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RELOCATION AND FOCAL MECHANISM SOLUTIONS OF THE 2021 MADUO, QINGHAI MS7.4 EARTHQUAKE SEQUENCE ZHAO Tao, WANG Ying, MA Ji, SHAO Ruo-tong, XU Yi-fei, HU Jing SEISMOLOGY AND EGOLOGY    2021, 43 (4): 790-805.   DOI: 10.3969/j.issn.0253-4967.2021.04.004 Abstract （1755）   HTML （52）    PDF（pc） （5589KB）（353）       Knowledge map    Save On May 22, 2021, an MS7.4 earthquake occurred in Maduo County, Guoluo Prefecture, Qinghai Province, which is the biggest earthquake in mainland China since the 2008 Wenchuan MS8.0 earthquake. It occurred in the Bayan Har block in the northern part of the Qinghai-Tibet Plateau, indicating that the Bayan Har block is still the main area for strong earthquakes activity in mainland China. In order to study the source characteristics and seismogenic structure of the Maduo earthquake, we used the double-difference location method to analyze the spatial distribution of earthquake sequences within 15 days after the mainshock. At the same time, the focal mechanism solutions of 15 aftershocks with MS≥4.0 are also obtained by full-waveform moment tensor inversion. We hope to provide seismological evidences with reference value for the study of the dynamic process of the Madao MS7.4 earthquake and the geological tectonic activities on the northern side of the Bayan Hala block. The results of moment tensor inversion show that the moment magnitude of the Maduo earthquake is about 7.24, the centroid depth is 13km, and the best double-couple solution is strike 283°, dip 59° and slip -4° for the nodal plane I, and strike 15°, dip 86° and slip -149° for the nodal plane Ⅱ, which indicates a strike-slip earthquake event. According to the strike of the fault and the distribution of aftershocks in the source area, we infer that the nodal plane I, which strikes NWW, is the seismogenic fault plane. The focal mechanism results of 15 aftershocks show that the aftershock sequence is mainly strike-slip type, which is consistent with the main shock. Meanwhile, there are also some other types reflecting the local complex structure. The differences in the direction and type of focal mechanism may reveal changes in the direction and characteristic of the fault from north to south. The azimuth of the P-axis is NE-SWW, and the azimuth of the T-axis is NNW-SSE. Both plunge angles are within 30° and close to horizontal, which shows that the activities of the Maduo earthquake sequence are mainly controlled by the horizontal compression stress field in the northeast-southwest direction. From NWW to SEE, the dip angle of fault plane increases gradually from 77° to 88°, and the northern segment dips to SW. Based on the results of relocation, moment tensor inversion and geological structure, preliminary conclusion can be drawn that the seismogenic fault of the Maduo earthquake may be the Kunlun Mountain Pass-Jiangcu Fault, which is a left-handed strike-slip fault. At the same time, there are certain segmental differences along the fault. The strike of the northern section is mainly NW, that of the middle section is NWW, and the southern section is near E-W, and the fault plane dips to the southwest with the dip angle increasing gradually from NWW to SEE. Table and Figures | Reference | Related Articles | Metrics

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RELOCATION AND FOCAL MECHANISM SOLUTIONS OF THE 2021 YANGBI, YUNNAN MS6.4 EARTHQUAKE SEQUENCE WANG Ying, ZHAO Tao, HU Jing, LIU Chun SEISMOLOGY AND EGOLOGY    2021, 43 (4): 847-863.   DOI: 10.3969/j.issn.0253-4967.2021.04.007 Abstract （1409）   HTML （41）    PDF（pc） （8652KB）（208）       Knowledge map    Save On May 21, 2021, a strong earthquake of magnitude 6.4 occurred in Yangbi County, Dali Prefecture, Yunnan Province. The focal depth of this earthquake is 8km. The earthquake broke the calm of magnitude 6 earthquake that had lasted for more than 6 years in Yunnan, and is a significant strong earthquake in the northwestern Yunnan region. Before the MS6.4 Yangbi earthquake, the foreshock activity near the epicenter was frequent, and the maximum magnitude of foreshock is 5.6. After the MS6.4 earthquake, another MS5.2 earthquake, and many aftershocks of magnitude 3 and 4 occurred. The earthquake sequence was very rich. In order to further study the spatio-temporal distribution, source characteristics and seismogenic structure of the magnitude 6.4 earthquake sequence in Yangbi, in this paper more than 2 800 seismic events of the Yangbi earthquake sequence were relocated using the double-difference relative positioning method based on the seismic phase data from the Seismic Cataloging System of China Earthquake Networks Center, and finally 2 116 precise location results were obtained. At the same time, based on the broadband digital waveform data provided by the China Earthquake Networks Center, focal mechanism solutions 31 earthquakes of the sequence were obtained by MTINV program. The results of the moment tensor inversion show that the moment magnitude of the Yangbi MS6.4 earthquake is MW6.0, the centroid depth is 10km, and the optimal double-couple solution is strike 135°, dip 81° and rake 176° for nodal plane I, and strike 226°, dip 86° and rake 9° for nodal plane Ⅱ. It is a strike-slip earthquake. Combining the strike of the fault in the earthquake source area and the distribution of aftershocks, it is inferred that the seismogenic fault is the nodal plane Ⅰ which strikes NW. Focal mechanism solutions of other 30 earthquakes of the sequence are mainly strike-slip type, which are consistent with the main shock. There are also a few events with mixed types. The focal mechanisms of several earthquakes close to the occurrence time of the MS6.4 main earthquake are in good agreement with the main earthquake. The relocation results show obvious linear distribution characteristics of the sequence. The overall strike is in the NW direction and the dip to the SW direction. The depth profile sequence is horizontally linear along the strike. The dip angles of the fault planes in the south and north sections are different. The dip angles of the northern section are approximately vertical, and that of the southern section is about 45° or so. However, the sequence of the northern section is more concentrated along the fault plane than southern section. The dominant strike of the Yangbi earthquake sequence is NW-SE, the dip angles are concentrated between 70° and 90°, and the rakes are distributed around 180°, indicating that the Yangbi earthquake sequence is mainly characterized by strike-slip faulting. The dominant azimuth of the P-axis is SN and that of the T-axis is EW. The plunge of P-axis and T-axis are near horizontal. This indicates that the activities of the Yangbi earthquake sequence are mainly controlled by the regional SN-direction horizontal compression stress field. The dominant directions of the sequence’s fault planes and P-axis parameters are single, indicating that it is less likely that complex fault activity and large-scale stress adjustment will occur in the source area of this earthquake. Integrating the results of relocations and focal mechanisms, it suggests that the seismogenic fault of Yangbi earthquake is a right-handed strike-slip active fault, striking northwest and dipping to the southwest, and the dip distribution is segmented. The dip angle of the northern segment is nearly vertical, and the dip angle of the southern segment is lower than that of the northern segment. There may exist rupture segmentation in the fault in the earthquake source area, and the structure morphology of local small areas may be more complicated. Table and Figures | Reference | Related Articles | Metrics

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NEW INSIGHT ON THE HOLOCENE ACTIVITY OF THE EASTERN MARGINAL FAULT OF DAXING UPLIFT, BEIJING PLAIN LI Zheng-fang, LI Yan-bao, ZHOU Ben-gang, ZHU Guo-jun, LIU Bao-jin, WU Jian SEISMOLOGY AND EGOLOGY    2021, 43 (6): 1671-1681.   DOI: 10.3969/j.issn.0253-4967.2021.06.018 Abstract （1385）   HTML （47）    PDF（pc） （8302KB）（314）       Knowledge map    Save The eastern marginal fault of Daxing Uplift is located in the southeast of the Beijing Plain, which is a boundary fault that controls the Daxing Uplift and the Langgu Sag. It intersects obliquely with the NNE-trending Xiadian Fault in the north where a magnitude 8 earthquake occurred in 1679. The overall strike of the fault is northeast, dipping southeast. Previous studies have suggested that the youngest stratum of the fault is the Mid Pleistocene of the Quaternary and it is not an active fault since the Late Quaternary. Based on high-precision shallow seismic exploration data, this study carried out high-density composite drilling geological section surveys and obtained evidence of obvious activity of the fault since the Late Quaternary. The fault is shown as an active normal fault in the composite drilling geological section. The top of the footwall of the fault is the 7m-thick silty clay marker layer buried at the depth of 74m and the top of the hanging wall is 102m deep, the amount of dislocation is about 28.0m. Fault slip surfaces were found in the cores of two of the boreholes, with depths of 54.2m and 39.4m, respectively. The buried depths of the top surface of the marker layer in the two boreholes with a horizontal distance of 2m are 8m and 10m, respectively, the dislocation amount is 2m. Combined with the observation of core deformation characteristics of the two boreholes, it is believed that the buried depth of the upper breakpoint of the fault may be shallower. This research has changed the understanding that the fault zone on the eastern margin of the Daxing Uplift is not active. This new discovery not only has great application value for understanding the risk of large earthquakes of this fault zone and the risk of earthquake disasters in Beijing, but also has scientific significance for the study of fault development and evolution and the deep-shallow coupling process in North China since the late Cenozoic. The main knowledge gained is as follows: 1)Through high-precision shallow seismic exploration, it is found that the Neogene and above strata in the study area generally show an inclined morphology which is deep in the south and shallow in the north. The strata below the Neogene are in angular unconformity contact with the bottom interface of the Neogene, and the depth of the shallowest upper breakpoint is about 38~43m. 2)The combined drilling geological section exploration reveals rich dislocation information of stratigraphic markers and further confirms the existence of active faults by borehole stratigraphic correlation. In the drill cores, fault slip surfaces were observed in the late Pleistocene strata at the depth of 39.4m, 51.5m and 54.2m, respectively. The stratigraphic comparison of the boreholes 5# and 9# with a hole spacing of 2m further reveals a fault throw of about 2m in the stratum at the buried depth of 8~10m, thus, it is inferred that the depth of the upper breakpoint on the fault may be 8m or shallower. According to the stratigraphic age data of adjacent boreholes in this area, it is considered that the fault is a Holocene active fault. The specific age of the latest activity and its activity parameters will be further studied through the subsequent borehole chronological tests and large-scale trench excavation. Table and Figures | Reference | Related Articles | Metrics

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THE SEISMOGENIC FAULT OF THE 2021 YUNNAN YANGBI MS6.4 EARTHQUAKE LI Chuan-you, ZHANG Jin-yu, WANG Wei, SUN Kai, SHAN Xin-jian SEISMOLOGY AND GEOLOGY    2021, 43 (3): 706-721.   DOI: 10.3969/j.issn.0253-4967.2021.03.015 Abstract （1174）   HTML    PDF（pc） （16261KB）（566）       Knowledge map    Save The May 21, 2021 Yangbi MS6.4 earthquake occurred at the western boundary of the Chuandian tectonic block in southeast Tibetan plateau. The structural background is complex, with multiple active faults distributed around the epicenter area. Focal mechanism and seismic waveform inversion reveal that this earthquake is right-lateral strike-slip type with a NW-trending rupture plane. This accords with the strike and motion directions of the Weixi-Qiaohou and Red River faults along the western boundary of the Chuandian block. We made a careful field investigation along the Weixi-Qiaohou Fault and around the epicenter area, and did not find any obvious earthquake surface rupture. But we observed a NW-trending ground fissure zone near the epicenter area to the west of the Yangbi County. This zone is divided into two sections, the Yangkechang-Paoshuitian section in the northwest and the Xiquewo-Shahe section in the southwest. These sections have a length of 2.5~3km and 3~3.5km, respectively, and are separated by a ~6km gap. They are characterized by NW-trending ground fissures with a width of several meters to tens meters. The formation of these fissures is inferred to be related to the tectonic movement under the ground, and the fissures have the following features: 1)they are not affected by the topography and cut the slope and range upward; 2)they are continuous and concentrated in a zone with a strike of NW 310°~320°, which is consistent with the belt of aftershocks and differs from the gravity fissures that usually have no regular strikes; 3)they usually have a plane dipping towards upslope(southwest), opposite to the valley; 4)they present shear property, not tensional. This zone thus is interpreted to be the surficial expression of the seismogenic fault of the Yangbi MS6.4 earthquake. Moreover, satellite image and field observation suggest that a~30km long linear structure with a NW strike traverses the epicenter area, which may suggest an undiscovered fault. Relocation of small earthquakes shows that the aftershocks are concentrated in a NW-trending belt that is consistent with the linear structure. Furthermore, the fissure zone lies in the northeast side of the aftershock belt, which suggests that the earthquake fault dips SW. Such a dip direction coincides with that of the observed fissure plane, and also agrees with the results from the focal mechanism and InSAR inversion. Both the focal mechanism and the waveform inversion result suggest that the Yangbi earthquake is a right-lateral strike-slip type, which is consistent with the type of the observed ground fissures. No displacement is observed on the fissures, with is also consistent with the InSAR inversion results that suggest the rupture did not break the surface. In addition, there is no coseismic deformation observed along the Weixi-Qiaohou Fault, which may indicate this fault did not move during this earthquake. Based on our field investigation, in combination with the focal mechanism, aftershock distribution, and InSAR and GNSS inversion results, the seismogenic fault for this Yangbi MS6.4 earthquake is believed to be a NW-trending(310°~320°)fault with a length of~30km, named as the Yangkechang-Shahe Fault. According to the location, size, and motion of the fault, it is suggested that the Yangkechang-Shahe Fault is a secondary fault of the Weixi-Qiaohou fault system. This fault has a slightly SW-dipping plane, and is dominated by right-lateral strike-slip motion, which may be a younger fault developed during the westward expansion of the western boundary of the Chuandian block. Reference | Related Articles | Metrics

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EVALUATION OF RYUKYU-MANILA TRENCH TECTONIC BACKGROUND AND SEISMOGENIC ABILITY LI Zheng-fang, ZHOU Ben-gang, XIAO Hai-bo SEISMOLOGY AND EGOLOGY    2021, 43 (6): 1381-1397.   DOI: 10.3969/j.issn.0253-4967.2021.06.003 Abstract （1098）   HTML （182）    PDF（pc） （13603KB）（131）       Knowledge map    Save After the Fukushima nuclear accident caused by the “3·11” earthquake tsunami in Japan, whether the coastal nuclear power stations in China are liable to similar earthquake tsunami impact has been widely concerned by the whole society. According to the previous results of earthquake tsunami impact assessment conducted by professional departments on coastal nuclear power plants, China's coastal areas do not have the conditions for the occurrence of large-scale earthquake tsunami, but in order to fully learn from the experience and lessons of the Fukushima nuclear accident caused by Japan's “3·11” earthquake tsunami, definite conclusions have been drawn on the offshore tsunami and its impact on nuclear power plants in the early assessment work of potential tsunami impact of coastal nuclear power stations in China, combined with the structural background, historical seismic data and tsunami impact analysis. However, whether the earthquakes in the Ryukyu trench, Manila trench and other areas can generate tsunami has not been systematically considered. Therefore, in this paper, the seismogenic capacity of the Ryukyu trench and Manila trench is evaluated based on the seismotectonic background and relevant seismic source parameters. Both Ryukyu and Manila trench belong to the west Pacific plate subduction zone, while the Japan's “3·11”earthquake is also located in the west Pacific plate subduction zone. Therefore, whether the former has the same tectonic background and conditions as the “3·11” earthquake does is the key factor to assess whether the Ryukyu trench and Manila trench have the same potential for M9 earthquake. Based on the analysis of a large number of data, this paper evaluates the tectonic background, segmentation characteristics and maximum potential earthquake generating capacity of the Ryukyu trench and the Manila trench. The Ryukyu trench and Manila trench are located in the west of the Philippine Sea plate. There are also subduction zones distributing in the east of the Philippine Sea plate from Izu-Ogasawara trench, Mariana trench to Yap Palau-Ayu trench. Since the Ryukyu trench-Manila trench subduction zones are not in the direct contact zone between the Pacific plate and the Eurasian plate, the plate tectonic setting is obviously different from the low-angle subduction zone where the Japan's March 11 earthquake locates. From the perspective of tectonic system, the Ryukyu trench belongs to the subduction tectonic system of trench-island arc-back arc basin. The island arc and trench are retreating eastward, showing the characteristics of weak coupling. The overall scale of the Manila trench is small, and affected by the “slab window” in the subduction slab formed by the ancient spreading ridge, the length of these two trench zones is much smaller than that of the subduction zones where MW≥9 earthquakes have occurred. Based on the comprehensive analysis of the differences in trench structure, earthquake data and etc., the Ryukyu trench can be divided into 6 rupture segments, and the section of the Manila trench concerned in this study can also be divided into 6 rupture segments. At the same time, the possibility of combined rupture of the rupture segment is considered from a conservative standpoint. The rupture segments RL5 and RL6 of the Ryukyu trench, RM2 and RM3 of the Manila trench all have the possibility of combined rupture, and rupture segments RM4, RM5 and RM6 also have the possibility of combined rupture. To sum up, the comprehensive estimation result of the maximum potential earthquake in the subduction zone is magnitude 8.5 in the Ryukyu trench and magnitude 8.8 in the Manila trench. Table and Figures | Reference | Related Articles | Metrics

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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 Abstract （1082）   HTML （23）    PDF（pc） （17177KB）（183）       Knowledge map    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. Table and Figures | Reference | Related Articles | Metrics

