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    RELOCATION AND FOCAL MECHANISM SOLUTIONS OF THE 2021 MADUO, QINGHAI M S7.4 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
    Abstract330)   HTML34)    PDF (5589KB)(161)       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.

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    RELOCATION AND FOCAL MECHANISM SOLUTIONS OF THE 2021 YANGBI, YUNNAN M S6.4 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
    Abstract285)   HTML20)    PDF (8652KB)(104)       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.

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    COSEISMIC DEFORMATION FIELD, SLIP DISTRIBUTION AND COULOMB STRESS DISTURBANCE OF THE 2021 M W7.3 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
    Abstract222)   HTML    PDF (9841KB)(300)       Save
    InSAR coseismic deformation fields caused by the Maduo M W7.3 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 M W7.3 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 M W7.45, 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.
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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
    Abstract204)   HTML197)    PDF (9559KB)(172)       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.

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    SEISMOGENIC FAULT AND COSEISMIC SURFACE DEFORMATION OF THE MADUO M S7.4 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
    Abstract198)   HTML    PDF (18089KB)(436)       Save
    At 02:04 a.m. on May 22, 2021, a M S7.4 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 M S7.4 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.
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    EARTHQUAKE GROUND MOTION INTENSITY MAP OF THE 21 MAY, 2021 M S6.4 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
    Abstract181)   HTML11)    PDF (4118KB)(62)       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.

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    THE SEISMOGENIC FAULT OF THE 2021 YUNNAN YANGBI M S6.4 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
    Abstract170)   HTML    PDF (16261KB)(398)       Save
    The May 21, 2021 Yangbi M S6.4 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 M S6.4 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 M S6.4 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.
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    DEEP SEISMOTECTONIC ENVIRONMENT OF THE 2021 MADOI M S7.4 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
    Abstract157)   HTML179)    PDF (5271KB)(248)       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.

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    THE CO-SEISMIC DEFORMATION CHARACTERISTICS AND SEISMOGENIC STRUCTURE OF THE YANGBI M S6.4 EARTHQUAKE
    XU Xiao-xue, JI Ling-yun, ZHU Liang-yu, WANG Guang-ming, ZHANG Wen-ting, LI Ning
    SEISMOLOGY AND EGOLOGY    2021, 43 (4): 771-789.   DOI: 10.3969/j.issn.0253-4967.2021.04.003
    Abstract143)   HTML174)    PDF (11808KB)(300)       Save

    A MS6.4 earthquake occurred on May 21th, 2021 at Yangbi, Yunnan. In this paper, high resolution InSAR coseismic deformation fields were obtained based on the ascending and descending track of Sentinel-1 SAR images. Based on the InSAR-derived deformation fields, the geometric model of the seismogenic fault was determined according to the aftershock relocation results. Then the fine coseismic slip distribution of the fault plane of Yangbi earthquake was inversed using a distributed sliding inversion method. Finally, the regional strain distribution and the Coulomb stress variation on the surrounding faults caused by coseismic dislocations and viscoelastic relaxation effect after earthquake were calculated, and the seismic risk of the seismogenic structure and the surrounding faults was discussed. The results show that the descending track co-seismic deformation field shows that the NE wall of the seismogenic fault moves close to the satellite, while the SW wall moves far away from the satellite, and the coseismic deformation is symmetrically distributed. The maximum LOS vectors were 8.6cm and 7.9cm, respectively, and the descending track profile showed a coseismic displacement up to 15cm. The fringes on the southwest side of the ascending track interferograms are relatively clear, showing movement close to the satellite, and the maximum LOS deformation magnitude is 5.7cm, while the interference fringes on the northeast side are not clear and the noise is obvious. The fault co-seismic dislocation is mainly of dextral strike-slip with a small amount of normal fault component. The coseismic slip mainly distributes at depths 2~10km, and the coseismic sliding rupture length is about 16km with the maximum slip of approximately 0.46m at a depth 6.5km. The average slip angle is 180° and the inverted magnitude is approximately MW6.1. The causative fault did not rupture the surface. From the analysis of regional strain distribution and tectonic dynamic background, the Yangbi earthquake occurred in the region where the Sichuan-Yunnan rhomboid block is blocked in its process of SE movement by the South China block and deforms strongly. Combined with the analysis of the geometric occurrence and movement properties of faults, our study suggests that the causative fault of the Yangbi earthquake maybe is a branch of the Weixi-Qiaohou Fault or an unknown fault that is nearly parallel to it on the west side. This earthquake has a significant impact on the Coulomb stress of the Longpan-Qiaohou Fault, Chenghai Fault and Red River Fault in the southwestern Sichuan-Yunnan rhombic block. The Coulomb stress in the northern section of Red River Fault is the most significant. The cumulative Coulomb stress variations of the coseismic and 10 years after the earthquake show that the Coulomb stress variation has increased in the northwestern Yunnan tectonic area. This earthquake is another typical seismic event occurring in the southwest of the Sichuan-Yunnan block after the Lijiang MS7.0 earthquake in 1996 and the Mojiang MS5.9 earthquake in 2018. The risk of strong earthquakes in the regional extensional tectonic system in northwest Yunnan and in the north section of the Red River fault zone cannot be ignored.

