regional gravity monitor network. The initial gravity value of the Jinzhai gravity baseline corresponds to the optimal combination of the gravimeter scale and control datum and the following conclusions are obtained:

    (1)The results of the mutual difference of gravimeters, section difference, and their accuracy calculated by different combinations of gravimeter scale and control datum show that the control datum is of great significance to reduce the inter-difference between gravimeters and obtain reliable measurement results, and the control datum used to the gravimeter scale calibration should form a complete coverage of the reading range of the regional gravity monitor network.

    (2)Gravity point values and their accuracy of the Jinzhai gravity baseline calculated by the different combinations of gravimeter scale and control datum show that the point values and accuracy are both affected by the gravimeter scale and control datum. When the solution point is located at the end of the control datum and the solution point is used as the control datum and participates in the solution calculation, the influence of the control datum and gravimeter scale on the solution is the same. When the solution point is not used as the control datum to participate in the solution control, the influence of the gravimeter scale on the solution is stronger than the control datum. When the solution points are between the control datum, the influence of the control datum on the solution is stronger than the gravimeter scale, and the more control points participate, the stronger the influence of the control datum on the solution.

    (3)Using the all control data which fully cover the reading range of the measuring network and its corresponding gravimeter scale to resolve the observation data, the point value has the highest accuracy. The solution obtained by a combination of the end control datum and its corresponding gravimeter scale is the second. The accuracies of other references and scale combinations are equivalent and the lowest. The points between the reference are strongly controlled, while the points out of the reference, the farther from the reference, the weaker the control, and the lower the point value accuracy which shows a monotone linear increase.

    (4)The data adjustment results of the Jinzhai gravity baseline based on the optimal gravimeter scale and control datum combination show that the accuracies of the section difference are better than 2μGal, and the average accuracy of the point value is better than 2.8μGal. The accuracies of the observation data of the three absolute gravity stations are better than 2.0μGal, and the accuracies about the section difference of the gravity vertical gradient are better than 1.0μGal. The results satisfy the project requirements and can be formally adopted.

    (5)The analysis of the difference between scale WYB and the optimal scale 123 shows that the gravimeter scales calibrated in the same reading section can be used directly with each other.

    (6)The Jinzhai gravity baseline can meet the requirements of gravimeter calibration for the gravity monitor network of Anhui. The construction and application of the Jinzhai gravity baseline are of great significance to improve monitoring efficiency and save economic cost. The data processing and analysis method of the Jinzhai gravity baseline field can also provide a reference for data processing of other baseline fields and regional gravity monitor networks. The Jinzhai gravity baseline extended to the absolute gravity point with a larger section difference can meet the gravimeter calibration requirements of the gravity monitor network with a larger gravity segment difference.

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    A WEB-BASED PROTOTYPE SYSTEM FOR THE THREE-DIMENSIONAL FAULT MODELS OF THE CHINA SEISMIC EXPERIMENTAL SITE
    WU Xi-yan, LU Ren-qi, ZHANG Jin-yu, SUN Xiao, XU Fang, CHEN Gui-hua
    SEISMOLOGY AND GEOLOGY    2024, 46 (1): 35-47.   DOI: 10.3969/j.issn.0253-4967.2024.01.003
    Abstract117)   HTML13)    PDF(pc) (6764KB)(116)       Save

    The data of active fault structure and three-dimensional(3D)fault models is essential for seismic risk analysis. With more and more requirement for complex 3D fault models, the demand for data sharing and related research increases dramatically. A web-based display system for three-dimensional fault models would improve data sharing and user experience. Moreover, constructing such a web-based system is also an important issue for data sharing.

    The 3D active fault models are built in a data modeling platform, while the web display system is constructed by the geographic information system(GIS)platform. Because the data structure, type, and content between data modeling and GIS platforms are different, the following questions are critical, for example, how to migrate 3D model data from the modeling platform to the GIS platform?and can the migrated data present the right attributions?In this paper we used the Web AppBuilder of ArcGIS 10.6 Enterprise Edition to build a Web prototype system to display 3D fault models of the China Earthquake Science Experimental Field(Sichuan-Yunnan region). The system implemented the basic functions of a 3D Web application and successfully tested the 3D scene display scheme, user interaction mode, and data migration scheme.

    The prototype system adopted a local scene, which can easily switch between the above-ground and underground viewing angles of the scene. The scene included 2D fault surface traces, 3D fault models, and earthquakes with or without focal depth. After data fusion, the 3D fault models were classified and displayed with active age, having a good visual fusion effect with 2D fault data. Earthquakes with or without focal depth were displayed in different colors. The earthquakes without focal depth were uniformly displayed at 17km depth according to the average focal depth of the earthquakes with focal depth. So the earthquakes without focal depth can be highly consistent with other elements in the 3D scene.

    The user interface interaction mode in the 3D scene of the prototype system was consistent with the common interaction mode of 2D map applications in the following aspects: 1)map browsing; 2)Navigation menu; 3)Geographical inquiry; and 4)Functional interactive tools. The system interface was simple, clear, logical, and unified. Users were easily acquainted with the three-dimensional scene interface according to the two-dimensional map interaction experience. It conformed to the user interface interaction principles of simple, consistent, predictable, and easy feedback.

    The prototype system had the basic functions of 3D scene browsing, zooming in and out, 3D object attribute viewing, geographic query, base map switching, layer control, legend, and distance measurement. However, the prototype system needed further development and more complex functions such as data attribute table browsing, space selection, and space query.