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EARTHQUAKE GROUND MOTION INTENSITY MAP OF THE 21 MAY, 2021 MS6.4 YANGBI, YUNNAN EARTHQUAKE CHEN Kun, WANG Yong-zhe, XI Nan, LU Yong-kun, LU Dong-hua SEISMOLOGY AND EGOLOGY    2021, 43 (4): 899-907.   DOI: 10.3969/j.issn.0253-4967.2021.04.010 Abstract （1006）   HTML （25）    PDF（pc） （4118KB）（257）       Knowledge map    Save On 21 May 2021, a great earthquake of M6.4 struck Southwest China. This catastrophic event caused extensive casualties, a large number of houses collapsed, traffic disrupted, and large bridges damaged in Yunnan Province. The epicentre of the Yunnan Yangbi earthquake is located near the NW trending Weixi-Qiaohou-Weishan Fault. After this earthquake, the Institute of Geophysics of China Earthquake Administration calculated the focal mechanism solution using the global network data, the result shows that the earthquake is a strike-slip faulting event with normal component. The result of the focal mechanism solution is consistent with the strike of the Weixi-Qiaohou Fault and the distribution of aftershocks. Therefore, the strike of seismogenic structure of this earthquake was determined to be NW. Based on the strong motion observation data of 21 strong motion seismographs and 304 seismic intensity meters, the earthquake ground motion intensity map of the 21 May, 2021 Yangbi, Yunnan earthquake was obtained using the deviation correction method of magnitude shift, considering the geological tectonic background of the seismogenic fault, the focal mechanism solution and the precise location of aftershocks of this event. A commonly used proxy VS30, the time-averaged shear wave velocity to 30m depth, was used to account for the local site effect of ground motion in the calculation of ground motion intensity map. We used VS30-based amplification terms, which depends on the amplitude and frequency of ground motion, to account for site amplification. The VS30 data of the macroscopic site classification was estimated using the correlation between topographic gradient and VS30. Ground motion prediction equations(GMPEs) was used to supplement sparse data in its interpolation and estimation of ground motions. The selection of GMPEs for ground motion estimation were the attenuation relation of peak acceleration in western China in the fourth generation seismic zoning map. The observations of the ground motion for this event show that the maximum horizontal peak ground acceleration is 720.3cm/s2 on the Yangbi station, 7.9km from the epicentre. Horizontal peak ground acceleration at 14 seismic stations is greater than 45cm/s2. A large number of remote observation records with small values of ground motion also revealed the attenuation characteristics of ground motion for this earthquake. Using strong motion observation data available, we computed an event bias that effectively removed the inter-event uncertainty from the selected GMPE. The deviation correction method of magnitude shift minimizes the systematic deviation between the observed and estimated data produced by ground motion prediction equation, and reduces the uncertainties of the ground motion estimation in the area without stations. After the ground motion observations were corrected(de-amplified) to “rock”, we flagged any data that exceed three times the sigma of the GMPE at the observation point as abnormal data. The bias was then recalculated using different earthquake magnitudes and the flagging was repeated until systematic deviation between the observed and estimated data produced by GMPE was minimal. The results of the earthquake ground motion intensity map show that the highest seismic intensity caused by Yangbi earthquake is Ⅷ. Cangshanxi Town in Yangbi County and Taiping Town are located in the seismic intensity Ⅷ area. The area of seismic intensity Ⅵ and above covers an area of about 6 500km2, spreading northwest in general. Many roads including Expressway G56 and national highway G215, pass through the estimated seismic intensity Ⅶ area, which may cause road damage and traffic disruption following this earthquake. On the other hand, the reliability of small amplitude observations recorded by far-field simple intensity meters need to be evaluated further. Finally, the seismogenic tectonic setting, the focal mechanism solution and the aftershock distribution of the earthquake also play a macro-control role in the distribution of the earthquake ground motion intensity. The results of this paper can provide theoretical basis and reference for earthquake emergency response decision-making and provide input for earthquake disaster emergency assessment. Table and Figures | Reference | Related Articles | Metrics

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SEISMOGENIC FAULT AND COSEISMIC SURFACE DEFORMATION OF THE MADUO MS7.4 EARTHQUAKE IN QINGHAI, CHINA: A QUICK REPORT LI Zhi-min, LI Wen-qiao, LI Tao, XU Yue-ren, SU Peng, GUO Peng, SUN Hao-yue, HA Guang-hao, CHEN Gui-hua, YUAN Zhao-de, LI Zhong-wu, LI Xin, YANG Li-chen, MA Zhen, YAO Sheng-hai, XIONG Ren-wei, ZHANG Yan-bo, GAI Hai-long, YIN Xiang, XU Wei-yang, DONG Jin-yuan SEISMOLOGY AND GEOLOGY    2021, 43 (3): 722-737.   DOI: 10.3969/j.issn.0253-4967.2021.03.016 Abstract （993）   HTML    PDF（pc） （18089KB）（634）       Knowledge map    Save At 02:04 a.m. on May 22, 2021, a MS7.4 earthquake occurred in the Maduo County, Qinghai Province, China. Its epicenter is located within the Bayan Har block in the north-central Tibetan plateau, approximately 70km south of the eastern Kunlun fault system that defines the northern boundary of the block. In order to constrain the seismogenic fault and characterize the co-seismic surface ruptures of this earthquake, field investigations were conducted immediately after the earthquake, combined with analyses of the focal parameters, aftershock distribution, and InSAR inversion of this earthquake. This preliminary study finds that the seismogenic fault of the Maduo MS7.4 earthquake is the Jiangcuo segment of the Kunlunshankou-Jiangcuo Fault, which is an active NW-striking and left-lateral strike-slip fault. The total length of the co-seismic surface ruptures is approximately 160km. Multiple rupture patterns exist, mainly including linear shear fractures, obliquely distributed tensional and tensional-shear fractures, pressure ridges, and pull-apart basins. The earthquake also induced a large number of liquefaction structures and landslides in valleys and marshlands. Based on strike variation and along-strike discontinuity due to the development of step-overs, the coseismic surface rupture zone can be subdivided into four segments, namely the Elinghu South, Huanghexiang, Dongcaoarlong, and Changmahexiang segments. The surface ruptures are quite continuous and prominent along the Elinghu south segment, western portion of the Huanghexiang segment, central portion of the Dongcaoarlong segment, and the Huanghexiang segment. Comparatively, coseismic surface ruptures of other portions are discontinuous. The coseismic strike-slip displacement is roughly determined to be 1~2m based on the displaced gullies, trails, and the width of cracks at releasing step-overs. Reference | Related Articles | Metrics

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TYPICAL CASE ANALYSIS ON SETBACK DISTANCE FOR URBAN BURIED ACTIVE FAULT: AN EXAMPLE SITE ALONG THE TANLU FAULT ZONE IN XINYI CITY CAO Jun, LI Yan-bao, RAN Yong-kang, XU Xi-wei, MA Dong-wei, ZHANG Zhi-qiang SEISMOLOGY AND GEOLOGY    2022, 44 (4): 1071-1085.   DOI: 10.3969/j.issn.0253-4967.2022.04.016 Abstract （993）   HTML （73）    PDF（pc） （11099KB）（523）       Knowledge map    Save With the acceleration of urbanization process, solving the earthquake and its associated disasters caused by buried active fault in urban areas has been a difficult issue in the construction of urban public security system. It is difficult to deal with the anti-seismic issues of cross-fault buildings using the existing techniques, therefore, reasonable setback distance for buried active fault in urban area is the only method for the planning and construction at the beginning. At present, theoretical research about setback for active fault is becoming more and more mature, and the mandatory national standard “Setback distance for active fault” will be enacted soon. As a result, how to work on the basis of these theories and national standards is in urgent. In recent years, the exploration of urban active faults was successively completed. However, there are no typical cases of how to make full use of the achievements of urban active fault projects in the follow-up work, and how to guide urban construction based on the project conclusions, so as to ensure urban safety and rational development of urban economy. In this paper, taking a site along the Anqiu-Juxian Fault in the Tanlu fault zone in Xinyi city as an example, based on the results of 1︰10 000 active fault distribution map, and referring to the stipulation of national standard “Setback distance for active fault”, 12 shallow seismic survey lines with a spacing of less than 50m were laid out firstly, and the results of shallow seismic exploration show the existence of two high-dip faults in the site. Secondly, considering the shallow seismic survey results and the geologic site conditions, five rows of borehole joint profiles were selected along five of the shallow seismic survey lines. Based on the location of the faults and stratigraphy in the site revealed by the borehole joint profiles, and considering the latest research results of Quaternary stratigraphy and the conclusion of urban active faults detection, the west branch fault is constrained to be a Holocene active fault and the east branch fault is an early Quaternary fault. As a result, we precisely mapped the trace, dip and upper breakpoint of the fault in the site based on the shallow seismic exploration and joint borehole profile. The accurate positioning of the plane position of the active fault differs by about 200m from the 1:1000 strip distribution map. According to the relevant national standards and scientific research results, active faults in the site shall be avoided. Based on the surface traces of active faults revealed by the accurate detection in the site, the active fault deformation zone was delineated, and the range of setback distance for active fault was defined outside the deformation zone. The detection results accurately determined the plane distribution of the active fault in the site, which meets the accuracy of the development and utilization of the site. Based on the accurately located active fault trace, and complying with the forthcoming national standard “Setback distance from active fault”, this study not only scientifically determines the setback distance for active fault in the site, but also releases the scarce land resources in the city. This result achieves the goal of scientifically avoiding potential dangerous urban hidden active fault and making full use of land. The case detection process confirms that the results of urban active fault detection are still difficult to meet the fault positioning accuracy required for specific site development, and the range of active fault deformation zone within the site must be determined based on the precise positioning method for hidden active faults as stipulated in the national standard “Setback distance for active fault”. The national standard “Code for seismic design of buildings” only specifies the setback distance for active faults under different seismic intensity, but does not provide any clear definition of the accuracy of active fault positioning, so it is difficult to define the required active fault positioning degree and boundary range of the deformation zone of active fault in practice. The national standard “Setback distance for active fault” clearly defines various types of active fault detection and positioning methods, determines the scope of active fault deformation zone and the accurate setback distance for active fault in different cases. The specific case proves that before developing and utilizing specific sites along urban concealed active faults, relevant work shall be carried out according to the national standard “Setback distance for active fault” to effectively resolve the issue about the relations between urban development and urban safety, so the promulgation and implementation of national standard should speed up. Table and Figures | Reference | Related Articles | Metrics

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SURFACE RUPTURE CHARACTERISTICS OF THE MENYUAN MS6.9 EARTHQUAKE ON JANUARY 8, 2022, QINGHAI PROVINCE LIANG Kuan, HE Zhong-tai, JIANG Wen-liang, LI Yong-sheng, LIU Ze-min SEISMOLOGY AND GEOLOGY    2022, 44 (1): 256-278.   DOI: 10.3969/j.issn.0253-4967.2022.01.016 Abstract （967）   HTML （28）    PDF（pc） （24460KB）（427）       Knowledge map    Save At 1:45 on January 8, 2022, a MS6.9 earthquake occurred in Menyuan County, Haibei Prefecture, Qinghai Province. The epicenter(37.77°N, 101.26°E)is located in the western segment of the Lenglongling Fault of the Qilian-Haiyuan fault zone, with a focal depth of 10km. The earthquake is located in the northwest of the MS6.4 Menyuan earthquake on January 21, 2016. According to the survey results of China Earthquake Administration, the highest intensity of this earthquake is IX degree, and the long axis of the isoseismic line is NWW-striking. The earthquake caused serious damage to the Daliang Tunnel between Haomen Station and Junmachang Station, and the Lanxin high-speed railway was interrupted. After the earthquake, the distribution of the earthquake surface rupture zone was quickly determined by interpreting the GF-7 satellite post-earthquake images, and the field surface rupture investigation was carried out at the epicenter site in the first time. The field investigation mainly includes the identification of surface rupture zones, the investigation of rupture characteristics, the survey of fault geomorphology, the high-precision aerial photogrammetry of typical rupture points, the identification and measurement of coseismic dislocation, and the investigation of earthquake disasters. Aerial photogrammetry realizes real-time difference through UAV linked network RTK, and takes high-definition photos from multiple angles. Pix4D software is used to complete calculation and point cloud encryption, etc. DSM (Digital Surface Model) and DOM (Digital Orthophoto Map) are generated for surface rupture space reproduction and feature measurement and analysis. According to the interpretation of high-resolution remote sensing images by GF-7 satellite and field investigation, the surface rupture of MS6.9 Menyuan earthquake can be divided into NW-striking western segment of Lenglongling Fault and EW-striking eastern segment of Tuolaishan Fault. The two surface ruptures are 291° and 86.9°, respectively, and their lengths are not less than 26km and 3.5km respectively. We made detailed observation and measurement on the Jingyangling site, Daogou site, east Daogou site, Shixiamen site, the seven sites along the Liuhuanggou on the Lenglongling Fault, and the Yangchangzigou site on the Tuolaishan Fault. The surface rupture zone is mainly a complex coseismic surface deformation zone formed by the combination of multiple types of fractures, such as tensional fracture, tensional shear fracture, compression bulge and seismic depression, and characterized by sinistral strike-slip motion and partly by thrusting. Generally, the NW-striking ruptures exhibit left-lateral strike-slip characteristics, while NW-striking branch ruptures exhibit a small amount of right-lateral strike-slip characteristics. At Shixiamen site, four pasture fences were continuously offset left-laterally by 2.0~2.15m. At the Daliang Tunel site, the rut was offset left-laterally by 2.77m measured by UAV, which is the largest co-seismic left-lateral displacement of this earthquake. Based on high-resolution remote sensing image interpretation, field investigation, InSAR inversion of focal mechanism, fault rupture model and small earthquake precision location, it is determined that the earthquake occurred at the deep intersection of the Tuolaishan Fault and Lenglongling Fault, and the main seismogenic structure is the western segment of Lenglongling Fault(strike 112°, dip 88°). The Tuolaishan Fault on its west side ruptured simultaneously at the east end. According to the distribution characteristics of the surface ruptures and the field investigation of this earthquake, we believe that the Lenglongling Fault continues to extend westward after passing through the Liuhuanggou No. 1 site until the Jingyangling site, and the NWW-striking Lenglongling Fault has a “Y”-shaped contact relationship with the EW-striking Tuolaishan Fault. The 1986 MS6.4 earthquake occurred at the northwestern end of the Lenglongling North Fault, which protrudes in an arc toward NE, and the 2016 MS6.4 earthquake occurred at the southeastern end of the fault. Affected by the left-lateral strike-slip movement of the Lenglongling Fault, the small block bounded by the Lenglongling Fault and the Lenglongling North Fault also moves in the direction of SEE relative to the northern block. Therefore, the 1986 MS6.4 earthquake showed tensile properties, and the 2016 MS6.4 earthquake showed compression properties. The seismogenic structure of the Menyuan MS6.9 earthquake is the Lenglongling Fault, so the earthquake is mainly characterized by left-lateral strike-slip. The MS6.4 earthquake in 1986, MS6.4 earthquake in 2016 and MS6.9 earthquake in 2022all occurred in the western section of Lenglongling Fault. Three strong earthquakes of M>6 occurred in a short period of time, indicating that this area is still an accumulation area of stress and deformation, and has the potential risk of large earthquakes. Due to the limitation of the data range of the Gaofen-7 satellite image and the inconvenience of traffic caused by the icing of the river, the location of the easternmost end point of the rupture and the exact length of the rupture have not been determined in this field investigation. We hope that follow-up studies will be carried out to confirm the rupture length when weather conditions are appropriate. Table and Figures | Reference | Related Articles | Metrics

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GEOMAGNETIC FIELD CHANGE BEFORE AND AFTER 2021 YANGBI MS6.4 EARTHQUAKE SONG Cheng-ke, CHEN Zheng-yu, ZHOU Si-yuan, XU Yu-jian, CHEN Bin SEISMOLOGY AND EGOLOGY    2021, 43 (4): 958-971.   DOI: 10.3969/j.issn.0253-4967.2021.04.014 Abstract （904）   HTML （21）    PDF（pc） （3460KB）（216）       Knowledge map    Save On May 21, 2021, at 21:48(Beijing time), an earthquake with magnitude MS6.4 occurred in Yangbi County of Yunnan Province. In order to obtain the geomagnetic field change before and after the MS6.4 earthquake, the geomagnetic repeat measurement was conducted on geomagnetic repeat stations surrounding the epicenter. In this paper, total intensity of geomagnetic field obtained on eight geomagnetic repeat stations is used to calculate the geomagnetic field change. The measurement period before and after earthquake are April 1 to May 10 and May 23 to May 31 respectively. The continuous data recorded on Lijiang geomagnetic observatory from April 1 to May 31 is used to correct the diurnal variation. The geomagnetic repeat data measured in different time is corrected to midnight(00~03 Beijing Time)of May 6 because the external disturbance was quiet then. Finally, the geomagnetic field changes before and after the earthquake are obtained by calculating the difference of geomagnetic data before and after the earthquake. There are two factors contributing to the error. The mounting error is less than 0.2nT on all stations and the corrected error is less than 0.5nT on six stations and less than 1nT on two stations. The error analysis shows that the amplitude of geomagnetic field change is more than twice the error on five observation stations, which means that the geomagnetic field change is convincing on these five stations. The observed result reveals the geomagnetic field with positive and negative change in the south and north side of epicenter. The amplitude of geomagnetic field change is related to the epicenter distance. The change amplitude is -2.82nT at a geomagnetic station in an epicenter distance of 20km, and there is no geomagnetic change higher than the error observed at geomagnetic stations in an epicenter distance of 100km. The observed stable geomagnetic changes before and after earthquake are generally interpreted in terms of piezomagnetic effect in rocks, which are most probably generated by sudden stress change resulting from earthquake rupturing. In order to explain the geomagnetic field change before and after the MS6.4 earthquake, especially the change of -2.82nT, a piezomagnetic model is constructed based on uniform slip model with a slip of 0.5m on a rectangular fault. In the piezomagnetic model, the stress sensitivity is 5×10-3MPa-1 and magnetization is 1A/m. The result shows that piezomagnetic field of the station closest to the epicenter is -0.3nT. It infers that the piezomagnetic effect cannot explain the observed geomagnetic field change of Yangbi earthquake because we cannot expect the stress sensitivity or magnetization are ten times the value used in our piezomagnetic model. The electrokinetic effect which results from the fluid motion in the vicinity of fault because of the stress change can also produce the geomagnetic field change. An electrokinetic model with parameters same as the piezomagnetic model is constructed to calculate the geomagnetic change resulting from electrokinetic effect. The result shows that the electromagnetic field is smaller than the observed geomagnetic field by 1 order of magnitude in the cases without high conductivity (0.1S/m) and pore pressure change(10MPa). It infers that the electrokinetic effect cannot explain the observed geomagnetic field change of Yangbi earthquake. Other seismo-magnetic effects may contribute to the observed geomagnetic field change before and after earthquake, such as thermomagnetic effect which results from the thermal demagnetization or remagnetization of rocks surrounding the epicenter. We can conduct the quantitative analysis of thermomagnetic effect in the future when we know how the earthquake influences the temperature change. Table and Figures | Reference | Related Articles | Metrics