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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
    Abstract138)   HTML24)    PDF (18555KB)(308)       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.

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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
    Abstract136)   HTML8)    PDF (7273KB)(79)       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.

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    TEMPORAL AND SPATIAL EVOLUTION OF THE 2021 YANGBI (YUNNAN CHINA) M S6.4 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
    Abstract129)   HTML17)    PDF (2494KB)(90)       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.

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    EARTHQUAKE CENTROID, SEISMIC MOMENT TENSOR AND DYNAMIC ENVIRONMENT ANALYSIS OF THE M S6.4 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
    Abstract129)   HTML28)    PDF (8319KB)(128)       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.

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    GEOMETRIC COMPLEXITY OF FAULT SYSTEM IN THE SOURCE REGION OF THE 2021 YANGBI, YUNNAN, M S6.4 EARTHQUAKE
    LIANG Shan-shan, XU Zhi-guo, ZHANG Guang-wei, ZOU Li-ye, LIU Yan-qiong, GUO Tie-long
    SEISMOLOGY AND EGOLOGY    2021, 43 (4): 827-846.   DOI: 10.3969/j.issn.0253-4967.2021.04.006
    Abstract121)   HTML22)    PDF (8044KB)(119)       Save

    Earthquake relocation and focal mechanism inversion can provide seismogenic structure information, especially in the source area without obvious fault trace on the surface, and further reveal the deep geometry of hidden faults. The Yangbi MS6.4 earthquake sequence recorded by Yunnan regional seismic network from May 18 to June 4, 2021 is relocated by using the double-difference location method. A total of 3 233 events, from 4 days before and 14 days after the main shock, are relocated and the b-value in the Yangbi source region is calculated accordinly. Then, using the waveform data recorded by the Yunnan and Sichuan regional broadband seismic stations, the full moment tensor solutions of 10 earthquakes (M≥4.0), including the main earthquake, are obtained using the near-field full waveform inversion method, and further the tectonic stress field is retrieved. The high-precision relocation of earthquakes shows that there are significant differences between the foreshocks and the aftershocks in the tempo-spatial distribution. The foreshocks are primarily in a belt-like distribution along the NW-SE direction, whose epicenters are in a back-and-forth migration. The aftershocks mainly occurred on asymmetric conjugate faults along NW and NE directions, and multi-groups of aftershocks with different strikes were distributed in the south end of the NW-striking seismic zone, implying the complexity of the medium and fault geometry in the focal area. The temporal distribution of the b-value shows that the b-value has a rising trend before the main earthquake, indicating that the stress accumulation in the source area had begun to release gradually at that time, which may be related to the fact that the sequence is of the foreshock-mainshock-aftershock type. After the main shock, the variation range of b-value is large, which may reflect very strong seismicity of the aftershocks and large release of the stress. The focal mechanism solutions show that the moderate earthquakes are mainly of strike-slip with a normal component and a significant non-double-couple component, which may indicate the staggered distribution of the NW- and NE-trending faults in the source region, and the earthquake rupture is not simply the slip along the fault plane. Taking into account for the above-mentioned results as well as the compressional stress field environment in nearly NS direction and the extensional environment in nearly EW direction, the seismogenic structure of Yangbi MS6.4 earthquake is a dextral strike-slip fault, NW striking with a high-dip angle, located in the Baoshan block, which may be a secondary fault parallel to the Weixi-Qiaohou-Weishan Fault and including multi-fault branches in NE direction in the southern segment. The tempo-spatial distribution characteristics of the earthquake sequence and the diversity of the fault plane rupture are controlled by the geometric complexity of fault system in the focal area.

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    STUDY OF CRUSTAL TOMOGRAPHY AND PRECISE EARTHQUAKE LOCATION IN YANGBI AREA, YUNNAN PROVINCE
    YIN Xin-xin, JIANG Chang-sheng, CAI Run, GUO Xiang-yun, JIANG Cong, WANG Zu-dong, ZOU Xiao-bo
    SEISMOLOGY AND EGOLOGY    2021, 43 (4): 864-880.   DOI: 10.3969/j.issn.0253-4967.2021.04.008
    Abstract114)   HTML19)    PDF (11709KB)(273)       Save