    This paper presented a data migration scheme from the modeling platform to the GIS platform. The data migration of this scheme can be divided into four steps: data format conversion, coordinate system conversion, 2D and 3D attribute information mapping, and 3D data attribute table construction. After transforming the data format and coordination system from the modeling platform to the GIS platform, 2D and 3D data fusion should be carried out to make 3D data and 2D data have the same attribution. The format conversion and coordinate system conversion steps can be automatically completed in batches. Otherwise, mapping the 2D and 3D attribute information and building the 3D data attribute table need manual handling.

    In summary, this paper presents a data migration scheme from the modeling platform to the GIS platform. Practice in reality shows that only after conversing data format and coordination system from the modeling platform, the 2D and 3D data fusion steps are caplable of ensuring a better visual integration of them. The Web-based prototype system of displaying 3D fault models of the China Seismic Experimental Site implements the basic functions of 3D scene application and tests the fused 2D and 3D data visualization. It is friendly and open to users, with a great demonstration significance.

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    REMOTE SENSING INTEPRETATION OF COSEISMIC LAND-SLIDES TRIGGERED BY 1976 LONGLING MS7.3 AND MS7.4 EARTHQUAKES AND THE TECTONIC SIGNIFICANCES
    LI Hao-feng, XU Yue-ren, GUO Ya-li, LIU Han, ZHAO Xin-yu, LU Ling-yu, TANG Jia-cheng
    SEISMOLOGY AND GEOLOGY    2024, 46 (1): 117-140.   DOI: 10.3969/j.issn.0253-4967.2024.01.008
    Abstract115)   HTML9)    PDF(pc) (17130KB)(100)       Save

    The Longling-Lancang seismotectonic belt in southwestern Yunnan is critical for accurately defining the boundaries of active blocks and evaluating seismic risks. Using pre- and post-earthquake high-spatial-resolution satellite imagery to study strong earthquakes retrospectively proves to be a practical method in such study. Strong earthquakes frequently cause secondary effects such as coseismic landslides, collapses, and debris flows, which lead to considerable loss of life and property. These secondary effects, often as the most dramatic manifestations of an earthquake, show geologic signatures providing evidence of historic or prehistoric seismic activities. The use of satellite imagery captured shortly after historic earthquakes to interpret these secondary effects is particularly beneficial in determining the intensity and influence radius of earthquakes, thereby helping study on seismogenic faults of earthquakes.

    On May 29, 1976, two strong earthquakes with MS7.3 and MS7.4 occurred in Longling county, southwest China, followed by intense aftershocks. The seismogenic structure of these earthquakes still remains undetermined to present. These earthquakes triggered numerous coseismic landslides in the regolith of the granitic rock mass. The seismic zone, located in subtropical regions, is characterized by high precipitation and dense vegetation. Apart from the ancient landslides in the northwest and southeast, no records of landslides and debris flows persisted in the epicenter zone for a century, making the occurrence of substantial landslides post the main earthquakes unexpected. Currently, these landslides have undergone reshaping by land surface processes and re-vegetation, which makes them indistinguishable in recent remote sensing images. Using Keyhole satellite images with a resolution of 0.6~1.2m offers a useful means to identify the coseismic landslides of the Longling mainshocks. In this study, we employ these images for a comprehensive visual interpretation of the coseismic landslides. To ensure the accuracy and reliability of the results, we used images captured in 1981(the most recent following the earthquakes)to extract coseismic landslides and substantiated them with images from 1974, field investigation photos from 1976, and relevant records. Finally, we have compiled an exhaustive database of coseismic landslides triggered by the 1976 Longling cases.

    Our results are summarized as follows: 1)A total of 14 448 landslides were interpreted, encompassing an overall area of 17.2km2. The area of individual landslides primarily ranged from several hundreds to one thousand m2, and most were superficial slides in the surface regolith with short sliding distances. The regional stratigraphy is complex, with 70.9% of the landslides occurring in the regolith of granitic rock mass, 15.3%in sandstones or siltstones, and a mere 13.8%in other areas such as limestones. Consequently, these landslides were relatively small compared to those in other regions like the Loess Plateau in north China, where the surface sediment is extremely loose. 2)A strong correlation exists between the intense area of coseismic landslides and the earthquake sequence, which tends to migrate from south to north. Notable aftershocks(e.g., MS6.2 on June 9 and MS6.6 on July 21)particularly exhibited the general NNW distribution direction of the earthquake sequence and triggered scattered landslides outside the epicenter zone. Through synthesizing field surveys, combining other records and the findings of this study, we believed that the two main earthquakes triggered numerous coseismic landslides, and the continuous strong aftershocks led to the destabilized regolith of the granitic rock mass creeping successively, resulting in subsequent landslides. 3)The concentration areas of coseismic landslides do not match the high-earthquake-intensity areas, instead, they are all located on one side of active faults, which suggests that the seismogenic fault is neither the Longling-Ruili Fault nor the Wanding Fault. The spatial distribution of the landslides suggests that the scope of the surface rupture zone is about 30km. The conjugated strong earthquake ruptures in southwestern Yunnan may limit the spatial scale of single strong earthquakes, so it is crucial to pay more attention to the intersection zone of NE and NW trending active faults when assessing regional strong earthquake risk in the future.

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