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PRELIMINARY INVESTIGATION AND RESEARCH ON SURFACE RUPTURE CHARACTERISTICS OF THE 2022 QINGHAI MENYUAN MS6.9 EARTHQUAKE GAI Hai-long, LI Zhi-min, YAO Sheng-hai, LI Xin SEISMOLOGY AND EGOLOGY    2022, 44 (1): 238-255.   DOI: 10.3969/j.issn.0253-4967.2022.01.015 Abstract （875）   HTML （34）    PDF（pc） （22330KB）（433）       Knowledge map    Save At 01:45 on January 8, 2022, Beijing Time, an MS6.9 earthquake occurred in Menyuan County, Haibei Prefecture, Qinghai Province, with a focal depth of 10km. The microscopic(instrument)epicenter is located at 37.77°N latitude and 101.26°E longitude in the intersection between the Toleshan fault zone and the Lenglongling fault zone in the northern Qilian-Qaidam block. The epicenter is 54km away from Menyuan County in Qinghai, 99km away from Qilian County, 100km away from Haiyan County, 83km away from Minle County in Gansu Province, 83km away from Yongchang County, and 141km away from Xining City. When the earthquake occurred, Menyuan County and Xining City, the capital of Qinghai Province, were strongly felt, and Yinchuan, Lanzhou, Xi'an and many other places were felt. At the same time, affected by the earthquake, the Lanxin high-speed rail line, an important railway transportation hub of the Belt and Road, was suspended. This earthquake is the largest earthquake in the world since 2022. It is also another earthquake of magnitude 6.0 or above in Qinghai Province following the Maduo MS7.4 earthquake on May 22, 2021. Besides, this earthquake is the event with the highest magnitude and the longest surface rupture in the region after the two M6.4 Menyuan earthquakes of August 26, 1986 and January 21, 2016. Therefore, this earthquake has attracted much attention from the society. The coseismic surface rupture distribution, combination characteristics, development properties and coseismic displacement of this earthquake were identified in time to help to have a correct understanding of the earthquake seismogenic structure, rupture process, and assessment of short-term earthquake hazards. It is also of great significance for major project route selection, earthquake fortification and rescue and disaster relief. On the basis of the on-site seismic geological investigation, based on the interpretation and analysis of high-resolution satellite remote sensing images, and combined with the low-altitude photogrammetry of unmanned aerial vehicles(DJI PHANTOM 4RTK), the author obtained the coseismic rupture data of five typical sites along the surface rupture zone generated by the earthquake. Using Agisoft Metashape Professional software to process the aerial photos of each section indoors, a high-resolution orthophoto map(DOM)was generated. At the same time, the five typical earthquake surface rupture sections were described in detail in ArcGIS Pro software based on the orthophoto map. Preliminary research shows that the surface rupture zone of the Menyuan MS6.9 earthquake is more than 22km long and consists of the main rupture of the northern branch and the secondary rupture of the southern branch. The north branch main rupture zone is distributed in the middle-western segment of the Lenglongling Fault of central Haiyuan fault zone, with a length of more than 18km and an overall strike of 295°. The maximum co-seismic horizontal displacement is located in the middle of the rupture zone at Liuhuangou(37.799°N, 101.2607°E), which is about 3.1m and gradually decays towards both ends. The secondary rupture of the southern branch is distributed on the local segments of the eastern Toleshan Fault in the central-western Haiyuan fault zone, with a length of about 4km and a strike of 275°, constituting a secondary branch rupture zone arranged in a left-stepped en-echelon pattern to the western segment of the main rupture zone. There are en-echelon extensional stepovers between the two rupture zones of the north and south branches. The whole surface rupture zone is mainly composed of linear shear cracks, oblique tension cracks, tension-shear cracks, compressional bulges and other structural types. The coseismic surface rupture has the characteristic of typical left-lateral strike-slip motion with a thrust component, and the maximum vertical dislocation is 0.8m. Table and Figures | Reference | Related Articles | Metrics

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ZHANG Zhi-liang, LIU Jin-rui, ZHANG Hao-bo, ZHANG Zhong-bao, HA Guang-hao, MIN Wei, NIE Jun-sheng, REN Zhi-kun SEISMOLOGY AND EGOLOGY    2021, 43 (6): 1351-1367.   DOI: 10.3969/j.issn.0253-4967.2021.06.001 Abstract （872）   HTML （203）    PDF（pc） （3722KB）（357）       Knowledge map    Save As the key area of interaction between land and sea, continental shelf is important for the tectonic evolution of continent, sea-land change, sea level eustacy and climate change. Due to the limits of different methods, the understanding of the chronology and potential geological information of the sediments on the continental shelf is not enough. The South China Sea, as the largest marginal sea of the West Pacific, is not only one of the most active areas of marine sedimentation in the world, but also the typical region of the interaction between land and sea. As the main sedimentary area of the East Asia, the South China Sea has received increasing academic research attention. At present, the researches mostly focus on the deep-sea sediments because they are continuous and can record stable signals, even though the relative slow deposition and low resolution. Comparatively, the shallow continental shelf deposits with faster sedimentary rate and higher resolution can provide important geological materials for studying the high-resolution chronology and paleoenvironment. However, the sedimentary signals recorded by the continental shelf sediments are unstable and even missing due to the turbulence of the sedimentary environment of the continental shelf. There are relatively few studies on the continental shelf sediments of the South China Sea, especially the high-resolution chronology of cores, thus limiting the understanding of tectonic and climate evolution of the South China Sea. In order to better constrain the geological chronology of the Late Pleistocene continental shelf sediments in northern South China Sea, study the paleoenvironmental signals in the continental shelf sediments and discuss the driving mechanism of the climate changes in East Asia and provide the chronological framework for the study of marine active tectonics in the South China Sea, the comparison between magnetic susceptibility and Marine Oxygen Isotope based on microscopic paleonotological fossils and carbon isotopic age(14C)was studied on the Core DG in this paper. Additionally, the results of sediments color and pollens were used to study the paleoclimatic implications. The results of magnetic susceptibility suggest that the chronology of the sediments of Core DG can be constrained from MIS 1 to MIS 9, with the age of the bottom being about 300ka. The relative high and low values of magnetic susceptibility correspond to interglacial and glacial periods, respectively. This is consistent with the paleoclimatic signals evidenced by the changes of pollen and color parameters in the DG core sediments. Therefore, we suggest that the magnetic susceptibility of continental shelf sediments can be affected by the changes of climate. During glacial periods, the relative cold weather, shallow water and increased transportation distance of the sediments resulted in the enhanced oxidation and the formation of minerals with weak magnetic susceptibility(such as hematite), thus the magnetic susceptibility decreased and the redness increased in the sediments. However, during interglacial periods, the relative warm and wet climate, together with the decreased transportation distance of the sediments, led to the formation of minerals with strong magnetic susceptibility(such as magnetite), thus the magnetic susceptibility enhanced significantly and the redness decreased in the sediments. Therefore, the variations of the magnetic susceptibility in the continental shelf sediments in the northern part of the South China Sea can reflect the glacial-interglacial cycles in the East Asia since the late Pleistocene. In conclusion, as a relative dating method used in the unconsolidated sediments in the late Quaternary, the comparison between magnetic susceptibility and Marine Oxygen Isotope is applicative and reliable in constraining the chronology of the Late Pleistocene continental shelf sediments in northern South China Sea, thus providing a new reference for studying and correlating the continental shelf sediments, which can be used reasonably in the Quaternary chronology. Table and Figures | Reference | Related Articles | Metrics

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COSEISMIC DEFORMATION FIELD, SLIP DISTRIBUTION AND COULOMB STRESS DISTURBANCE OF THE 2021 MW7.3 MADUO EARTHQUAKE USING SENTINEL-1 INSAR OBSERVATIONS HUA Jun, ZHAO De-zheng, SHAN Xin-jian, QU Chun-yan, ZHANG Ying-feng, GONG Wen-yu, WANG Zhen-jie, LI Cheng-long, LI Yan-chuan, ZHAO Lei, CHEN Han, FAN Xiao-ran, WANG Shao-jun SEISMOLOGY AND GEOLOGY    2021, 43 (3): 677-691.   DOI: 10.3969/j.issn.0253-4967.2021.03.013 Abstract （851）   HTML    PDF（pc） （9842KB）（489）       Knowledge map    Save InSAR coseismic deformation fields caused by the Maduo MW7.3 earthquake occurring on May 22, 2021 were generated using the C-band Sentinel-1A/B SAR images with D-InSAR technology. The spatial characteristics, magnitude of coseismic deformation and segmentation of the seismogenic fault were analyzed. The surface rupture trace was depicted clearly by InSAR observations. In addition, the coseismic slip distribution inversion was carried out constrained by both ascending and descending InSAR deformation fields and relocated aftershocks to understand the characteristics of deep fault slip and geometry of the seismogenic fault. The regional stress disturbance was analyzed based on coseismic Coulomb stress change. The results show that the Maduo MW7.3 earthquake occurred on a secondary fault within the Bayan Har block which is almost parallel to the main fault trace of the Kunlun Fault. According to field investigation, geological data and InSAR surface rupture traces, the seismogenic fault is confirmed to be the Kunlunshankou-Jiangcuo Fault. The rupture length of seismogenic fault is estimated to be~210km. The NWW direction is followed by the overall displacement field, which indicates a left-lateral strike-slip movement of seismogenic fault. The maximum displacement is about 0.9m in LOS direction observed by both ascending and descending InSAR data. The inversion result denotes that the strike of the seismogenic fault is 276°and the dip angle is 80°. The maximum slip is about 6m and the average rake is 4°. The predicted moment magnitude is MW7.45, which is overall consistent with the result of GCMT. An obvious slip-concentrated area is located at the depth of 0~10km. The coseismic Coulomb stress change with the East Kunlun Fault as the receiver fault shows that the Maduo earthquake produced obvious stress increase near the eastern segment of the East Kunlun Fault. Thus the seismic risk increases based on the high interseismic strain rate along this segment, which should receive more attention. In addition, the coseismic Coulomb stress change with the Maduo-Gande Fault as the receiving fault indicates that the Maduo earthquake produced an obvious stress drop near the western part of the Maduo-Gande Fault, which indicates that the Maduo earthquake released the Coulomb stress of the Maduo-Gande Fault, and its seismic risk may be greatly reduced. However, there is a stress loading effect in the intersection area of the Maduo-Gande Fault and the Kunlunshankou-Jiangcuo Fault. Considering that aftershocks of Maduo earthquake will release excess energy, the greater earthquake risk may be reduced. Reference | Related Articles | Metrics

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THE RELATIONSHIP BETWEEN ACTIVITY OF JINSHA RIVER FAULT ZONE AND LARGE-SCALE LANDSLIDES: A CASE STUDY OF THE SECTION BETWEEN NARONG AND RONGXUE ALONG THE JINSHA RIVER CHANG Hao, CHANG Zu-feng, LIU Chang-wei SEISMOLOGY AND EGOLOGY    2021, 43 (6): 1435-1458.   DOI: 10.3969/j.issn.0253-4967.2021.06.006 Abstract （845）   HTML （24）    PDF（pc） （19495KB）（364）       Knowledge map    Save The relationship between large-scale landslides and active faults has attracted much attention. From the point of view of active tectonics and disaster geology, the late Quaternary activity of the Jinsha River fault zone is investigated and studied, and the relationship between large-scale landslides and activity of the Jinsha River fault zone is emphatically analyzed. The Jinsha River fault zone was formed during the closure of the Paleotethys Ocean. According to the distribution of the 5km-wide ophiolitic melange zone, the ultramafic rock zone, and the local migmatization and progressive metamorphism around the Variscan intermediate acid intrusive rock mass distributed along the fault, it is inferred that the fault zone was once a strongly active superlithospheric fault zone with obvious compressive properties. The Jinsha River fault zone is a large-scale, long-term active suture structure, with many branches, forming a 50km wide structural fracture zone. Affected by the eastward compression of the Tibet Plateau, it has changed into a strike-slip fault zone characterized by dextral shear since Pliocene. In the study area, the fault landforms are clear along the Zengdatong, Xulong, Nizhong, Lifu-riyu, Langzhong and Guxue faults, which are mainly manifested as straight fault trough, linear ridge, fault scarp, and directional aligned fault facets. Results of field geological and geomorphological investigation and chronology show that the late Pleistocene and Holocene deposits are faulted, indicating the faults are active during the late Quaternary and dominated by dextral strike-slip with an average horizontal slip rate of 3.5~4.3mm/a in Holocene. The study area is located in the middle and north of the world-famous Jinsha River suture of the north-south structural belt in Sichuan, Yunnan and Tibet, and the geological structural conditions are very complex. The main structural line is distributed in NS direction, interwoven with NE and NW faults and fold axes in network shape, and the structure is complex. Strong neotectonic movement, huge topographic elevation difference, steep mountains, dry-hot valleys microclimate and other factors have caused serious internal dynamic geological disasters on both banks of Jinsha River. The landslide in the area has the characteristics of high frequency, large scale and serious damage. There are 23 large-scale and super large-scale landslides in the main stream and its tributaries of Jinsha River within the 38km-long section from Narong to Rongxue. Most of them are super large-scale landslides with a volume of more than 10 million cubic meters, even have a volume of more than 100 million cubic meters. Most of the landslides are located within 1km on both sides of faults, and many of them are developed on the fault zone. The occurrence of these large-scale landslides is closely related to the long-term activity, evolution history and complex structure of Jinsha River fault zone along the river, as a result, the rock mass structure gets fragmented and the continuous tectonic activity becomes the main cause of landslides. Active faulting is the fundamental controlling factor for the occurrence of large landslides along the river, especially for large landslides, and is an important internal dynamic condition for the formation of landslides. Further analysis of the fault structure shows that landslide is closely related to the movement evolution history of Jinsha River fault zone. Special structural combination parts(mechanical mechanism)such as closely adjacent faults, acute angle area of fault intersection, right turning parts of the faults and the intersection area between the main faults and the transverse faults are the key sites where the tectonic stress is easy to concentrate, thus conducive to generating large-scale landslides. Many large landslides occur in these structural parts. The controlling effect of active faults on landslides is not only embodied in the process of large earthquakes, but also can lead to the intensive occurrence of large and super large landslides in a natural state(non seismic action). This research has positive scientific significance for understanding the formation mechanism and development law of landslides on both sides of Jinsha River, and for understanding the relationship between fault activities and large landslides. Table and Figures | Reference | Related Articles | Metrics

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SIMULATION OF STRONG GROUND MOTION FROM THE 2021 YANGBI, YUNNAN MS6.4 EARTHQUAKE HE Xin-juan, PAN Hua SEISMOLOGY AND EGOLOGY    2021, 43 (4): 920-935.   DOI: 10.3969/j.issn.0253-4967.2021.04.012 Abstract （832）   HTML （38）    PDF（pc） （3540KB）（220）       Knowledge map    Save In this study, we simulated the strong ground motion from the MS6.4 earthquake that occurred in Yangbi, Yunnan Province on May 21, 2021, with stochastic finite fault method. The peak ground acceleration(PGA)distribution within the range of (25.25°~26.15°N, 98.5°~100.8°E) of 920grid points was synthesized and the impact field of this earthquake ground motion was also obtained. According to the information of strong ground motion records released officially by Institute of Engineering Mechanics, China Earthquake Administration, it can be seen that stations are sparse in the area near the epicenter, therefore, we chose 4 strong motion stations(53YBX, 53YPX, 53BCJ, 53LKT)with horizontal peak ground acceleration(PGA)greater than 10cm/s2 in the range of 150km of epicentral distance to simulate the strong ground motions and obtained the synthetic acceleration time series and acceleration response spectrum(PSA)with a damping ratio of 5%for the four stations. Simultaneously, we compared the synthetic results with observed ones and found that the value of synthetic peak ground acceleration and the shape of accelerograms fit well with the records of stations 53YBX, 53YPX, and 53BCJ. Besides, the simulated acceleration response spectrums for the above three stations are consistent with the observed ones, and the average logarithmic error is between ±0.5, which suggests a good agreement for both high and low frequency. However, for station 53LKT, although the value of synthetic peak ground acceleration fits well with the observed one, the shape of acceleration time series has some differences with the observed, and the fitting degree is not as good as other three stations. Besides, there is a better agreement for high frequency than low frequency in the acceleration response spectrum, the simulated result is higher than the observed in low frequency for station 53LKT. The reason for this phenomenon is complex and may be associated with the site condition and so on, so further study is needed for the specific reasons. For the above mentioned four stations, the results show that there are some differences in duration between the synthetic acceleration time series and the recorded data. The cause for such differences may be described as follows: In the stochastic finite fault method, a time window is added to white Gaussian noise to control its shape and make sure that the windowed white Gaussian noise is similar to the real acceleration time series, and also, the path duration is expressed by a simplified theoretical duration, thus there are something different between the windowed white Gaussian noise and the real acceleration time series. Therefore, the simulation results cannot reflect the complex propagation process of seismic waves well. The result of PGA distribution shows a maximum peak ground acceleration of 875cm/s2 located near the epicenter. The value of simulated maximum peak ground acceleration is beyond the range of 186~372cm/s2, which is the peak ground acceleration range corresponding to intensity Ⅷ. The peak ground acceleration of 720.3cm/s2 recorded by station 53YBX is also beyond the range of 186~372cm/s2. To a large extent, the cause of this phenomenon may be related to the topography of the region of Yangbi County. About 98.4% of the area of Yangbi County is mountainous area. Consequently, mountain topography is the most widely distributed terrain in this county, and geological disasters are frequent. However, most buildings around the epicenter are not high, the intensity obtained from the damage degree could not reflect the value of peak ground acceleration clearly. Other earthquakes in Yunnan also have similar phenomena. For example, the Ludian MS6.5 earthquake that happened in Yunnan in 2014 had a maximum intensity of Ⅸ, but the actual peak acceleration recorded by strong motion station reached 949.1cm/s2, which is also beyond the peak ground acceleration range corresponding to intensity Ⅸ. The impact field of ground motion obtained in this study is approximately(25.25°~26.15°N, 99.3°~100.5°E), which is consistent with both the predicted ground motion influence area for Yunnan Yangbi MS6.4 earthquake from Institute of Geophysics, China Earthquake Administration and the area shown in the seismic intensity map issued by Yunnan Earthquake Agency for the Yangbi MS6.4 earhtquake, which suggest the effectiveness of the simulation results. Although the simulation results are consistent with the observed ones on the whole, there are still a few stations that have a little deviation. The main reason is that some parameters in the process of simulation are obtained by empirical formula and the site conditions are not well reflected, these are the aspects that need to be improved in the process of simulation. And it’s necessary to establish a more accurate model in order to realize accurate simulation and forecast the strong ground motions in the future. Using stochastic finite fault method to simulate ground motion can fill the gap of strong motion records to some extent and the synthetic ground motion results of this study can provide a scientific basis for post-earthquake relief, post-disaster reconstruction, and seismic design in this area. Table and Figures | Reference | Related Articles | Metrics