    The occurrence of strong earthquake is closely related to the distribution of crustal velocity anomalies. Some studies have shown that strong earthquakes occur in the transition zone between high-velocity anomalies and low-velocity anomalies in the middle and upper crust or inside the low-velocity anomaly zone. Thus, high-resolution imaging of the velocity structure in the seismic source area and accurate earthquake location can assist the evaluation of seismogenic settings of strong earthquakes. On May 21, 2021, an MS6.4 earthquake occurred in Yangbi, Yunnan with casualties and property losses. The epicenter region of the Yangbi earthquake is in the western Yunnan area of the Sichuan-Yunnan block, which is located on the southeastern edge of the Qinghai-Tibet Plateau and characterized with intensive tectonic activity. Previous studies in this area are mostly on regional scales, and lacking on the three-dimensional fine crustal velocity structure in the Yangbi earthquakes area. To investigate the seismogenic environment and source characteristics of the 2021 Yangbi MS6.4 sequence in Yunnan, we used the P-wave and S-wave arrival data of 12 652 earthquakes recorded by both the Yunnan regional digital network and the mobile observation arrays over a 10-year period(May 1, 2011, to May 31, 2021) and obtained the average VP/VS ratio of 1.79 via fitting the P-wave and S-wave arrival-time curves with the Wadati method. The magnitude ranges from MS0.0 to MS6.4, and the original focal depth ranges from 0 to 35km. To ensure the reliability of the calculation results, at least 4 stations records are required, and the maximum station azimuth gap allowed is 120°. Furthermore, the event-station distance is restricted to 400km and only earthquakes with travel time residuals<0.5s are retained. Our final velocity model is further refined via gridding(i.e., nodes)with an optimal horizontal grid of 0.25°×0.25° and a range between 0~65km vertically. A checkerboard test is also conduced to validate our inversion results. The test results showed that the recovery degree is high except for the depths of 0 and 65km, which were impacted by the uneven seismic distribution and rays. The high degree of recovery of 5~45km suggests high-resolution and robust imaging at these depths. Finally, the double-difference tomography method(TomoDD)was used to invert the three-dimensional P-wave and S-wave velocity structures in the Yangbi and its surrounding areas(24.5°~26.5°N, 99°~101°E). According to the result of precise location, the MS6.4 main shock is located at 99.89°E, 25.70°N with a focal depth of 7.9km. The Yangbi MS6.4 earthquake sequence is mainly distributed along the NW direction. Least-squares fitting prefers a~20km long axis with a strike of 312°, and the hypocenter depths are 5~20km. In general, the studied sequence is shallow and located within the upper crust, consistent with the depth distribution characteristics of historical earthquakes in this area. According to the spatio-temporal evolution characteristics of the aftershock sequence, the aftershocks of the MS6.4 earthquake mainly spread unilaterally toward SE direction. Thus, we speculate that the overall medium in the NW of the mainshock is rigid and hinders aftershocks evolution. On the north side of the MS6.4 mainshock epicenter, a group of earthquakes spread along the NNE direction and extended to the Weixi-Qiaohou Fault that hosted the MS4.1 earthquake on May 27, 2021. Considering the geological and structural background, we believe this earthquake occurred on a parallel but unmapped fault on the SE side of the Weixi-Qiaohou Fault. In contrast, the earthquakes spreading in the NNE direction on the north side of the main shock maybe occurred on an unknown fault in the NNE direction. Therefore, the two faults form a conjugate structure. From the imaging results, the upper crustal velocity structure in the study area is consistent with the geological structure changes and the active faults, where the velocities are low. At 0km depth, the extremely low P-wave and S-wave velocities may reflect impacts from surface sediments. A velocity contrast is observed at a depth of 5km near the mainshock. In addition, a high-velocity anomaly was observed to the southeast side of the mainshock at 10-km depth, with a length of about 0.6°(EW)and a width of about 0.2°(SN). Within the depth range of 10~20km, the distribution of earthquakes near the mainshock shows a clear strip-like distribution, delineating the geometry of the fault. The velocity structure and seismic relocation results at 10-km depth suggest that majority of the events locate around the high-velocity anomaly on the west side of the Weixi-Qiaohou Fault. From the AA' profile, both P- and S-wave velocities suggest high-velocity anomalies in the SE direction of the mainshock. Combining with the distribution characteristics of aftershocks, the non-uniform variations of velocity structure are probably the major factor controlling the distribution of aftershocks, leading to the aftershock distribution extending along the SE direction.