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THE DEEP STRUCTURAL CHARACTERISTICS AND THE SEIS-MOGENIC STRUCTURE OF THE YANGBI EARTHQUAKE REGION AND ITS SURROUNDING AREAS WU Gui-ju, YU Bing-fei, HAO Hong-tao, HU Min-zhang, TAN Hong-bo SEISMOLOGY AND EGOLOGY    2021, 43 (4): 739-756.   DOI: 10.3969/j.issn.0253-4967.2021.04.001 Abstract （831）   HTML （236）    PDF（pc） （9559KB）（329）       Knowledge map    Save Three earthquakes occurred in Yangbi County, Dali, Yunnan Province with the maximum magnitude M6.4, on May 21, 2021, and caused huge economic losses and human casualties. In this paper, the existing high-precision gravity data, mobile gravity survey data and EGM2008 model data were fused into high-precision grid data with 2.5km point distance to clarify the seismogenic structure and seismogenic environment of Yangbi earthquake. With the Yangbi earthquake as the center location, two long gravity profiles and 10 short gravity profiles are extracted, and the three-dimensional crustal imaging characteristics in the study area are obtained by the normalized full gradient imaging method, and the deep and shallow contact relationship and deep seismogenic environment along the northern section of Honghe fault zone, Weixi-Weishan Fault, Yongsheng-Binchuan Fault, Eryuan-Heqing Fault in the Yangbi earthquake area are analyzed. In this paper, the vertical and transverse characteristics of the upper crustal structure of the northern section of Red River Fault in Yangbi and its surrounding areas along the gravity profiles were obtained, the deep structural differences of the southern Yunnan block, Sichuan-Yunnan block and large faults were revealed, and the seismogenic structure and environment of the three Yangbi earthquakes were analyzed and discussed. The results of the study are as follows: (1)The sudden change zone of dip angle and dip direction of the normalized gravity gradient is in good agreement with the medium and large geological faults, such as Nujiang Fault, Lancangjiang Fault, Red River Fault, Anninghe Fault, and Zemuhe Fault, etc. (2)When the continuity of normalized gravity gradient of the middle and lower crust is good, and the middle and upper crust is in the high-low transition zone, earthquakes greater than M6.0 will occur frequently, especially in the intersection area of Weixi-Weishan Fault, Yongsheng-Binchuan Fault and the northern section of Red River Fault. (3)Near the epicenter of Yangbi earthquake, there is a strong deformation belt of high and low normalized gravity gradients in the upper crust converging at a depth of about 15km, and the epicenter projection intersected with the Weixi-Weishan Fault and the secondary fault at a depth of about 10km, the continuity of normalized gravity gradient values is very well below the depth of 20km in the crust, it is inferred that the seismogenic structure of the three earthquakes in Yangbi are the Weixi-Weishan Fault and its secondary fault. (4)Earthquakes of M6.0 or higher normally occur where the geological strata connect and are relatively young. Strong earthquakes occurred at the junction of the Triassic and Permian in the east of Dali. At the same time, analyzing the distribution characteristics of the normalized gravity gradient value(Gh)can provide a reference for the division and correction of stratigraphic boundaries. (5)In the deformation process of geological structure, when the high-low gradient deformation zones of Gh value are formed in the middle and upper crust, whilst Gh values have good continuity in the middle and lower crust, earthquakes of M6.0 or higher normally occur. These features can be used as an important marker to judge the preparation and occurrence of strong earthquakes. Based on the geological and geophysical characteristics and the distribution characteristics of M≥6.0 earthquakes, the relationship between the change of Gh values and the occurrence of moderate and strong earthquakes, the stratigraphic boundary, the strike and dip angle of structural faults in the study area were analyzed, and the seismogenic structure and environment of the three Yangbi earthquakes on May 21 in 2021 were discussed. This study can provide a scientific basis and important reference value for determining the seismogenic mechanism and location of moderate-strong earthquakes. Table and Figures | Reference | Related Articles | Metrics

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ANALYSIS ON THE SEISMIC CHARACTERISTIC DISPLACEMENT OF ANQIU-JUXIAN FAULT BASED ON DEXTRAL HORIZONTAL DISLOCATION OF GULLY JI Hao-min, LI An, ZHANG Shi-min SEISMOLOGY AND GEOLOGY    2021, 43 (3): 471-487.   DOI: 10.3969/j.issn.0253-4967.2021.03.001 Abstract （828）   HTML    PDF（pc） （11209KB）（451）       Knowledge map    Save The Tanlu fault zone(TLFZ)is the largest strike-slip fault system in eastern China, which is composed of five main faults in Shandong and Jiangsu Provinces. Among them, the Anqiu-Juxian Fault(AJF)is the only fault with obvious activity since the late Quaternary, and it is also the seismogenic structure of the Anqiu M7 earthquake in 70BC. It is of great significance to understand the tectonic activity of the TLFZ by analyzing the co-seismic displacement of this earthquake and studying the long-term activity behavior of the fault. According to the spatial distribution characteristics and seismic activity, the northern segment of the AJF between Juxian and Changyi(NAJF)is divided into four sub-segments, which are, from south to north, the Juxian-Mengyan segment, the Qingfengling segment, the Anqiu-Mengtong segment and the Changyi-Nanliu segment, respectively. However, paleoearthquake studies in the NAJF are not ideal, and only suggested that this segment was active in the Holocene. In addition, there is also no competent evidence of coseismic displacement in the previous researches. In this study, we interpreted the geomorphic trace of the fault through remote sensing images and found that there were a large number of gullies where dextral horizontal dislocations are discovered, which are concentrated in the Anqiu-Mengtong segment and Qingfengling segment. Later, we used the high-resolution UAV-SfM photogrammetry technology to map the typical geomorphic areas from Anqiu to Juxian in the field investigation, and obtained the DEM of areas with offset gullies. Then we measured the offsets of the gullies by the measurement software, LaDiCao_v2, and acquired 79 horizontal dislocations. Combined with 5 measurement results from the previous research, we finally obtained 84 horizontal dislocations, including 26 data in the Anqiu-Mengtong segment and 58 in the Qingfengling segment. According to the statistical results of the cumulative offset probability distribution(COPD), the horizontal displacements in the Anqiu-Mengtong segment mainly concentrated in 5 intervals with the peak values of 5m, 10.4m, 15.5m, 20.6m and 25m, respectively; the horizontal displacements in the Qingfengling segment mainly concentrated in 4 intervals with the peak values of 5m, 9.7m, 16m and 19.7m, respectively. The bigger data is of less statistical significance due to large time span and small amount. The smallest dextral horizontal displacements of gullies on these two segments are both about 5m, and the larger offsets are also multiples of 5m. In addition, as the increase of the interval peak value, the number of gullies in the interval decreases. Therefore, the minimum dislocation of 5m should represent the latest activity event of these two secondary faults and be the coseismic displacement of the earthquake; the large dislocations represent the cumulative displacements of multiple seismic events, which reveal the characteristic displacement of about 5m for the two secondary faults. However, due to the unclear paleoearthquake sequence, it is also unclear whether these sub-segments were active at the same time. In addition, based on the statistical analysis on the strike-slip seismic events, there are a series of empirical formulas among the coseismic displacement, magnitude, and surface rupture length about the strike-slip faults. We used the coseismic displacement of 5m to infer the magnitude and surface rupture length of the Anqiu earthquake, and the results show that the earthquake magnitude mostly ranges from 7.5 to 7.7 and the surface rupture length is about 100km. According to previous historical records, when the 70BC Anqiu earthquake struck, the quake was felt strongly in the city of Xi 'an, hundreds of kilometers away. Therefore, combined with the calculation results and the fact that only the 70BC Anqiu earthquake was recorded in the NAJF, if the coseismic displacement of 5m was caused by the Anqiu earthquake, its magnitude may be undervalued, and the actual magnitude should be above 7.5. At the same time, the latest paleoearthquake event on Juxian-Mengyan segment is(2 140±190)a BP ago, close to the Anqiu earthquake in 70BC. Therefore, due to the calculation results of the surface rupture length of 100km, the Anqiu earthquake may have caused the cascade rupture of Anqiu-Mengtong, Qingfengling, and Juxian-Mengyan segments. Or the characteristic displacement of 5m indicates another paleoearthquake event, and the seismogenic fault of the 70BC Anqiu M7 earthquake is the Changyi-Nanliu segment, because there are more evidences of Holocene activity observed in this segment. However, since there has been no strong earthquake in this segment for more than 2 000a and various evidences have indicated that this segment has the ability of generating strong earthquake, high attention should be paid to the seismic risk in this area in the future. Reference | Related Articles | Metrics

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FEATURES OF ANCIENT LANDSLIDES AND THEIR SEISMIC-GEOLOGICAL SIGNIFICANCE ALONG THE SOUTHERN SEGMENT OF XIAOJIANG FAULT IN THE SOUTHEASTERN YUNNAN, CHINA GAO Fan, HAN Zhu-jun, YUAN Ren-mao, DONG Shao-peng, GUO Peng SEISMOLOGY AND EGOLOGY    2021, 43 (6): 1412-1434.   DOI: 10.3969/j.issn.0253-4967.2021.06.005 Abstract （814）   HTML （30）    PDF（pc） （11799KB）（212）       Knowledge map    Save Historical records with time information are useful for determining the time of earthquake events, while the investigation of historical damage phenomena such as earthquake-triggered landslides can help determine the magnitude of historical earthquakes by analyzing the correlation among historical earthquake-caused landslides, historical earthquakes and related active faults. A series of small basins were developed along the southern segment of the Xiaojiang Fault(XJF), with relatively flat and open topography and concentrated human activities. In most of the southern segment of the XJF, the terrain is relative flat, but some landslide accumulations are still clear, which are obviously different from the surrounding settings and are easy to be identified. Based on remote sensing interpretation and field investigations, landslides with different scales have developed in more than 10 locations along the southern segment of the XJF. Some of them are large with a volume of more than 1 million m3, and some are small with a volume of less than 100 000m3. They are the ancient landslides with a stable state. These landslides are mainly distributed in basins and their border areas with gentle terrain slopes. They are likely to be earthquake landslides rather than rainfall induced. The main scarp angles of these landslides are relatively concentrated, most of which are between 29~31 degrees, indicating that these landslides are caused by one geological event. We use light detection and ranging(LiDAR) measurement technology to obtain the digital elevation model(DEM)data of the landslide development section. The generated three-dimensional topographic shadow map presented in this paper suggests that there is a close relationship between these landslides and the latest surface ruptures of the southern segment of the XJF, indicating that these landslides should be triggered by the latest seismic event along the southern segment of XJF. The fault section was faulted in the latest earthquake events on the surface, triggering clusters of landslides. Based on the age test results of samples from the trench on the landslide body and historical literature data, the co-seismic landslides were triggered in 1606AD. According to the latest research results of the earthquake surface rupture zone in the southern segment of the XJF and empirical formula, combined with the comparative analysis on the intensity of geological disasters and the number of casualties of different earthquake cases, the authors re-assess the magnitude of the 1606 Jianshui earthquake and find that the magnitude of this historical earthquake could not be less than 7½(≥7.5). It means that the southern segment of the XJF, as a part of Xianshuihe-Xiaojiang fault(XSH-XJF) system, shows strong activity and has the ability to generate large earthquakes. GPS observations have verified that the crustal material on the southeastern margin of the Tibetan plateau rotates clockwise around the Eastern Himalaya Syntaxis(EHS), which requires a continuous left-lateral strike-slip fault system as the eastern boundary. The results presented in this paper are useful for deeper study of such an eastern boundary. Table and Figures | Reference | Related Articles | Metrics

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SEDIMENTARY PROCESS AND TECTONIC SIGNIFICANCE OF WUWEI BASIN DURING THE QUATERNARY HE Xiang, DU Xing-xing, LIU Jian, LI Yi-hao, LI Qun SEISMOLOGY AND GEOLOGY    2022, 44 (1): 76-97.   DOI: 10.3969/j.issn.0253-4967.2022.01.006 Abstract （810）   HTML （18）    PDF（pc） （13184KB）（298）       Knowledge map    Save The Wuwei Basin is located in the eastern part of the Hexi Corridor Basin group. It belongs to the frontier of uplift and expansion of the northeastern margin of the Qinghai-Tibet Plateau. The Quaternary sedimentary strata in the Wuwei Basin are hundreds of meters thick, which records the geological information of uplift and expansion of the northeastern margin of the Qinghai-Tibet Plateau during the Quaternary period. In order to study the sedimentary process of the Wuwei Basin since Quaternary and the characteristics of the uplift and extension of the northeastern margin of the Qinghai-Tibet Plateau, the sedimentary stratigraphy and stratigraphic chronology methods are applied to analyze the Quaternary sedimentary strata in the Wuwei Basin. Firstly, the Quaternary sedimentary sequence of the Wuwei Basin is established by studying the stratum characteristics and OSL ages of the Quaternary sedimentary strata. Secondly, through the analysis of the paleocurrent direction and zircon U-Pb isotopic age of the sedimentary gravel, the paleocurrent direction of different deposition periods and sediment source are restored. Finally, based on the above analysis, the sedimentary process of Quaternary strata in the Wuwei Basin and its response to the uplift and extension of the Qinghai-Tibet Plateau are discussed. The Quaternary sedimentary strata in the Wuwei Basin are the lower Pleistocene Yumen conglomerate, the middle Pleistocene Jiuquan gravel bed and the Upper Pleistocene-Holocene Gobi gravel bed from bottom to top. The Gobi gravel bed can be subdivided into the Upper Pleistocene gravel bed, the Upper Pleistocene-Holocene loess layer and the Holocene gravel bed. In the early Pleistocene, the Yumen conglomerate’s source material is mainly Mesozoic and Paleozoic rocks. The main provenance area of the Yumen conglomerate is located in the Qilian Mountains south to the Wuwei Basin. The main sedimentary area of the Yumen conglomerate is located in the Zoulang Nan Shan located in the southern part of the Wuwei Basin and the southern part of the northern fault basin. In the middle Pleistocene, the Jiuquan gravel bed’s source material is mainly Cenozoic and Mesozoic rocks. In the early sedimentary stage of the Jiuquan gravel bed, the main provenance area is located in the Zoulang Nan Shan and the main sedimentary area is located in the northern part of Wuwei Basin. In the late sedimentary stage, the main provenance area of the Jiuquan gravel bed is located in the Zoulang Nan Shan and the Fenmen Mountain located in the northwestern margin of the Wuwei Basin, and the main sedimentary area is located in most of the northern fault basin and the surrounding area of the Fenmen Mountain. Since late Pleistocene, the Gobi gravel bed’s source material is mainly early Paleozoic rocks. The main provenance area of the Gobi gravel bed is located in the Zoulang Nan Shan, the Fenmen Mountain and the Longshou Mountain, and the main sedimentary area is located around the source area. The uplift boundary of the northeastern margin of the Qinghai-Tibet Plateau continued to expand towards the northeast direction. The uplift boundary was located in the Qilian Mountains south to the Wuwei Basin in the early Pleistocene. It extended northward to the Zoulang Nan Shan and the Fenmen Mountain in the middle Pleistocene, and reached the Longshou Mountain north to the Wuwei Basin in the late Pleistocene. The main provenance and sedimentary areas of the Quaternary sediments in the Wuwei Basin show the migration characteristics from south to north, which indicates the uplift and expansion of the northeastern margin of the Qinghai-Tibet Plateau. The uplift time was early in the south and late in the north, and the uplift intensity was strong in the south and west and weak in the north and east. Table and Figures | Reference | Related Articles | Metrics