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    COSEISMIC SURFACE DEFORMATION AND SLIP MODELS OF THE 2021 M S6.4 YANGBI(YUNNAN, CHINA)EARTHQUAKE
    WANG Shao-jun, LIU Yun-hua, SHAN Xin-jian, QU Chun-yan, ZHANG Guo-hong, XIE Zhao-di, ZHAO De-zheng, FAN Xiao-ran, HUA Jun, LIANG Shi-ming, ZHANG Ke-liang, DAI Cheng-long
    SEISMOLOGY AND GEOLOGY    2021, 43 (3): 692-705.   DOI: 10.3969/j.issn.0253-4967.2021.03.014
    Abstract110)   HTML    PDF (7636KB)(159)       Save
    Due to the ongoing collision between Indian and Eurasian plates, the internal blocks of the Tibet plateau are experiencing eastward extrusion. Resulting from the blocking of the Sichuan Basin along the eastern boundary of the Bayanhar block, the plateau begins to rotate clockwise around the eastern syntaxis, and continues to move toward the IndoChina Peninsula. Such process forms the Hengduan Mountains with thousands of gullies in the Sichuan-Yunnan region, and generates major earthquakes across the entire Red River Fault, where infrastructures and residents are seriously threatened by the frequent earthquakes. InSAR observations feature a high spatial resolution and short intervals, ranging from several days to over a month, depending on the satellite revisit period.
    On May 21, 2021, an earthquake struck the Yangbi city. This event provides a rare opportunity to look at the local tectonic and seismic risk in the north of the Red River Fault. We processed the Sentinel-1 SAR data with D-InSAR technology and generated the surface deformation caused by the Yangbi M S6.4 earthquake occurring on May 21, 2021. Due to the abundant vegetation and moisture in Yunnan, significant atmospheric noise needs to be corrected for the derived InSAR displacement field. The results show a maximum deformation of~0.07m in line-of-sight for ascending track and~0.08m for descending track. The quality of interferogram on the ascending track is low, and only one of the quadrans can be distinguished, the rest of the interferogram is regarded as phase noise. However, the descending interferogram contains two deformation regions, with its long axis roughly along the NW-SE direction. The northeast part of interferogram moves towards the satellite, while the southwest part moves away from the satellite. The InSAR interferograms pattern shows a right-lateral strike-slip movement. Then, we combined coseismic displacement data obtained from the Global Navigation Satellite System(GNSS)and InSAR(both the ascending and descending)to invert the coseismic slip model of the Yangbi earthquake. The inversion test shows that our data cannot give strong constraints for the dip orientations, and the two slip models with opposite dip orientation can explain the observations within the noise level. No matter what the dip orientation is, the slip models show that the coseismic slip concentrated at depth of 2~10km, with a maximum slip of~0.8m, which corresponds to a moment magnitude of M S6.4, and is consistent with body-wave-based focal mechanism. But the relocated aftershocks in 3 hours immediately after the mainshock reveal a SW-dipping fault plane 10km away to the west of Weixi-Qiaohou-Weishan Fault, we therefore conclude that the Yangbi earthquake ruptured a SW-dipping dextral fault, which is previously unknown. To analyze the effects of the Yangbi earthquake on the seismic risk of the regional dextral faults, we estimated the Coulomb stress change caused by our preferred slip model. The Coulomb stress at 7.5km depth is negative, indicating stress unloading, while the Coulomb stress at 15km depth is positive, indicating slightly loading, but still less than the empirical triggering threshold. The results indicate that Yangbi earthquake partially relieved the strain accumulated on the nearby faults, thus restraining the seismic risk of these faults.
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    SIMULATION OF STRONG GROUND MOTION FROM THE 2021 YANGBI, YUNNAN M S6.4 EARTHQUAKE
    HE Xin-juan, PAN Hua
    SEISMOLOGY AND EGOLOGY    2021, 43 (4): 920-935.   DOI: 10.3969/j.issn.0253-4967.2021.04.012
    Abstract109)   HTML14)    PDF (3540KB)(71)       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.

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    STUDY ON GRAVITY VARIATION OF THE EPICENTER AND SURROUNDING AREAS OF MADUO AND YANGBI EARTHQUAKES DURING 2002-2021 BY GRAVITY SATELLITE
    ZOU Zheng-bo, ZHANG Yi, TAN Hong-bo, CUI Li-lu, YIN Peng, WU Gui-ju
    SEISMOLOGY AND EGOLOGY    2021, 43 (4): 999-1012.   DOI: 10.3969/j.issn.0253-4967.2021.04.017
    Abstract109)   HTML11)    PDF (6574KB)(61)       Save

    The Yangbi MS6.4 earthquake in Yunnan Province and Maduo MS7.4 earthquake in Qinghai Province occurred in western China on May 21 and May 22, 2021, respectively, which caused huge loss of life and property. Gravity changes in the epicentral area and its surroundings before the two earthquakes can provide important reference for studying the seismogenic environment and background. The ground gravity observation is relatively sparse in western China and satellite gravity can supplement this deficiency. GRACE(Gravity Recovery and Climate Experiment)(from March 2002 to June 2017)and GRACE-FO(GRACE Follow-on)(from May 2018 to March 2021)can produce the wide-area space, quasi-real-time, long-term and near-continuous observation data, which will provide large-scale background information for the ground gravity research.

    In this paper, the epicenters of the two earthquakes and their surrounding area were taken as the study area(18°~45°N, 83°~115°E). We used GRACE, GRACE-FO and GLDAS(Global Land Data Assimilation System)data to calculate long-term gravity spatial-temporal distribution in the study area with 300km fan filter. We presented the gravity rate, cumulative gravity changes, differential gravity changes in the study area for about 20 years, and the gravity time series of Maduo earthquake and Yangbi earthquake. We simulated the theoretical co-seismic gravity variation of Maduo earthquake and evaluated the possibility of detecting the co-seismic gravity signal for GRACE-FO. The research results showed that:

    (1)Long-term gravity changes in the study area were mainly characterized by positive-negative-positive-negative spatial layout in four quadrants. Gravity increased in Qinghai-Tibet block and South China block, and gravity decreased in Indian block and North China block. However, the North-South seismic belt and Bayankala block were located at the low-value areas in four quadrants and their gravity changes were relatively small. This was the large-scale gravity seismogenic background in western China.

    (2)The epicenters of Maduo earthquake and Yangbi earthquake were both located in the center of the four quadrants and also at the corner of the high gradient zone of satellite gravity change. And their gravity changes were very small in the last 20 years, which was consistent with the basic characteristics of the ground gravity location prediction. After a year of continuous increase in the last two years before the Maduo earthquake, the gravity in Maduo area experienced a four-month period of decrease, then it increased again. This was similar to the process of gravity change before the Tangshan earthquake.

    (3)MS≥7.0 Earthquakes in the study area since 2002, such as Wenchuan MS8.0 earthquake, Yushu MS7.1 earthquake, Lushan MS7.0 earthquake, Jiuzhaigou MS7.0 earthquake, Maduo MS7.4 earthquake and Nepal MS8.1 earthquake, basically occurred in the central area of the four quadrants or at the corner of the tectonic-related high gradient zone, which was consistent with the earthquake case results of earthquake prediction based on the ground gravity observations. This study provided more earthquake cases for ground gravity prediction.

    (4)Based on dislocation theory simulation, the magnitude of co-seismic gravity change of Maduo earthquake in Qinghai Province reached -40~151μGal. It is difficult for GRACE-FO to detect the co-seismic gravity change of Maduo earthquake with the current accuracy. And it could be possible only when the time-variable gravity accuracy of gravity satellite was improved by 1-2 orders of magnitude. This research provided earthquake case supports for the demand demonstration of gravity satellites in China in the future.

    In this study, the temporal and spatial evolution of gravity in the western region of China and its surrounding areas from March 2002 to March 2021, which coverd the epicenters of Yangbi and Maduo earthquakes, was obtained by using the satellite gravity and global hydrological data after considering the influence of periodic signals. The theoretical coseismic effects of the Maduo earthquake on the local gravity field were analyzed and the accuracy of gravity satellite to detect this seismic signal was evaluated. This study provided the important background information of large-scale gravity field for the study of Maduo earthquake in Qinghai Province and Yangbi earthquake in Yunnan Province, and also provided valuable material for the seismic demand analysis of gravity satellite in China.

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    THE RAPID DETERMINATION OF RADIATED SEISMIC ENERGY OF M S6.4 YANGBI EARTHQUAKE
    WANG Zi-bo, LIU Rui-feng, SUN Li, LI Zan, KONG Han-dong
    SEISMOLOGY AND EGOLOGY    2021, 43 (4): 908-919.   DOI: 10.3969/j.issn.0253-4967.2021.04.011
    Abstract100)   HTML15)    PDF (4109KB)(131)       Save

    Radiated seismic energy is a fundamental physical quantity that can be used to represent the dynamic characteristic of the earthquake source, and it is a reliable indicator of the high-frequency component radiated by the seismic source which goes into the seismic waves. Estimating radiated seismic energy has an important role in seismic hazard assessment, quantification of earthquake research, and engineering seismology. Therefore, it is of great significance for the rapid determination of the seismic radiated energy after an earthquake as a reference. After the Yangbi earthquake on May 21, 2021 in Yunnan Province, we measured the radiated seismic energy for the Yangbi earthquake based on the broadband recordings provided by the Global Seismograph Network. The results show that the radiated seismic energy is estimated to be 1.6×1013J; the corresponding Me value is 5.9, which is smaller than MW; The difference between the single station value and the average value is less than 0.2 units in the determination of energy magnitude for more than 70% of the stations; Based on the results of the seismic moment obtained by Global Centroid Moment Tensor(GCMT), it can be concluded that the energy-moment ratio is 9.9×10-6, which is considerably weaker than the global average value of (4×10-5) for the strike-slip earthquakes.