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HOLOCENE ACTIVITY AND PALEOEARTHQUAKES OF THE WEIXI-QIAOHOU FAULT CHANG Zu-feng, CHANG Hao, LI Jian-lin, MAO Ze-bin, ZANG Yang SEISMOLOGY AND EGOLOGY    2021, 43 (4): 881-898.   DOI: 10.3969/j.issn.0253-4967.2021.04.009 Abstract （795）   HTML （45）    PDF（pc） （18555KB）（476）       Knowledge map    Save The Weixi-Qiaohou Fault is located in the west boundary of Sichuan-Yunnan rhombic block, and also the north extension segment of active Red River fault zone. Strengthening the research on the late Quaternary activity of Weixi-Qiaohou Fault is of great theoretical and practical significance for further understanding the seismogeological background in northwest Yunnan and the structural deformation mechanism of the boundary of Sichuan-Yunnan block. Based on the 1︰50 000 active fault mapping and the research results of the National Natural Science Fund project, this paper mainly elaborates the latest active times of the fault and paleoseismic events along it revealed by exploration trenches at Matoushui, Shiyan, and Yushichang. Matoushui trench revealed three faults developed in late Pleistocene and Holocene pluvial fan accumulation, and the latest ages of faulted strata are(638±40)a BP and(1 335±23)a BP, respectively. The Shiyan trench revealed six faults, three in the western section and three in the eastern section. The three faults in the western section dislocated the late Pleistocene and Holocene accumulation, and the 14C ages of the latest faulted strata are(4 383±60)a BP, (4 337±52)a BP and(4 274±70)a BP, respectively; the other three faults revealed in the eastern part of the trench offset the Holocene fluvial facies accumulation, the 14C age of the latest faulted strata in the footwall of the main fault is(9 049±30)a BP, and the 14C ages of two sets of faulted sag pond deposits in the hanging wall are(1 473±41)a BP and(133±79)a BP, separately. Five active faults are revealed in Yushichang trench. Among them, the F1 and F2 dislocated the gray-white gravelly clay layer and the black peat soil layer. The 14C age of the gray-white gravelly clay layer is(1 490±30)a BP, and 14C ages of the upper and lower part of the black peat soil layer are(1 390±30)a BP and(1 190±30)a BP, respectively. The F3 and F4 faults offset the gray-white gravelly clay layer, the black peat soil layer and the brown yellow sand bearing clay, and the OSL age of brown yellow sand bearing clay is(0.6±0.2)ka. The F5 fault dislocated the gray-white gravelly clay layer, its 14C age is(1 490±30)a BP. According to the relationship between strata and the analysis of dating data, the Yushichang trench revealed two seismic events, the first one occurred at(1 490±30)~(1 390±30)a BP, as typified by the faulting of F5, the second paleoseismic event is represented by the faulting of F1, F2, F3 and F4.The F1 and F2 faulted the gray-white gravelly clay layer and the black peat soil. Fault F3 and F4 dislocated the gravelly clay, the peat soil and the sandy clay, and a seismic wedge is developed between fault F3 and F4, which is filled with the brownish yellow sandy clay. The OSL dating result of the brownish yellow sandy clay layer is(0.6±0.2)ka. Judging from the contact relationship between strata and faults, F3 and F4may also faulted the upper brownish yellow sandy clay layer, but the layer was eroded due to later denudation. Therefore, fault F1, F2, F3 and F4 represent the second event. Combined with the analysis of fault scarps with a height of 2~2.5m and clear valley landform in the slope near the fault, it is estimated that the time of the second paleoearthquake event is about 600 years ago, and the magnitude could reach 7. The trench at Gaichang reveals that the seismic wedge, soft sedimentary structure deformation and the medium fine sand uplift(sand vein)and other ancient seismic phenomena are well developed near the fault scarp. All these phenomena are just developed below the fault scarp. The vertical dislocation of the strata on both sides of the seismic wedge is 35cm, and 14C ages of the misinterpreted peat clay are(36 900±350)a BP and(28 330±160)a BP, respectively, so, the occurrence time of this earthquake event is estimated to be about 28 000a BP. If the fault scarp with a height of 2m was formed during this ancient earthquake, and considering the 0.35m vertical offset revealed by the trench, the magnitude of this ancient earthquake could reach 7.The Matoushui trench revealed three faults, which not only indicated the obvious activity of the faults in late Pleistocene to Holocene, but also revealed two paleoseismic events. Among them, the OSL age of the faulted sand layer by fault F1 is(21.54±1.33)ka, which represents a paleoearthquake event of 20 000 years ago. The faulted strata by fault F2 and F3 are similar, which represent another earthquake event. The 14C dating results show that the age of the latest faulted strata is(638±40)Cal a BP, accordingly, it is estimated that the second earthquake time is about 600 years ago. A clear and straight fault trough with a width of several ten meters and a length of 4km is developed from Meiciping to Matoushui. Within the fault trough, there are fault scarps with different heights and good continuity, the height of which is generally 3~5m, the lowest is 2~3m, and the highest is 8~10m. Tracing south along this line, the eastern margin of Yueliangping Basin shows a fault scarp about 5m high. After that, it extends to Luoguoqing, and again appears as a straight and clear fault scarp several meters high. In addition, in the 2km long foothills between Hongxing and Luoguoping, there are huge rolling stones with diameters of 2~5m scattered everywhere, the maximum diameter of which is about 10m, implying a huge earthquake collapse occurred here. According to the length, height, width and dislocation of the rupture zone, and combined with the experience of Yiliang M≥7 earthquake and Myanmar Dongxu M7.3 earthquake, this earthquake magnitude is considered to be ≥7. Table and Figures | Reference | Related Articles | Metrics

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STUDY ON FOCAL MECHANISM SOLUTION AND STRESS FIELD CHARACTERISTICS IN SICHUAN AND YUNNAN AREA ZHANG Zhi-wei, LONG Feng, ZHAO Xiao-yan, WANG Di SEISMOLOGY AND GEOLOGY    2022, 44 (1): 170-187.   DOI: 10.3969/j.issn.0253-4967.2022.01.011 Abstract （793）   HTML （27）    PDF（pc） （10401KB）（429）       Knowledge map    Save Based on the focal mechanism solutions of 2 600 ML≥3.0 earthquakes in Sichuan and Yunnan area from January 2000 to March 2017, the focal mechanism quantitative classification and stress field inversion are carried out for the sub blocks and fault zones with relatively dense focal mechanisms. Using the focal mechanism solutions of 727 ML≥4.0 earthquakes from January 1970 to March 2017, the regional stress tensor damping method is used to inverse the spatial distribution of principal compressive stress in Sichuan and Yunnan area before and after Wenchuan MS8.0 and Lushan MS7.0 earthquakes, and the temporal and spatial evolution characteristics of current stress field are discussed. The focal mechanisms are distributed mainly in Longmenshan fault zone, Xianshuihe-Anninghe-Zemuhe-Xiaojiang fault zone, Mabian-Yanjin fault zone, Lijiang-Xiaojinhe fault zone, the central Yunnan block, the west Yunnan block and the southwest Yunnan block in Sichuan and Yunnan area. The focal mechanism is mainly strike slip type in Sichuan and Yunnan area, but there are local differences. The Longmenshan fault zone is dominated by thrust type earthquakes, while in the Mabian-Yanjin fault zone, there are relatively more strike slip and thrust type earthquakes. The types of earthquakes in Sichuan Basin are complex, and there is no obvious dominant type. In general, the focal mechanisms of the Longmenshan fault zone and Sichuan Basin earthquakes are affected by strong earthquake and other factors, and the focal mechanism types have good inheritance in Sichuan and Yunnan area. The stress field in Sichuan and Yunnan area has obvious subarea characteristics, and it rotates clockwise from north to south. The compressive stress in Longmenshan fault zone and Sichuan Basin shows nearly EW direction. It shows NWW direction in the eastern boundary of Sichuan and Yunnan rhombic block and NNW direction in the inner part of rhombic, while it shows NNE direction in the western and southern Yunnan blocks. The principal compressive stress in Sichuan is more complex than that in Yunnan. The principal compressive stress direction in Sichuan experiences EW-NW-EW rotation from west to east, the dip angle is steep in the west and slow in the east, and the stress regime also experiences the transition from normal faulting to strike-slip to thrust. The principal compressive stress direction in Yunnan is NNE in the west and NNW in the east, forming an inverted “V” shape in space, the stress regime is mainly strike-slip and the dip angle is horizontal. Before and after the Wenchuan MS8.0 and Lushan MS7.0 strong earthquakes, the stress field in the Longmenshan fault zone changed greatly, followed by the Sichuan Basin and its surrounding areas, and there was no obvious change in other areas of Sichuan and Yunnan. The stress field in the Longmenshan fault zone experienced a complete transformation process from basic stress field to variable stress field to basic stress field. Table and Figures | Reference | Related Articles | Metrics

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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 Abstract （790）   HTML （22）    PDF（pc） （14676KB）（573）       Knowledge map    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. Table and Figures | Reference | Related Articles | Metrics

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THE 2022 M6.8 LUDING EARTHQUAKE: A COMPLICATED EVENT BY FAULTING OF THE MOXI SEGMENT OF THE XIANSHUIHE FAULT ZONE LI Chuan-you, SUN Kai, MA Jun, LI Jun-jie, LIANG Ming-jian, FANG Li-hua SEISMOLOGY AND GEOLOGY    2022, 44 (6): 1648-1666.   DOI: 10.3969/j.issn.0253-4967.2022.06.017 Abstract （788）   HTML （75）    PDF（pc） （16086KB）（345）       Knowledge map    Save The September 5, 2022, M6.8 Luding earthquake occurred along the southeastern segment of the Xianshuihe fault zone. Tectonics around the epicenter area is complicated and several faults had been recognized. Focal mechanisms of the main shock and inversions from earthquake data suggest that the earthquake occurred on a northwest-trending, steeply dipping strike-slip fault, which is consistent with the strike and slip of the Xianshuihe fault zone. We conducted a field investigation along the fault sections on both sides of the epicenter immediately after the earthquake. NW-trending fractures that were recognized as surface ruptures during the earthquake, and heavy landslides along the fault section between Ertaizi-Aiguocun village were observed during the field investigations. There are no surface ruptures developed along the fault sections north of the epicenter and south of Aiguocun village. Thus it can be concluded that there is a 15.5km-long surface rupture zone developed along the Moxi Fault(the section between Ertaizi and Aiguo village). The surface rupture zone trends northwest and shows a left-lateral strike slip, which is consistent with the strike and motion constrained by the focal mechanism. The coseismic displacements were measured to 20~30cm. Field observations, focal fault plane, distribution of the aftershocks, GNSS, and InSAR observation data suggest that the seismogenic structure associated with the M6.8 Luding earthquake is the Moxi Fault that belongs to the southeastern segment of the Xianshuihe fault zone. Slip along the segment south of the epicenter generated this earthquake, and also triggered slip along a northeast-trending fault and the northwestern section of the Moxi Fault in the epicenter. So, the M6.8 Luding earthquake is an event that is nucleated on the section south of the epicenter and then triggered an activity of the whole fault segment. Table and Figures | Reference | Related Articles | Metrics

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CHARACTERISTICS AND IMPLICATIONS OF SEISMIC ACTIVITY AROUND MINSHAN ACTIVE BLOCK IN EASTERN MARGIN OF QINGHAI-TIBET PLATEAU LI Jia-ni, HAN Zhu-jun, LUO Jia-hong, GUO Peng SEISMOLOGY AND EGOLOGY    2021, 43 (6): 1459-1484.   DOI: 10.3969/j.issn.0253-4967.2021.06.007 Abstract （787）   HTML （24）    PDF（pc） （10954KB）（299）       Knowledge map    Save Minshan active block is located in Bayan Har block of Qinghai-Tibet Plateau. It is bounded by the Huya Fault and Minjiang Fault on the east and west sides of the block. In less than 100 years, there have been four earthquakes with MS≥7.0 occurring along the eastern and western boundary faults, namely, the Diexi earthquake with M7.5 in 1933, two Songpan earthquakes with MS7.2 in 1976, the Jiuzhaigou earthquake with MS7.0 in 2017, and several earthquakes with M6.0~6.9. Such intensity and frequency of seismicity on either side of a relatively small intraplate active block is rare. Because the landforms along the active fault are mostly relatively gentle valleys with dense population and there is large terrain difference between the two sides of the valleys, each of the major earthquakes and the large-scale landslides it triggered were liable to cause serious casualties and property losses. Therefore, how does the destructive seismic activity around the active block migrate in space, and is it closely related to the segmentation and coalescence of active faults?And what are the temporal development characteristics of major earthquake activities and earthquake sequences?The discussion of these questions will not only deepen our understanding of the location and time of future destructive earthquakes, but also promote the development of the hypothesis of active block theory. Compared with the Bayan Har block, the Minshan active block located in the eastern margin of the Qinghai-Tibet Plateau provides a unique experimental field for studying the temporal and spatial regularity of earthquake occurrence in the active block. In this paper, 39 076 small earthquakes in Minshan active block and its adjacent areas from 2000 to 2019 were relocated using the double-difference location method, and 48, 110 seismic events in the study area were obtained by combining the earthquake catalogues recorded by instruments in the same area from 1972 to 1999. For the major earthquakes since the 1933 Diexi M7.5 earthquake, a thorough analysis was made on the spatial distribution characteristics of earthquake sequences in different periods, especially on the basis of formation of small earthquake bands, and the results show that: Since the Diexi M7.5 earthquake in 1933, the four M≥7.0 earthquake sequences are all distributed along the boundary zone of Minshan active block in space, indicating that the active block plays a controlling role in the process of large earthquake preparation. In terms of the determination of seismogenic structure, the strike of the seismogenic fault of the 1976 Songpan MS7.2 earthquake is basically the same with that of the 2017 Jiuzhaigou MS7.0 earthquake, but differs by 60°~70° with that of the 1976 Pingwu MS7.2 earthquake. So, it is more reasonable that the seismogenic faults of these three major earthquakes belong to two earthquake rupture segments, among them, the seismogenic fault of Jiuzhaigou MS7.0 earthquake in 2017 and Songpan MS7.2 earthquake in 1976 is the NW-trending Shuzheng Fault, and that of the 1976 Pingwu MS7.2 earthquake is the north segment of the Huya Fault. From the perspective of seismicity, the seismogenic fault of the 1933 Diexi earthquake should be the southern segment of Minjiang Fault. The 2017 Jiuzhaigou MS7.0 earthquake occurred in the gap between the 1976 Songpan MS7.2 earthquake and the Minjiang Fault. There are probably two seismic hazard areas around Minshan active block, which are located in the southern segment of Huya Fault and the middle segment of Minjiang Fault. The large earthquakes around Minshan block probably belong to foreshock-main shock-aftershock type. Therefore, from the perspective of earthquake prediction, it is suggested to strengthen monitoring of these two seismic gaps. Table and Figures | Reference | Related Articles | Metrics

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THE CHARACTERISTICS OF MAJOR FAULTS AND STRESS FIELD IN WEIHE-YUNCHENG BASIN CONSTRAINED BY SEISMIC ACTIVITY AND FOCAL MECHANISM SOLUTIONS YU Zhan-yang, SHEN Xu-zhang, LIANG Hao, ZHENG Wen-jun, LIU Xu-zhou SEISMOLOGY AND GEOLOGY    2022, 44 (2): 395-413.   DOI: 10.3969/j.issn.0253-4967.2022.02.008 Abstract （778）   HTML （18）    PDF（pc） （8440KB）（176）       Knowledge map    Save In this paper, the seismic phase bulletin of 14381 earthquakes from January 1, 2009 to June 30, 2018 in the Weihe-Yuncheng Basin and its adjacent region were selected and analyzed. After removing the records with incomplete event information and insufficient station information, 11856 seismic events remained. A basic requirement for the double difference location method is that the distance between the pairs of seismic events is much smaller than the distance between the events and the stations and the linear scale of the velocity inhomogeneous body on the wave propagation path, so that the travel time difference between two earthquakes and the same station is only determined by the relative position between the two seismic events and the velocity of the seismic wave. In this case, the error caused by insufficient understanding of crustal structure can be effectively reduced and the result of relocation can be more accurate. Due to the large area, the whole study region was divided into three smaller parts for relocation of the events in order to reduce the influences of local structures. 8106 seismic events recorded by 52 stations were relocated using the double-difference location algorithm. It is found that the results constrained by the grid searching method are basically consistent with those obtained by other methods. The reliability of focal mechanism is affected by the number of initial motion and the azimuth distribution of the station. Therefore, when inversion of focal mechanism solution is carried out, earthquakes with more than 10 clear initial motion phases are selected, and the maximum azimuth gap between two stations with clear initial motion is required to be less than 90°. The azimuth coverage of the initial motion on the source sphere was measured according to azimuth and take-off angle distributions, and the focal mechanism solutions with poor coverage were eliminated. The contradiction ratio of focal mechanism solutions is less than 0.2. The average difference of b-axis of the best fitting solutions is less than 20°. Finally, the focal mechanism solutions of 346 seismic events with ML≥2 were determined with initial motion of P and S waves. Normal type and strike-slip type earthquakes are widely distributed, accounting for more than 60% of all seismic events, and most of them are concentrated near fault zones. Before the formal inversion, the study area was divided into 1°×1° grids, and a series of damping coefficients were set to obtain the trade-off curve between the residual error of data fitting and the length of the stress field inversion model. The crustal stress field of 1°×1° grid in Weihe-Yuncheng Basin was obtained based on focal mechanism solution and stress tensor damping inversion method, and a certain number of depth profiles vertical to the faults were constructed for the analysis. The results show that compared with the original locations of seismic phase bulletin, the distribution of seismic events after relocation is more concentrated along the fault strike in plane. Vertically, they are densely distributed along the fault plane. There are more earthquakes in and around Shanxi graben, but the magnitude is generally small. The seismic activity in Weihe rift is relatively weak. Before the relocation, the focal depth distribution was concentrated in 5~10km, but after the relocation, the focal depth distribution changed significantly. The earthquakes were concentrated in the range of 10~25km, the overall focal depth was concentrated in the range of 20km, and a small number of earthquakes occurred in the range of 25~35km. The focal depth in the basin is relatively shallow with depth range of 5~15km. The focal depth at both ends of the basin tends to deepen, and the deepest depth can reach about 30km, which is consistent with the results of previous studies. The results of the depth profiles show that most of the fault planes in the study area have a large dip angle, similar to the occurrence of the surface, and some fault planes are even nearly vertical. The motion properties of fault structure and focal mechanism indicate that the faults in the study area are mainly normal and strike-slip ones. The results of stress field inversion indicate that the R values, which indicate the stress state, of the other regions are all less than 0.5 except for some areas in the southeastern margin of the research area. The stress state of Weihe-Yuncheng Basin tends to be tensile, and the maximum horizontal principal stress direction is nearly EW in Weihe rift and NNE and NEE in southern Shanxi rift, which is basically consistent with previous studies. Table and Figures | Reference | Related Articles | Metrics