    Several previous studies showed that due to the difference in the source rupture process, even earthquakes with small differences in seismic moment and focal mechanism that occurred in the same seismotectonic area may have obviously different amounts of radiated seismic energy. Therefore, we compared this event with Jinggu earthquake that occurred on October 7, 2014. Although the moment magnitudes and focal mechanisms of the two earthquakes are similar, and the epicenter distance is only 290km apart, the area of the same isoseismal for the Jinggu earthquake is significantly larger than the Yangbi earthquake. Earthquake intensity indicates the level of ground shaking, which is associated with the high-frequency components of the seismic waves radiated from the seismic source. Thus, we compared the energy release characteristic of the two earthquakes. The results show that the energy-moment ratio of the Jinggu earthquake(1.58×10-5)is 1.6 times higher than that of this earthquake(9.9×10-6); Comparing with the time-domain analysis alone, the time-frequency analysis performed with the S-transform allows us to quantify important details about the source process information provided by the seismic recordings. According to the results of the time-frequency analysis via the S-transform of these two earthquake waves recorded by the same station, the recording of the Jinggu earthquake had much larger spectral amplitudes at frequencies between 0.5 and 1.5Hz during 30~50s after the P-arrival, while this phenomenon was not observed in the Yangbi earthquake. It can be seen that the high-frequency energy of the Jinggu earthquake is higher than that of the Yangbi earthquake.

    In summary, the Yangbi earthquake belongs to a low energy release efficiency strike-slip event, the difference in seismic energy radiated per unit seismic moment led to the different area of the same isoseismal. The relatively high-frequency seismic wave is firmly related to the near-fault seismic hazard. Therefore, we suggest that apart from the difference in focal depth, geological condition and building structure, the difference in energy release efficiency is one of the reasons for the different areas of the same isoseismal with the earthquakes that have the same moment magnitude, the low energy release of the Yangbi earthquake limits the capacity to generate enormous numbers of casualties, damage to the environment and critical infrastructure.