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RESEARCH ON IDENTIFICATION OF SEISMIC EVENTS BASED ON DEEP LEARNING: TAKING THE RECORDS OF SHANDONG SEISMIC NETWORK AS AN EXAMPLE ZHOU Shao-hui, JIANG Hai-kun, LI Jian, QU Jun-hao, ZHENG Chen-chen, LI Ya-jun, ZHANG Zhi-hui, GUO Zong-bin SEISMOLOGY AND GEOLOGY    2021, 43 (3): 663-676.   DOI: 10.3969/j.issn.0253-4967.2021.03.012 Abstract （766）   HTML    PDF（pc） （3002KB）（379）       Knowledge map    Save In order to realize the rapid and efficient identification of earthquakes, blasting and collapse events, this paper applies the Convolutional Neural Network(CNN)in deep learning technology to design a deep learning training module based on single station waveform recording of single event and a real-time test module based on multiple stations waveform recording of single event. On the basis of ensuring that the data is comprehensive, objective and original, the three-component waveforms of the first five stations that recorded the P-wave arrival time of each event are input, and the current mainstream convolutional neural network structures are used for learning test. The four main convolutional neural network structures of AlexNet, VGG16, VGG19 and GoogLeNet are used for learning training, and the learning effects of different network structures are compared and analyzed. The results show that in the training process of various convolutional neural network structures, the accuracy rate and the cost function curve of the training set and the test set of each network are basically the same. The accuracy rate increases gradually with the increase of the training times and exceeds 90%, and finally stabilizes around a certain value. The cost function curve decreases rapidly with the increase of the training times, and eventually the stability does not change near a relatively small value. At the same time, over-fitting occurred in all convolutional neural network structures during training, except for AlexNet. In the end, the cost function of each type of structural training set and test set is finally lower than 0.194, and the recognition accuracy of each type of structure for training sets and test sets is over 93%. Among them, the recognition accuracy of AlexNet network structure is the highest, the accuracy of the training set of AlexNet network structure is as high as 100%, the test set is 98.51%, and no overfitting occurred; the accuracy of VGG16 and VGG19 network structure comes second, and the recognition accuracy of GoogLeNet network structure is relatively low, and the trend curves of the accuracy and cost function in training and test set of each network in the training process are basically the same. Subsequently, in order to test the event discrimination efficiency of the CNN in deep learning in the real-time operation of the digital seismic network, we select the trained AlexNet convolutional neural network to perform event type determination test based on the waveform recording of multiple stations of a single event. The final result shows that the types of a total of 89 events are accurately identified in the 110 events with M ≥0.7 recorded by Shandong seismic network, and the accuracy rate is about 80.9%. Among them, the accuracy rate of natural earthquake is about 74.6%, that of explosion is about 90.9%, and that of collapse is 100%. The recognition accuracy of collapse and explosion events is relatively high, and it basically reaches or exceeds the recognition accuracy of manual determination in the daily work of the seismic network. The accuracy of natural earthquake identification is relatively low. Among the 18 misidentified natural earthquakes, up to 13 events were judged as blasting or difficult to identify due to distortion of waveforms recorded by some stations(They are determined to be explosion and earthquake each by the records of two of the five stations). If sloughing off the recognition type error events caused by waveform distortion due to the background noise interference that overwhelms the real event waveform or waveform drift, the recognition accuracy of earthquake will become 91.4%, and the recognition accuracy of all events will increase from 80.9%to 91.7%, which is basically equivalent to the recognition accuracy of manual judgment in the daily work of the seismic network. This indicates that deep learning can quickly and efficiently realize the type identification of earthquake, blasting and collapse events. Reference | Related Articles | Metrics

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GEOMORPHIC ANALYSIS OF STRIKE-SLIP FAULTING AT THE TOP OF ALLUVIAL FAN: A CASE STUDY AT AHEBIEDOU RIVER ON THE EASTERN MARGIN OF TACHENG BASIN, XINJIANG, CHINA MIAO Shu-qing, HU Zong-kai, ZHANG Ling, YANG Hai-bo, YANG Xiao-ping SEISMOLOGY AND GEOLOGY    2021, 43 (3): 488-503.   DOI: 10.3969/j.issn.0253-4967.2021.03.002 Abstract （760）   HTML    PDF（pc） （11430KB）（315）       Knowledge map    Save The top of the piedmont alluvial fan has the characteristic of fan-shaped terrain and gradually descending terrain in the downstream direction. Faulting of various natures will result in different geomorphic features of alluvial fan surface. The variation of slope aspect and height of the pure sinistral fault scarp at the top of the alluvial fan is analyzed firstly under the three conditions, namely, the fault plane is vertical, the fault plane inclines toward the upper stream of the river, and the fault plane inclines toward the downstream of the river. We have also analyzed the variation of slope aspect and height of the fault scarps at the top of the alluvial fan under different fault inclination conditions of inverse sinistral strike-slip fault and the sinistral strike-slip normal fault. The seven geomorphic types we analyzed above cover the geomorphic features caused by the activity of strike-slip faults at the top of alluvial fans, which can help us to analyze the formation of the landforms. Based on drone-measured terrain data, Google satellite images and field investigations, we found that the Dongbielieke Fault, which strikes northeast-southwest and is located in the eastern margin of the Tacheng Basin, Xinjiang, almost vertically passes through the Ahebeidou River which develops from southeast to northwest. The direction of central axis of the alluvial fan at Ahebedou River is northwest, with a north-facing slope. The fault activity has caused the development of an uphill-facing scarp that has a height of~5.2m and a slope aspect facing southeast on the top of the alluvial fan at the Ahebiedou River section of the Dongbielieke Fault. And on the piedmont alluvial fan 1km away on both sides of the river bed, the sinistral fault scarps have a northwest-facing slope aspect and a height of 1~5m. The river terraces are divided into five levels, the T2 on the left bank, T4 on the right bank and T5 terraces on the left and right banks of Ahebeidou River were affected by fault activity. Sinistral offsets and southeast-facing fault scarps were developed on the geomorphic surface. By using DispCalc_Bathy_v2, a script based on Matlab, we get the offsets of the river terraces from the high-resolution DEM data obtained by using UAV photogrammetry technology. The sinistral horizontal offsets of T2 on the left bank, T4 on the right bank and T5 terraces on the left and right banks of Ahebeidou River are(10.1±0.2)m, (10.6±0.7)m, (29.1±0.2)m and(20.0±0.7)m, respectively. The vertical displacements are(1.5±0.1)m, (3.6±0.3)m, (4.7±0.2)m and(5.2±0.1)m, respectively. The asymmetrical development of terrains on both sides of the river is affected by topography and fault activity. The terraces on the lower elevation right bank of the river are misplaced into the channel by sinistral strike-slip faulting to receive more erosion, so the offsets we measured on the left bank of the river are more reliable than that on the right bank. Through field surveys, we found two fault outcrops, indicating that the fault plane is inclined to the southeast. The young river terrace T2 was effected by faulting and a uphill-facing scarp was developed, which indicates that the latest faulting was of sinistral strike-slip with a normal component, but the fault scarp's aspect changed twice within a short area of two kilometers, which is not consistent with the geomorphological type caused by the strike-slip faulting on the top of the alluvial fan as we previously analyzed. According to the landform features and the strike-slip fault geomorphic model, a model for the geomorphic surface development of the Ahebiedou River section is established. In this model, we think the Dongbielieke Fault was an inverse sinistral strike-slip fault after the formation of an older phase geomorphic surface S1 in the area. The early fault activity formed a northwest-facing fault scarp at S1, the height of the scarp is about 10m. Then the alluvial fan(Fan1)began to develop, and the material brought by the flowing water deposited and buried the fault scarp at the exit of piedmont, resulting in the disappearance of the existing fault scarp in the piedmont. Then the characteristic of fault changed into left-lateral strike-slip with a normal component. The activity of normal fault with the fault plane dipping to SE would form a fault scarp facing SE on the geomorphic surface. With the gradually cutting of the river, river terraces began to form on both sides of the river, and the corresponding geomorphic features were formed under the influence of fault activities. A fault scarp with a slope facing southeast formed at both banks of the river's mountain outlet with a height of about 5.2m through several fault activities, and sinistral horizontal offsets of river terraces increased at the same time. And the height of the pre-existing northwest-facing scarp 1~2km away from both banks of the river's mountain outlet decreased to about 5m, which can be observed in the field. The eventual geomorphic surface is characterized by the features of downhill-facing scarp-no scarp-uphill-facing scarp-no scarp-downhill-facing scarp from southeast to northeast. Reference | Related Articles | Metrics

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THE BASIC CHARACTERISTICS AND TYPICAL PHENOMENA OF THE SURFACE RUPTURE ZONE OF THE MADUO MS7.4 EARTHQUAKE IN QINGHAI YAO Sheng-hai, GAI Hai-long, YIN Xiang, LI Xin SEISMOLOGY AND EGOLOGY    2021, 43 (5): 1060-1072.   DOI: 10.3969/j.issn.0253-4967.2021.05.002 Abstract （747）   HTML （26）    PDF（pc） （13580KB）（297）       Knowledge map    Save At 02:04, May 22, 2021, an earthquake with MS7.4 occurred in Maduo County, Guoluo Tibetan Autonomous Prefecture, Qinghai Province. The epicenter of the earthquake is about 70km(34.59°N, 98.34°E)south of the east Kunlun fault zone on the northern boundary of the Bayan Har block, with a focal depth of 17km. The Maduo MS7.4 earthquake is the largest in China after the 2008 Wenchuan MS8.0 earthquake. As of 07:00 on June 12, 2021, 58 aftershocks of M≥3.0 had been recorded, including 0 earthquakes of M7.0~7.9, 0 earthquakes of M6.0~6.9, 1 earthquake of M5.0~5.9, 17 earthquakes of M4.0~4.9 and 40 earthquakes of M3.0~3.9. Field geological surveys after the earthquake showed that the earthquake occurred in the Yematan area, which is more than 30 kilometers south of the county seat of Machali Town. The seismic surface rupture shows obvious segmentation, which can be initially divided into 3~4 segments. The rupture spreads from east to west in a left step, gradually approaching the middle of the Yematan Basin. The nature of the fault is mainly left-lateral strike-slip. The earthquake produced a large-scale continuous surface rupture in the area from the west of National Highway 214 to the south of Eling Lake, with a length of about 45km and a strike of N95°~105°E. The surface rupture zone is composed of a series of compressional bulges and right-hand echelon fractures, forming large-scale seismic bulges(ridges), seismic fissures, left-lateral displacement and other geomorphic features, and producing the seismic geological disasters such as sand and water gushing, soft soil seismic subsidence and so on. From the east of National Highway 214 to the east of Xueluodong, the fracture zone strikes N100°E, which is composed of discontinuous, small-scale tension shear cracks and small-scale bulge(ridge). In the vicinity of Xuema village, Changmahe Township, a section of about 10km long, N75°E striking, large-scale tension shear fracture and seismic bulge(ridge) with good continuity is developed. The earthquake caused left-handed displacement of geological bodies, water system gullies, roads, etc. and formed strike-slip scratches in the strata. Through measurement, the horizontal displacement of this rupture is 1.5m in the Langmajiaheri area, 1.3m in the area of Yematanshangtou, and 1.1m west of Xuema Village. There is an obvious vertical displacement of 1.4~0.8m near Yematanshangtou, and the vertical displacement of other sections is not obvious. Generally speaking, the horizontal displacement is greater than the vertical displacement, and the rupture is dominated by strike-slip.Based on the field geological survey results, it is considered that the seismic rupture of this earthquake is large in scale and has a good continuity at its both ends, while the rupture scale is small and the continuity is poor in the middle. The preliminary inversion results of seismic rupture process, InSAR processing results and small earthquake precise positioning results show that the Maduo earthquake is a bilateral rupture with a rupture length of about 170km. The field geological investigation results are basically consistent with the geophysical inversion results. The Maduo MS7.4 earthquake(the instrument epicenter is located at 34.59°N, 98.34°E)occurred inside the Bayan Har block on the south side of the main Arak Lake-Tosuo Lake section of the east Kunlun fault zone. Existing data show that a number of nearly parallel NW-trending strike-slip faults are developed around the earthquake sequence. According to previous studies and this geological survey, the seismogenic structure of this earthquake is determined to be the Jiangcuo Fault. According to a comprehensive survey of the scale and length of the earthquake surface rupture and the damage to the buildings, it is believed that surface rupture zone in the Langmajiaheri area is large in scale with good continuity and multi types of surface ruptures. The area can be preliminarily determined as the macro-epicenter. The geographic coordinates of the macro-epicenter are 34.736°N, 97.794°E, which is nearly 50km away from the micro-epicenter. The difference is mainly due to the sparse seismic stations and weak monitoring capability in the area. The fact that the Maduo earthquake occurred inside the Bayan Har block on the south side of the east Kunlun main fault demonstrates the possibility of generating earthquakes with magnitude 7 or greater in the interior of this block. Therefore, the seismogenic conditions and mechanism of strong earthquake activity inside the Bayan Har block should be a scientific issue that needs more attention in the future. Table and Figures | Reference | Related Articles | Metrics

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DISCUSSION ON SEISMICITY CHARACTERISTICS OF WEIXI-QIAOHOU FAULT IN THE NORTHERN SECTION OF RED RIVER FAULT ZONE BASED ON THE DENSE SEISMIC ARRAY OBSERVATION WANG Zhi-wei, MA Sheng-li, LEI Xing-lin, WANG Kai-ying SEISMOLOGY AND EGOLOGY    2021, 43 (6): 1524-1536.   DOI: 10.3969/j.issn.0253-4967.2021.06.010 Abstract （746）   HTML （19）    PDF（pc） （5754KB）（235）       Knowledge map    Save Based on the seismic data from temporary stations and regional stations in the northwestern area of Yunnan, the paper performs high-resolution detection and high-precision location on continuous waveforms recorded from February 25, 2018 to July 31, 2019 using waveform correlation methods and analyzes the seismicity characteristics of the Weixi-Qiaohou Fault in the northern section of the Red River fault zone. Studies have shown that the Weixi-Qiaohou Fault exhibits weak seismic activity currently, except for some special fault locations(such as terraces, intersections, etc.), but there may be a hidden steep-dip right-lateral strike-slip fault along the west side of the fault. Small earthquakes are frequent along the fault. The distribution of seismic activity and focal mechanism solutions indicate that this fault is a right-lateral strike-slip fault with a steep dip. Statistical parameters, such as seismic frequency, energy release rate and b-value, indicate that the seismic activity in the Weixi-Qiaohou Fault and its surrounding areas is relatively stable, and the regional stress enhancement is not obvious. The b-value is relatively high in most areas, and low b-value areas are mainly distributed in some special fault locations(such as terraces, intersections, etc.), but the scale is generally small. The statistical results of the ETAS model show that more than 40% of seismic activity may be affected by external factors such as deep fluid disturbance and remote strong earthquake triggering. This shows that the role of external trigger mechanisms in seismic activity cannot be ignored. The external triggering seismic activity factors are related to the disturbance of deep fluid activity and the dynamic triggering of long-distance strong earthquakes. Therefore, we believe that the Weixi-Qiaohou Fault is currently not active, but on the hidden branch fault to its west, small earthquake activity is clustering and has a tendency to increase. So, when assessing the seismic risk of the fault, comprehensive analysis shall be made on the activity of the main fault and the branch fault to its west. Table and Figures | Reference | Related Articles | Metrics

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TEMPORAL AND SPATIAL EVOLUTION OF THE 2021 YANGBI (YUNNAN CHINA)MS6.4 EARTHQUAKE SEQUENCE WANG Kai-ying, JIN Ming-pei, HUANG Ya, DANG Wen-jie, LI Wen-tao, ZHUO Yan-qun, HE Chang-rong SEISMOLOGY AND EGOLOGY    2021, 43 (4): 1030-1039.   DOI: 10.3969/j.issn.0253-4967.2021.04.019 Abstract （745）   HTML （32）    PDF（pc） （2494KB）（257）       Knowledge map    Save In 2018, a short-period seismic network was set up in Eryuan area of Yunnan Province to carry out continuous field observation of the sub-instability process of the earthquake. The relevant data of the Yangbi MS6.4 earthquake sequence are mainly from the waveforms recorded by this network, combined with some other stations from Yunnan regional seismic network. The Yangbi earthquake sequence shows that the events in this area began to occur intensively on May 18. A total of 2 000 earthquakes with M>0.1 were recorded from May 18 to 23, including 770 foreshocks. Seismicity analysis shows that two clusters of foreshocks occurred successively in the adjacent area of the main earthquake in the northwest segment of the rupture strip within 3 days, then in the subsequent impending period(within 1 hour before the main shock)about 60 events spread symmetrically from the center of the fracture zone to the ends. The spatial distribution of foreshocks in different periods shows the spatial migration of local fractures and accelerated expansion prior to the main shock. The spreading speed is about 5km/d from foreshock clustering process to 96km/d in impending earthquake period. The epicenter of the main shock is located at the edge of the cluster foreshocks and the northwest end of the final rupture zone. Subsequent aftershocks extend southeastward to the whole fracture zone in about half an hour, and the final fracture zone is more than 20 kilometers long, showing unilateral propagation of the rupture. Since 2018, b-value in the Yangbi area has been stable(0.9~1.1)for the past three years. After March this year, the b-value abnormally decreased to 0.6 before the main shock, reflecting that there was a significant process of continuous increase of local stress before the Yangbi earthquake. The identification of short-term precursors and somehow definite information is one of the focus problems in earthquake prediction research. On the basis of the experimental results, Ma Jin proposed the theory of seismic meta-instability stage based on the characteristics of the load stress after the peak value from rock experiments and the corresponding change of related physical field, and considered that the degree of fault activity synergy was a sign to determine the stress state of the fault. When the fault activity changes from the expansion and increase of the stress releasing points in the early stage of meta-instability to the connection between the released segments at the late stage of meta-instability, that is, the quasi dynamic instability stage, the stress release on the fault will accelerate, and the acceleration mechanism is the strong interactions between the fault segments. In the context that the macroscopic stress state cannot be known directly, the original intention of the “meta-instability” test area is to try to capture the characteristic signal of the meta-instability stage described by the experimental phenomenon through the deformation and seismicity of the actual faults during the earthquake preparation process. It is clear that in this stage, the fault will continue to expand in the pre-slip zone theoretically, and it will enter into the quasi dynamic fracture expansion before the impending earthquake. This theory is obviously embodied in the foreshocks of this earthquake, forming the phenomenon of rapid migration of small earthquakes as mentioned above. From the current understanding of the meta-instability, it can be seen that the seismogenic fault is in the state of overall stress release at this stage, rather than the continuous increase of stress. Therefore, the decrease of b value before the earthquake shows that local faults have been activated and entered the final stage of nucleation process. The quasi dynamic spreading phenomenon before this kind of moderate-strong mainshock displayed by small earthquake activity can be identified as the precursor of a kind of earthquakes. Table and Figures | Reference | Related Articles | Metrics

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CHARACTERISTICS OF LATE QUATERNARY ACTIVITY OF THE SOUTHERN RIYUESHAN FAULT ZHANG Chi, LI Zhi-min, REN Zhi-kun, LIU Jin-rui, ZHANG Zhi-liang, WU Deng-yun SEISMOLOGY AND GEOLOGY    2022, 44 (1): 1-19.   DOI: 10.3969/j.issn.0253-4967.2022.01.001 Abstract （739）   HTML （110）    PDF（pc） （22131KB）（659）       Knowledge map    Save Due to the collision between the Indian plate and the Eurasian plate, the Tibetan plateau has experienced violent uplift and strong intraplate deformation inside the plateau, which has a great impact on the tectonic evolution of the surrounding areas. The northeastern edge of the Tibetan plateau is the forefront of the northeastward expansion of the Tibetan plateau, which is the ideal place to study the deformation of the plateau as well as the far-field deformation associated with continental collision between the Eurasia and India plates. In recent years, scholars have gained a certain understanding of the characteristics of late Quaternary tectonic activity in the northeast margin of Tibetan plateau. Within the northeastern margin of Tibetan plateau, there are two major fault systems: One is the near EW-trending left-lateral strike-slip fault system, including the Kunlun, Haiyuan and western Qinling faults, the other one is the NNW-trending right-lateral strike-slip fault system, including the Elashan and Riyueshan faults. They are sub-parallel to each other. Since the Riyueshan Fault is one of the major right-lateral strike-slip faults in the northeastern margin of Tibetan plateau, its activity is of great significance for understanding the plateau expansion. Previous studies mainly focused on its northern part which is believed to be active during Holocene. However, its southern part is believed to be active during late Pleistocene, but not active since Holocene. Therefore, there are little studies focusing on the late Quaternary activities of the southern part of the Riyueshan Fault. Hence, our understanding about the characteristics of the late Quaternary activity is insufficient. During our preliminary field survey along the southern Riyueshan Fault, we found distinct deformation of Holocene landforms, such as the young alluvial fan, terrace risers and channels, which indicate its late Quaternary activity. In this study, we firstly analyze the fault geometry of the southern Riyueshan Fault based on high-resolution Superview-1 remote sensing images and carry out field verification. Based on fault geometry characteristics, fault strike orientation etc., we divided the southern Riyueshan Fault into two segments from north to south. One is the Guide segment(generally trending in NW 20°)and the other is the Duohelmao segment(generally striking in NS). During our field investigation, we found two typical sites for slip rate studies, the Rixiaolongwa site on the Guide segment and the Niemari site on the Duohemao segment, respectively. We collected high-resolution images using UAV, and then generated high-resolution DEM of these two sites. By measuring the offsets and corresponding dating results of multi-level terrace risers, we obtained the displacements of the three-level and two-level terraces at Rixiaolongwa and Niemari site, respectively. Then we collected the OSL and 14C samples on different terrace risers to constrain the age of each terrace. In the Rixiaolongwa area, the corresponding offsets of T1, T2 and T3 terraces are(26.3±3.1)m, (32.7±7.1)m and(38.6±8)m, and the age sequence is(7840±30)a BP, (9 350~10 700)a BP and(11.9±1.3)ka BP, respectively. In the Nimari area, the corresponding offsets of T1 and T2 terraces are(6.3±0.7)m and(9.7±1.7)m, and the ages are(2 860±30)a BP and(3 460±30)a BP, respectively. By applying Monte Carlo method, we obtained the corresponding slip rates of(3.37+0.55/-0.68)mm/a and(2.69+0.41/-0.38)mm/a for the Guide and Duohemao segment, which is comparable to the previously suggested slip rate of northern Riyueshan Fault. Finally, we discussed the role of the Riyueshan Fault in the tectonic deformation of northeastern Tibetan plateau. Table and Figures | Reference | Related Articles | Metrics

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EARTHQUAKE CENTROID, SEISMIC MOMENT TENSOR AND DYNAMIC ENVIRONMENT ANALYSIS OF THE MS6.4 EARTHQUAKE SEQUENCE IN YANGBI, YUNNAN ON MAY 21, 2021 GUO Xiang-yun, YIN Hai-quan, WANG Zhen-jie, YANG Hui SEISMOLOGY AND EGOLOGY    2021, 43 (4): 806-826.   DOI: 10.3969/j.issn.0253-4967.2021.04.005 Abstract （720）   HTML （40）    PDF（pc） （8319KB）（210）       Knowledge map    Save According to the Unified Earthquake Catalogue of China Earthquake Networks, using the seismic phase data compiled by the Seismic Data Center, the observations of 101 fixed and mobile seismic stations in the Yunnan region and its surrounding seismic network from May 18 to 28, 2021, we conducted precise positioning research on the foreshock-mainshock-aftershock sequence of the Yangbi earthquake using the double-difference positioning method, and obtained the precise locations of 2 144 earthquakes. It is found that the distribution of the main aftershocks and the long axis orientation of the intensity isoseismal are not consistent with the image position of the Weixi-Qiaohou Fault, and the strike intersects with a small angle. The seismogenic fault of this earthquake may be a secondary fault of the Weixi-Qiaohou Fault. On the basis of the precision positioning results, the Bayesian Bootstrap Optimization(BABO)algorithm is used to perform a moment tensor inversion on the M6.4 earthquake and the M3.6 and above earthquake sequences in Yangbi, Yunnan from May 18 to 28, 2021. The results show that the sequence of Yangbi earthquake in Yunnan has obvious segmentation. The M6.4 Yangbi main shock is of right-handed strike-slip type with a small amount of normal dip-slip component, and the centroid depth is 5.9km. Most aftershocks have the same focal mechanism as the main shock, mainly right-handed strike-slip, except for the earthquakes in the west branch of the southeast section of the aftershock area, where the source property is obviously different, showing a normal strike-slip motion. The centroid depth of the entire earthquake sequence is 3.5~8.2km. The inversion results show that the principal compressive stress field of the earthquake area is in the near NS direction and the strike-slip dislocation is associated with a slight normal dip-slip component. The spatial distribution of earthquakes shows that this earthquake sequence gradually developed from NW to SE, and the seismic density gradually dispersed from NW to SE. Therefore, it can be considered that the stress was mainly concentrated in the NW direction before the earthquake, and then gradually spread to the SE. Therefore, the main power source of this earthquake may be the southeastward extrusion of the Sichuan-Yunnan block. The rupture process and rupture pattern of this earthquake represents the “relaxation” process of the Sichuan-Yunnan rhombic block after being extruded. The southern part of the Sichuan-Yunnan block is the material diffusion zone resulting from the eastward extrusion of Qinghai-Tibet Plateau. In this area, the subduction and westward retreat of the Indian plate led to the absence of lateral restraint on the Sichuan-Yunnan block, which may be the main reason causing the earthquake sequences in this area changing from convergence to dispersion, from strike-slip to normal fault type. The Sichuan-Yunnan block is one of the most insensive areas where the Qinghai-Tibet Plateau squeezes out and escapes southeastward. Regarding the regional dynamic mode, we believe that under the background of continuous eastward extrusion of the material in the eastern part of the Qinghai-Tibet Plateau, and due to the lack of rigid blocks in the horizontal direction, it is more prone to velocity migration in the horizontal direction when the Sichuan-Yunnan block extrudes in the direction of SE and crosses the eastern structural junction and Longmen Mountains. The velocity migration in the study area may be caused by plate subduction or mantle underplating. The study of the lithospheric structure in the Sichuan-Yunnan area found that the crustal thickness of the sub-blocks in central Yunnan gradually thinned from north to south, and the lithospheric thickness in the area west of the Honghe fault zone shows a gradual thinning trend from east to west. It may be related to the intrusion of hot mantle material caused by the subduction and westward retreat of the Indian plate. Tomography results show that in the Ailaoshan-Honghe fault zone, the Yangtze block subducted downwards accompanied by mantle disturbance and asthenosphere upwelling, which led to magmatic activity and intrusion in the Cenozoic. Both of the above two effects can make the western boundary of the Sichuan-Yunnan block have a tensile stress background, and make the focal mechanism of major earthquakes on the nearby tectonic belt possible to appear normal. In summary, the dynamic source of earthquakes in the study area mainly comes from the escape of the Sichuan-Yunnan block to the southeast, and plate subduction or mantle underplating is possibly the deep-seated dynamic background for the lateral velocity migration of the study area. Table and Figures | Reference | Related Articles | Metrics

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CHARACTERISTICS OF TECTONIC STRESS FIELD AND DYNAMIC SIGNIFICANCE IN THE SHANDONG-JIANGSU-ANHUI SEGMENT OF TANCHENG-LUJIANG FAULT ZONE AND ITS ADJACENT AREAS SUN Ye-jun, HUANG Yun, LIU Ze-min, ZHENG Jian-chang, JIANG Hao-lin, LI Ting-ting, YE Qing, FANG Tao SEISMOLOGY AND EGOLOGY    2021, 43 (5): 1188-1207.   DOI: 10.3969/j.issn.0253-4967.2021.05.010 Abstract （697）   HTML （25）    PDF（pc） （7273KB）（193）       Knowledge map    Save The Shandong-Jiangsu-Anhui segment of Tancheng-Lujiang fault zone is a key seismic monitoring and defense area in China due to its complex structural deformation and intense seismic activity. With the accumulation of digital seismic data from the digital seismic networks of provinces and cities in the area and its adjacent regions, the waveform quality is steadily advanced, and the calculation methods for the focal mechanism solution and the inversion methods of stress field are constantly improved, which makes it possible to obtain more reliable focal mechanism solution and more accurate stress field. Based on the seismic waveform data recorded by regional seismic network, we calculated and obtained focal mechanism solutions of 825 moderate and small earthquakes in Shandong-Jiangsu-Anhui segment of Tancheng-Lujiang fault zone and its adjacent areas from 2001 to 2016, by using the initial motion and amplitude information of P wave, SH wave and SV wave. In addition, we collected focal mechanism solutions of 323 earthquakes from 1970 to 2000. A total of 1 148 focal mechanism solutions were obtained. With the focal mechanism solutions as the input data, we adopted the damped regional-scale stress method to inverse and calculate the spatial variation characteristics of the stress field by 1.0°×1.0°grid region of the study area, and discussed the structural boundary, block difference, stress environment, seismicity and related dynamic problems. The results show that the maximum principal stress direction of the study area presents continuous change spatially, with an overall rotation trend in EW, NEE and NE direction from west to east, and there are differences locally. The dominant stress type is strike-slip, followed by normal strike-slip, indicating that the study area is generally under the action of horizontal stress field, and the difference of stress types mainly reflects the difference of local geological tectonic environment and fault activity mode to a certain extent. Taking the Tancheng-Lujiang fault zone as the boundary, the stress fields of the Ludong-Yellow Sea block and the North China Plain on the both sides are different. The direction of maximum principal stress in the North China Plain block on the west is near-EW and NEE, while that on the east is NEE and NE. The analysis shows that the near EW-directed stress field in the North China Plain block generally inherits the stress field pattern resulting from the eastward extrusion of the Qinghai-Tibet block, but is more influenced by the near EW compression of the Qinghai-Tibet block. The stress field of the Ludong-Yellow Sea block is obviously affected by the westward subduction of the Philippine Sea plate. Although the whole North China block is controlled by the combined action of the northward push of the Indian plate and the westward subduction of the Philippine Sea plate, the effects of various driving forces on different secondary blocks in the block are different due to the existence of the Tancheng-Lujiang fault zone which extends obliquely to the top of the upper mantle. It reflects significantly that the Tancheng-Lujiang fault zone plays a significant role as a block boundary fault. Along the 33°N latitude of Tancheng-Lujiang fault zone, there is a significant difference in the stress field between the north and the south. The direction of the maximum principal stress at the 33°N and its north area begins to deflect anticlockwise from west to east; while in 32°N and to the south, it is deflected clockwise from west to east. The direction of the maximum principal stress gradually transits from NE in North China to NW in South China, showing the characteristics of the stress field in South China to some extent. It indicates that 31°~32°N latitude is the transition zone of the two primary blocks, the North China block and the South China block. The direction of the maximum principal stress of the area between 31°~33°N and 120°~122°E is complex and characterized by radial distribution. This region locates in a very complex tectonic environment and may be influenced by the dextral strike-slip of Tancheng-Lujiang Fault caused by the near EW—NEE movement of the North China Plain block as well as the westward subduction of the Philippine Sea plate. The moderate-strong seismicity in the study area is obviously related to the tectonic stress environment. The area with complex tectonic stress field is usually the area with moderate-strong earthquake activity. Table and Figures | Reference | Related Articles | Metrics

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PRESENT SLIP AND STRESS DISTRIBUTION OF BLOCK BOUNDARY FAULTS IN THE SICHUAN-YUNNAN REGION WAN Yong-kui, SHEN Xiao-qi, LIU Rui-feng, LIU Xia, ZHENG Zhi-jiang, LI Yuan, ZHANG Yang, WANG Lei SEISMOLOGY AND EGOLOGY    2021, 43 (6): 1614-1637.   DOI: 10.3969/j.issn.0253-4967.2021.06.015 Abstract （693）   HTML （34）    PDF（pc） （8665KB）（254）       Knowledge map    Save Previous studies have shown that M≥8 earthquakes and more than 80% M≥7 earthquakes occurred in the boundary zones of active blocks. Therefore, studies on the slip rate and stress distribution of the boundary faults can provide the basis for assessing the risk of strong earthquake. It also can help us understand the regional tectonic deformation, motion and dynamic process. Based on current cognition of the division of active block and fault system in the Sichuan-Yunnan region, we build a two-dimensional finite-element contact model, which includes ten small blocks and the primary block boundary faults, such as East Kunlun Fault, Minjiang Fault, Huya Fault, Xianshuihe-Xiaojiang Fault and Red River Fault. Slip rate and stress distribution of the primary block boundary faults are obtained by using long-term GPS observation data from 1991 to 2015 and “block-loading” method. This loading method can reflect interaction between the block and the boundary. Compared with the direct loading of GPS results, it can avoid local distortion caused by the large single-point error. Comparing GPS observation results with simulation results, the residual error less than 1mm accounts for 66%, and the error less than 2mm accounts for 86%. The direction angle residual error less than 5° accounts for~56%, and that less than 10° accounts for 82%, which means that simulation results of this study are reasonable. In addition, by collecting the relevant information on seismic activity and focal mechanism solutions in the Sichuan-Yunnan region, and combining with the simulation results, we discuss the relationship between slip rate distribution, transfer and stress transformation in large left-lateral strike-slip fault zones, the tectonic mechanism with normal fault type, as well as the probable cause of the seismic discrepancy between the northern and southern segments of the Red River Fault. The main conclusions are as follows: (1)As the strike of the left-lateral strike-slip East Kunlun and Xianshuihe-Xiaojiang fault zones turns sharply from NW to near north-south-direction, the strike-slip component is partially absorbed by the fault-bend parts and then converted into strain accumulation, resulting in high stress distribution in the fault-bend areas. Among them, the area from the easternmost end of East Kunlun Fault to Huya Fault absorbs a strike-slip rate of~0.15mm/a. The accumulative rates of compressional stress are 3 711.7Pa and 699.3Pa, respectively. And the area from southeastern end of Xianshuihe Fault to Anninghe and Daliangshan Faults absorbs a strike-slip rate of~1mm/a. The accumulative rates of compressional stress are 3 051.7 Pa and 2 844.6 Pa, respectively. (2)Affected by the left-lateral shear of Xiaojiang Fault, the south-central segment of the Red River Fault is dominated by right-lateral strike-slip with weak compression. The right-lateral strike-slip rate is 1.20~2.68mm/a. The right-lateral strike-slip rate of north segment of Red River Fault is 0.71~1.54mm/a. This indicates that right-lateral strike-slip in the northern segment of Red River Fault is caused by traction of the south-central segment. The Red River Fault constitutes a right-lateral shear deformation zone arranged in right-step en echelon pattern with the Jinsha River Fault and Deqin-Zhongdian Fault. In the vicinity of Deqin-Zhongdian Fault, the Yulong snow mountain eastern piedmont fault, the southern segment of the Lijiang-Xiaojinhe Fault and the Ninglang-Yongsheng-Binchuan Fault, form a tectonic pull-apart zone. The normal focal mechanisms are predominantly distributed within this zone. This deformation pattern is not consistent with imbricated thrust conversion-limited extrusion model, which suggests that the current movement mode of Jinsha River and Lijiang-Xiaojinhe fault zones and their effect on regional deformation may have changed. (3)The north segment of the Red River Fault appears to be slightly tensional, while the south segment is weakly compressional. According to Coulomb's criterion, the shear stress required for fault rupture in the northern section should be lower than that in the southern section. As a result, the north section is more likely to rupture and the seismic activity is significantly stronger than that of the south-central part. Table and Figures | Reference | Related Articles | Metrics