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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
    Abstract95)   HTML    PDF (3002KB)(138)       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.
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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
    Abstract95)   HTML    PDF (11209KB)(323)       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.
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    A SPECIAL MAGMATISM: PHREATOMAGMATIC ERUPTION
    ZHANG Wen-qian, LI Ni
    SEISMOLOGY AND GEOLOGY    2021, 43 (1): 105-122.   DOI: 10.3969/j.issn.0253-4967.2021.01.007
    Abstract92)   HTML    PDF (2820KB)(78)       Save
    Phreatomagmatic eruption is a kind of special eruption, which usually occurs when hot magma rises and contacts with the ground water. The water/melt interaction produces explosive eruption and base-surge deposits, which resulted in maars. Monogenetic maar is a common volcano type on continents and islands. This kind of volcanoes is widely distributed in many countries. Researchers have studied eruption process and products of phreatomagmatic eruptions by means of petrological, sedimentological, volcanic physical and geochemical methods and techniques. Additionally, they have also explored influence factors over eruption process through experimental and computer simulations.
    Phreatomagmatic eruptions can be considered as the natural equivalent of a class of physical processes termed fuel-coolant interactions(FCI)by investigators of large industrial explosions. Initially, a small volume of water is vaporized due to contact with the melt, with pulsating increasing in the high-pressure steam volume within the aquifer, the dominant effect of the vaporization energy is to fragment the melt and country rocks. Subsequently, the steam, melt, and country rock mix and vapor explosion occurs after vaporization energy increases beyond the confinement strength in chamber. Finally, maar and base-surge deposits form when the overlying layer is broken by the impact of the explosion. Two contrasting environments exist with respect to groundwater availability for the phreatomagmatic explosions. 1)In hard rock environment, the wall rocks are cut by joints and faults, many of which are hydraulically active. Under such groundwater conditions, a maar-diatreme volcano would form. 2)When the magma rises into soft-rock environment rich in water or with high permeability, it will lead to the formation of tuff-rings.
    Maar consists of the crater at the surface(which is cut into the pre-eruption land surface), the tephra ring surrounding the crater and the cone-shaped diatreme, root zone and feeder underlies the maar crater. This tephra ring is easily eroded in the late evolution process, and it usually contains base-surge deposits with obvious dune-like bedding, fallout deposits and individual blocks and bombs that were emplaced ballistically, in which the base-surge deposits are dominant. Besides, the base-surge deposits and individual blocks and bombs are deposited near the crater. Maar lake usually forms in the center of the maar crater. It may form in many years after the phreatomagmatic eruptions. After the eruptions, the maar craters may be filled with groundwater and surface water. Maar lake is different from other crater lakes. For example, its surface of crater is often lower than the pre-eruption surface. In addition, the hydrology and sedimentary environment of maar lake are relatively simple. Archives from sediments of maar lakes, especially annually laminated sediments, will provide high-resolution dataset and are conducive to the study of paleoenvironment and paleoclimate. Compared with the maar-diatreme volcano, the tuff-rings volcano is formed in water-rich and shallower environments and has a wider crater which is not cut into the pre-eruptive land surface. The tuff-rings ejecta usually contain less than 5% of country rock clasts only.
    Base surge is a kind of pyroclastic density currents with great velocity, and it carries debris further than ballistic fragments. Base surge transports lapilli, magma fragments, broken country rocks and ash formed by the explosion. When the base surge flows move, it generates shear force to the lower ground. The base surge can be subdivided into two parts by the interface where the shear stress is zero. The density of lower base surge currents is relatively large and the particles are coarser. In contrast, the upper currents have less density, and the particles accumulate slowly with the decrease of energy. Some indicative sedimentary structures, such as climbing bedding, dune-like bedding, and accretionary lapilli, would form in the base-surge deposits due to their special genetic mechanism. The climbing bedding helps to determine the location of the crater during field investigations. Accretionary lapilli indicate the distant source facies.
    The entire eruption process of phreatomagmatic eruption is relatively complicated. This process may be influenced by several factors, such as the characteristics of the magma, the location and topography of the explosion, the country rocks, and the amount of water involved in the explosion. Foreign scientists have carried out many quantitative studies on the dynamic process of phreatomagmatic eruption through field geology and simulation experiments, while domestic scientists mainly focus on the analysis of the structure, particle size, composition and morphology of base-surge deposit, and the study of the dynamic process is relatively rare. Quantitative studies of the process of phreatomagmatic eruption will be the key in the future research.
    Volcanic hazard is one of the major disasters in the world. Base surge generated by phreatomagmatic eruptions owns great energy and velocity. It would generate great damage to people’s life and the environment due to its special transportation process. Further, volcanoes formed by phreatomagmatic eruptions are common in China and relevant research is important. This paper introduces the research progress concerning phreatomagmatic eruptions and their products, aiming to advance our understanding of this special eruption so as to improve our strategies for preventing future volcanic hazards and protect people’s lives and properties.
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    PROGRESS AND RESEARCH OF PALEOALTITUDE RECON-STRUCTION OF CENOZOIC BASINS IN THE SOUTHEASTERN TIBET PLATEAU
    TANG Mao-yun, LIU-ZENG Jing, LI Cui-ping, WANG Wei, ZHANG Jin-yu, XU Qiang
    SEISMOLOGY AND GEOLOGY    2021, 43 (3): 576-599.   DOI: 10.3969/j.issn.0253-4967.2021.03.007
    Abstract92)   HTML    PDF (6314KB)(139)       Save
    The elevation evolution history of the southeastern Tibet Plateau is of great significance for examining the deformation mechanism of the plateau boundary and understanding the interior geodynamic mechanics. It provides an important window to inspect the uplift and deformation processes of the Tibet Plateau, and also an important way to test two controversial dynamic end-element models of the Plateau boundary. In recent years, some breakthroughs have been made in the study of paleoaltitudes in the southeastern Tibet Plateau, which allows us to have a clearer understanding of its evolution process and dynamic mechanism. By reviewing and recalculation of the latest achievements of paleo-altitude studies of the basins in the southeastern Tibet Plateau from north to south, including the Nangqian Basin, Gongjue Basin, Mangkang Basin, Liming-Jianchuan-Lanping Basin, Eryuan Basin, Nuhe Basin and Chake-Xiaolongtan Basin, we discuss the surface elevation evolution framework of the Cenozoic geomorphology and dynamics in the southeastern Tibet Plateau. The results show as follows:
    (1)There was an early Eocene-Oligocene quasi plateau with an altitude of at least 2.5km from the north to middle of the southeastern Tibet Plateau(north of Dali), while the surface elevation in the south(south of Dali to Yunnan-Guizhou Plateau)was relatively low, even close to sea level. Until Miocene, the north to middle of the southeastern Tibet Plateau reached the present altitude, while the southern part of the Tibet Plateau showed a differential surface uplift trend, which established the present geomorphologic pattern. But it cannot be completely ruled out that this trend was probably caused by the accuracy of the calculation results.
    (2)The quantitative constraints on the uplift process of the southeastern Tibet Plateau during Cenozoic provide certain constraints for the dynamic mechanism of geomorphic evolution in the southeastern Tibet Plateau. The northern and central parts of the southeastern Tibet Plateau can be well explained by the plate extrusion model. In this model, the collision and convergence between India and Eurasia plate or Qiangtang block and Songpan-Ganzi block resulted in the shortening and thickening of the upper crust in the region, and making the early stage(early Eocene)surface uplift. Subsequently, due to delamination or the continuous convergence between the Qiangtang block and the Songpan-Ganzi block resulting in the shortening and thickening of the crust, the plateau continued to grow northward and rose to its present altitude around Miocene. In the Eocene, the area from the south of the southeastern Tibetan plateau to the Yunnan-Guizhou Plateau mainly showed a low altitude. It seems that it may be in the peripheral area not affected by the shortening and thickening of the upper crust during the early stage India-Eurasia plate collision or plate extrusion and escape. In addition, as proposed by the lower crustal channel flow model, the lower crust material made the low-relief upland surface extending thousands of kilometers in the region uplift gradually towards the southeast, which seems to explain the low elevation landform of the region in the early stage, but it could not explain the whole uplift process of the southeastern Tibet Plateau. Therefore, a single dynamic model may not be able to perfectly explain the Cenozoic complex uplift process of the southeastern Tibet Plateau, and its process may be controlled by various dynamic processes.
    (3)According to the paleoaltitude reconstruction results, if most areas of the ancient southeastern Tibet Plateau, especially the area to the north of Jianchuan Basin, had been uplifted in a certain scale and became part of the early plateau in the early Cenozoic, and reached to the current surface altitude around Miocene, the widely rapid surface erosion in this area since Miocene probably would be a continuous lag response to the finished surface uplift process, and the lag time may correspond to the sequential response process of surface uplift, the decline of river erosion base level and the gradual enhancement of river erosion capacity. Therefore, it is not proper to regard the rapid denudation and rapid river undercutting as the starting time of plateau uplift, as proposed in the previous thermochronological study.
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    GEOMAGNETIC FIELD CHANGE BEFORE AND AFTER 2021 YANGBI M S6.4 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
    Abstract91)   HTML11)    PDF (3460KB)(47)       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.