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PALEOEARTHQUAKE CHARACTERISTICS IN DUNHUANG SEGMENT OF THE SANWEISHAN FAULT LIU Xing-wan, YUAN Dao-yang, YAO Yun-sheng, ZOU Xiao-bo SEISMOLOGY AND EGOLOGY    2021, 43 (6): 1398-1411.   DOI: 10.3969/j.issn.0253-4967.2021.06.004 Abstract （684）   HTML （43）    PDF（pc） （7033KB）（277）       Knowledge map    Save The Sanweishan Fault is located in the front of the northwest growth of the northern margin of Tibetan plateau, a branch fault of the Altyn Tagh Fault which extends to the northwest. The latest seismic activity of the Sanweishan Fault reflects the tectonic deformation characteristics of the northern plateau. Meanwhile, it is of great significance for the seismic risk assessment of Dunhuang and its adjacent areas to understand the characteristics of earthquake recurrence. The Sanweishan Fault runs along the western piedmont of the Sanwei Shan, with a total length of 175km. The fault is characterized by left-lateral strike-slip and reverse faulting, with local normal fault features. Based on the geometry, the fault can be divided into three segments, i.e. the Shuangta-Shigongkouzi, the Shigongkouzi-Shugouzi and the Shugouzi-Xishuigou segment from east to west. Previous studies about the paleoearthquakes on the Sanweishan Fault mainly focus on the middle and east segments of the fault, while the west segment of the fault has been less studied. Meanwhile, the available research does not involve the recurrence characteristics and possible magnitude of the paleoearthquakes. Based on high-resolution satellite images, we found that the main fault has grown toward the basin and formed fault scarps in the western segment of the Sanweishan Fault. We have carried out a detailed study on these fault scarps. Based on trench excavation and chronological study on the latest fault scarps, this paper determines the sequence of the paleoseismic events on the fault and discusses the recurrence characteristics and possible magnitude of earthquake for the Sanweishan Fault. In the western segment of the fault, through satellite image interpretation and field investigation, we found new fault scarps distributed on the alluvial fan in front of the mountain near Gedajing. We called it Dunhuang segment of the Sanweishan Fault. The activity characteristics of the fault scarps may reflect the latest seismic events in the western part of the Sanweishan Fault. Different from the sinistral strike slip of the main Sanweishan Fault, this fault segment shows the characteristics of thrust with low angle. According to the differential GPS survey, the height of the fault scarp is approximately 2.2m. The paleoseismic trench was excavated across the fault scarp. Based on the analysis of paleoseismological trenching and optical stimulated luminescence dating, two paleoseismic events are determined. Event E1 occurred at approximately(35.1±3.7)~(36.7±4.1)ka; event E2 occurred at approximately(76.5±8.8)~(76.7±8.3)ka. According to the strata offset and corresponding age, the vertical slip rate of the Sanweishan Fault is determined to be(0.03±0.01)mm/a, with a corresponding shortening rate of(0.09±0.01)mm/a. Together with the previous results, we consider that the Sanweishan Fault is characterized by segmentation. The middle and east segments may have the ability of independent rupture, and also the characteristics of cascading rupture with the Dunhuang segment. According to the existing results, we conclude that the recurrence interval for cascading rupture behavior on the Sanweishan Fault is approximately 40ka, which shows a characteristic of low slip rate and long-term recurrence. The best estimated magnitude is inferred to be in the range between MW7.1 and MW7.5 based on the empirical relationships between moment magnitude and rupture length. Table and Figures | Reference | Related Articles | Metrics

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NEW DISCOVERY OF XIARIHA FAULT ZONE AROUND DULAN AREA, QINGHAI PROVINCE AND ITS TECTONIC IMPLICATIONS HA Guang-hao, REN Zhi-kun, LIU Jin-rui, LI Zhi-min, LI Zheng-fang, MIN Wei, ZHOU Ben-gang SEISMOLOGY AND GEOLOGY    2021, 43 (3): 614-629.   DOI: 10.3969/j.issn.0253-4967.2021.03.009 Abstract （684）   HTML    PDF（pc） （19276KB）（317）       Knowledge map    Save The deformation pattern in the northeastern margin of Tibetan plateau is characterized by NE compression, clockwise rotation and eastward extrusion, forming the NNE trending dextral strike-slip faults which further divide the region into several sub-blocks. The deformation of Qaidam secondary block is dominant by northwestward extrusion and rotation, which is controlled by the Elashan and East Kunlun faults. However, the deformation style of Dulan area, the junction of these two faults, remains unclear. We discovered a new active fault zone with a length of 60~70km west to Elashan Fault during our recent field geological survey around Dulan area, named Xiariha fault zone(XFZ), which is a dextral strike-slip fault zone trending NW, consisting of the Xiariha and Yingdeerkang faults. According to the remote sensing interpretation and field investigation, it is found that the Xiariha fault zone showed distinct linear characteristics, reverse scarp, sag pond and ridge dislocation on the satellite images and displaced multi-levels of alluvial fans and river terraces. According to previous studies, the exposed age of T1 terraces is Holocene in the Elashan area, which is located at east of Dulan. During the field investigation, we used the unmanned aerial vehicle(UAV)to get the fine geomorphology features along the XFZ. Also, to define the active era, we tried to find the fault section of the XFZ that could provide the information of the contact between the fault and late Quaternary strata. Based on the high-resolution DEM obtained by UAV, the offset of T1 is about 2.5m, indicating its activity in Holocene compared with the Elashan area. Along the XFZ, the fault displaced late Quaternary strata revealed on the section. The geomorphic features and fault section show that the XFZ is a late Pleistocene to Holocene active fault. The Dulan area is located at the convergence of East Kunlun Fault and Elashan Fault, the southeastern end of Qaidam secondary block, which is affected by the regional NE and SW principal compressive stress and shear stress. Under this circumstance, the Qaidam block is experiencing extrusion and rotation and there are a series of NW-trending dextral strike-slip faults parallel to the Elashan Fault and EW-trending sinistral strike-slip faults parallel to the East Kunlun Fault, such as Reshui-Taosituo River Fault, developed in the Dulan area. Therefore, we suggest that the Xiariha Fault and the nearly EW trending, Holocene sinistral Reshui-Taosituo River Fault adjust the extrusion rotation deformation jointly at the southeast end of the Qaidam block under the control of the Elashan Fault and the East Kunlun Fault, respectively. Meanwhile, the new discovery of Xiariha Fault and its activity in Holocene is not only of great significance to understand the regional tectonic deformation model, but also leads to a great change in the understanding of regional seismic risk because of its capabliliby of generating strong earthquakes. Therefore, it is urgent to carry out further research work in this area, improve the understanding of regional strain distribution mode, and provide reference for regional seismic safety issues. Reference | Related Articles | Metrics

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IDENTIFYING FAULT LINEAMENT BASED ON ALOS-PALSAR DEM: A CASE STUDY FROM THE WEST QINLING MOUNTAINS ZHANG Bo, WANG Ai-guo, TIAN Qin-jian, GE Wei-peng, JIA Wei, YAO Yun-sheng, YUAN Dao-yang SEISMOLOGY AND EGOLOGY    2022, 44 (1): 130-149.   DOI: 10.3969/j.issn.0253-4967.2022.01.009 Abstract （673）   HTML （18）    PDF（pc） （22166KB）（160）       Knowledge map    Save The most significant feature of active faults on remote sensing images is fault lineament. How to identify and extract fault lineament is an important content of active fault research. The rapid development of remote sensing technology has provided people with extremely rich remote sensing data, and has also created the problem of how to choose suitable data for fault interpretation. In the traditional fault interpretation, people pay more attention to high-resolution optical images and high-resolution DEM, but optical remote sensing images are greatly affected by factors such as weather condition, vegetation and human impacts, and the time and economic costs for obtaining high-resolution DEM are relatively high. Due to the low resolution, the medium-resolution DEM(such as Aster GDEM, SRTM1, SRTM3, etc.)is generally used to automatically extract structural lineament, and then analyze the overall regional structural features, but it is rarely used to visually interpret active faults. ALOS-PALSAR DEM is generated from SAR images acquired by the phased array L-band synthetic aperture radar mission sensor of the Japanese ALOS satellite. It is currently a free DEM with the highest resolution(resolution of 12.5m)and the widest coverage. Based on ALOS-PALSAR DEM and ArcGIS 10.4 software, this paper generates a hillshade map and visually interprets the fault lineaments in the West Qinling Mountains. When generating a hillshade map, we set the light azimuths to be oblique or orthogonal to the overall trend of the linear structures, the light azimuths to be consistent with the slope direction of the hillslope, and the light dips to be a medium incident angle. Based on the hillshade map generated from ALOS-PALSAR DEM, this paper summarizes the typical performance and interpretation markers of fault lineaments on the hillshade map(generated by DEM), and visually interprets the V-shaped fault system in West Qinling Mountains where the research on fault geometry is limited based on the interpretation markers. The results of the research are as follows: First, this study discovers a number of fault lineament zones, including the fault lineament located between the Lintan-Dangchang Fault and the Guanggaishan-Dieshan Fault, the NE-directed fault lineament zone between the Lixian-Luojiapu Fault and the Liangdang-Jiangluo Fault, and the arc-shaped dense fault lineament zones distributed south of the Hanan-Daoqizi Fault and the Wudu-Kangxian Fault; Second, this study completes the geometric distribution images of the known active faults, such as the western and eastern sections of the Lintan-Dangchang Fault, the western and eastern sections of the Liangdang-Jiangluo Fault; Third, fault lineaments in the West Qinling Mountains exhibit a “V” shape, with two groups of fault lineaments trending NW and NE, whose tectonic transformation mainly consists of two kinds: mutual cutting and arc transition. The Lintan-Dangchang Fault cuts the Lixian-Luojiapu Fault, the Lintan-Dangchang Fault and the Guanggaishan-Dieshan Fault are connected with the Liangdang-Jiangluo Fault in arc shape, and the Tazang Fault is connected with the Hanan-Daoqizi Fault in arc shape. The research results show that ALOS-PALSAR DEM has an outstanding capability to display fault lineaments due to its topographic attributes and strong surface penetration. In circumstances when the surface is artificially modified strongly, the spectrum of ground objects is complex and the vegetation is dense, the ALOS-PALSAR DEM can display fault lineament that cannot be displayed on optical remote sensing images, indicating that the medium-resolution DEM is an effective supplement to high-resolution optical remote sensing images in the fault lineament interpretation. The research results are of great significance for improving the geometric image of the V-shaped fault system in the West Qinling Mountains. It is also the basis for further research on fault geometry, kinematics, regional geodynamics and seismic hazard. Table and Figures | Reference | Related Articles | Metrics

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COSEISMIC DISPLACEMENT MEASUREMENT AND DISTRIBUTED DEFORMATION CHARACTERIZATION: A CASE OF 2021 MW7.4 MADOI EARTHQUAKE SHAO Yan-xiu, LIU-ZENG Jing, GAO Yun-peng, WANG Wen-xin, YAO Wen-qian, HAN Long-fei, LIU Zhi-jun, ZOU Xiao-bo, WANG Yan, LI Yun-shuai, LIU Lu SEISMOLOGY AND GEOLOGY    2022, 44 (2): 506-523.   DOI: 10.3969/j.issn.0253-4967.2022.02.014 Abstract （671）   HTML （26）    PDF（pc） （7392KB）（183）       Knowledge map    Save The coseismic displacements are required to characterize the earthquake rupture and provide basic data for exploring the faulting mechanism and assessing seismic risk in the future. Detailed field investigation is still an important way to acquire reliable coseismic displacements comparing to geodetic measurements. Combining with previous research on other earthquakes, this study tries to discuss distributed deformation along the strike rupture and its implications. The MW7.4 Madoi earthquake ruptured the southeast section of the Kunlun Shankou-Jiangcuo Fault on May 22, 2021, in Qinghai Province. It is a typical strike slip event, and its epicenter locates at~70km south of the East Kunlun Fault, which is the north boundary of the Bayan Har block. Field investigation results show that the surface rupture extends along the piedmont. The deformation features mainly include compression humps, extensional and shear fissures, and scarps. After the earthquake, we used the unmanned aerial system to survey the rupture zone by capturing a swath of images along the strike. The swath is larger than 1km in width. Then we processed the aerial images by commercial software to build the orthoimage and the digital elevation model(DEM)with high resolutions of 3~5cm. We mapped the surface rupture in detail based on drone images and DEM along the western section. Meanwhile, we also got the commercial satellite images captured before the earthquake, on 2nd January 2021. The images were processed with geometrical rectification before comparison. The spatial resolution of satellite images before earthquake is about 0.5m. At the south of the Eling Hu(Lake), the clear offset tire tracks provide an excellent marker for displacement measurement. We located the positions of tracks precisely based on remote sensing images, and compared between the tracks lines after earthquake and the corresponding positions before earthquake, then extracted distance difference, which is defined as coseismic displacements. The results show that the total displacement is about 3.6m, which contains the distributed deformation of about 0.9m. The off-fault deformation is about 33% of the on-fault and about 25% of the total deformation. The ratios are similar to previous studies on earthquake worldwide. The fault zone width is probable about 200m. The total horizontal displacement measured by this study is similar to the slip in depth by InSAR inversion, which implies that there is no slip deficit at the west rupture section of the earthquake. The results also present the asymmetry of distributed deformation that most distributed deformation occurs at the south of the surface rupture zone. Comparing with other earthquakes in the world, it is likely that the asymmetrically distributed deformation is common in strike-slip earthquakes and the asymmetric feature is not related to the property of the material. The characteristics of distributed deformation might be related to fault geometry at depth or local stress state. More work is needed to resolve this question in the future. This study implies that we probably underestimated the slip rates resulting from ignoring distributed deformation in the past. In order to avoid underestimation of slip rates, we can correct the previous results by the ratio of distributed deformation to total slip. It is also suggested that the study sites should be on the segment with narrow deformation and simple geometry. Table and Figures | Reference | Related Articles | Metrics

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DEEP SEISMOTECTONIC ENVIRONMENT OF THE 2021 MADOI MS7.4 EARTHQUAKE SONG Xiang-hui, WANG Shuai-jun, PAN Su-zhen, SONG Jia-jia SEISMOLOGY AND EGOLOGY    2021, 43 (4): 757-770.   DOI: 10.3969/j.issn.0253-4967.2021.04.002 Abstract （669）   HTML （197）    PDF（pc） （5271KB）（737）       Knowledge map    Save On May 22, 2021, an MS7.4 earthquake occurred in the Madoi area of Banyan Har block, with a focal depth of about 8km. The seismogenic fault is deduced as the Jiangcuo Fault, a branch of the east Kunlun strike-slip fault. Different with previous strong earthquakes which located at the boundary faults around the Bayan Har block, the Madoi MS7.4 earthquake occurred inside the block and about 70km away from the boundary fault. Furthermore, there is a contradiction between the small strike-slip component of the seismogenic fault and the large earthquake magnitude. The above phenomena indicate that the Madoi earthquake may have special seismotectonic background and seismogenesis. Strong earthquakes in Tibetan plateau are always closely related to the deep crustal structure and dynamic process. Therefore, it is of great significance to study the crustal structure and the distribution of deep faults in the Madoi area in order to reveal the deep tectonic background and genesis of the Madoi MS7.4 earthquake. To research the deep seismotectonic environments of the MS7.4 Madoi earthquake, we reinterpret the deep seismic sounding(DSS)results in Madoi area. The DSS profile reveals fine crustal structure beneath the Madoi area, and divides the crust into 3 crustal layers. From the crustal velocity structure of the Madoi and adjacent area, we found the generation of the Madoi earthquake is closely connected with the deep structure and crustal medium. Through analysis on the velocity structures, we get the following understanding: 1)There is an interface in the upper crust of the Madoi area, which represents the velocity changing from 5.8km/s to 5.6km/s and divides the upper crust into two layers. The upper layer is composed of high velocity structure, indicating a brittle medium environment, while the lower layer consists of low velocity zone and provides the strain accumulation condition for the Madoi earthquake. In addition, the transition between local high velocity zone(HVZ)and the normal crust in the focal area provides an ideal medium environment for earthquake preparation. 2)A wedge-shaped low velocity zone(LVZ)exists in the lower crust south of Madoi, which provides an environment for the movement of weak materials from the SW to NE direction. However, the high-velocity lower crust beneath Madoi area resists the crustal flow and thus transforms the horizontal movement to vertical upwelling, resulting in the stress concentration of the upper crust beneath Madoi area, which may provide dynamic for the preparation of the Madoi MS7.4 earthquake. 3)The Jiangcuo Fault merges into the East Kunlun Fault in the deep crust, forming a reverse thrust fault structural style dominated by the East Kunlun strike-slip fault. As a branch of the East Kunlun Fault, the strike slip of the Jiangcuo Fault is the adjustment results of strain and movement of the East Kunlun Fault. Moreover, the Jiangcuo Fault and adjacent faults constitute the horsetail-shaped fault zone, combined with the imbricated thrust fault zone profile, reflecting the compressive stress of Modoi area that facilitates the strain concentration. Therefore the occurrence of the Madoi earthquake is related to the left-lateral strike-slip movement of the East Kunlun Fault and the special imbricated thrust fault assemblages. On the other hand, the upwelling of the lower crustal flow and the corresponding sliding of the upper crust may be related with the occurrence of the Madoi earthquake. In conclusion, the Madoi MS7.4 earthquake is closely related to the ideal medium environment of the upper crust, the lower crustal flow and vertical upwelling beneath Madoi area, as well as the left-lateral strike-slip of the East Kunlun Fault. Table and Figures | Reference | Related Articles | Metrics