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    A SEISMIC DAMAGE IDENTIFICATION METHOD BASED ON IMPROVED ANT COLONY ALGORITHM AND UNMANNED AERIAL VEHICLE IMAGES AND ITS APPLICATION TO YANGBI EARTHQUAKE
    DU Hao-guo, LIN Xu-chuan, ZHANG Jian-guo, DU Hao-biao, ZHANG Fang-hao, DU Zhu-quan, LU Yong-kun, DAI Bo-yang
    SEISMOLOGY AND EGOLOGY    2021, 43 (4): 1013-1029.   DOI: 10.3969/j.issn.0253-4967.2021.04.018
    Abstract87)   HTML9)    PDF (12917KB)(45)       Save

    Earthquake is one of the most destructive natural disasters, it can not only cause heavy casualties and economic losses, but also may even lead to serious secondary disasters. As the main bearing body in earthquake, buildings often suffer serious damage, so they can be used as an important reference for post-earthquake disaster loss assessment. Timely and accurate acquisition of regional earthquake damage information after an earthquake is of great significance for scientific and effective emergency rescue and disaster loss assessment. At present, the main methods for earthquake damage identification can be roughly divided into two categories: 1) Manual visual interpretation investigation method. It takes a lot of time for manual field investigation or manual identification of earthquake damage images to process a large amount of seismic damage information in a short period of time, and it is likely to lead to inconsistent discrimination standards for seismic damage of buildings. 2)Image recognition method based on satellite image or UAV image. The recognition method based on satellite remote sensing image after the quake identifies earthquake damage by the texture, brightness and other characteristics of the image of the seriously collapsed buildings, thus, it can quickly get the seismic damage situation in a large area, but as results of offset, low resolution and poor timeliness of the satellite image, it is hard to identify the slightly overlaying and cracking of tiles on the roof of buildings. The combination of high-resolution image obtained by UAV and machine learning algorithm can not only reduce the labor input, but also bring a high accuracy rate. Therefore, based on ant colony algorithm(ACO)and high-resolution remote sensing image of UAV, this paper proposes a new method to efficiently identify the earthquake damage of buildings in the study area, which was applied and verified in the recent Yangbi M6.4 earthquake in Yunnan Province. By improving the update strategy of pheromone concentration in ant colony algorithm and introducing the optimization operator, the better identification rules are established, and the speed and accuracy of earthquake damage identification are enhanced. The UAV high-resolution image of Yangbi county seat was obtained the first time after the Yangbi, Yunnan Province, M6.4 earthquake took place, and taking the image as experimental data, the extraction effect of regional earthquake damage is verified, and compared with ant colony algorithm and maximum likelihood method. The results show that the proposed earthquake damage identification method based on improved ant colony algorithm and UAV high-resolution image can effectively improve the identification accuracy and efficiency of damaged buildings in the region, which is of great significance for post-earthquake emergency rescue and providing accurate disaster information.

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    A STUDY ON THE SEISMOGENIC STRUCTURE OF GAOYOU-BAOYING M S4.9 EARTHQUAKE
    ZHAO Qi-guang, SUN Ye-jun, HUANG Yun, YANG Wei-lin, GU Qin-ping, MENG Ke, YANG Hao
    SEISMOLOGY AND GEOLOGY    2021, 43 (3): 630-646.   DOI: 10.3969/j.issn.0253-4967.2021.03.010
    Abstract86)   HTML    PDF (10586KB)(72)       Save
    The Gaoyou-Baoying M S4.9 earthquake on July 20, 2012 occurred in the Gaoyou Sag in the Subei Basin. This earthquake was a relatively rare medium-strength earthquake in the weak seismicity region of eastern China. Although studies on the seismogenic structure of this earthquake have been conducted previously, the seismogenic structure itself is still under debate and needs to be further studied. This paper uses the methods such as distribution of seismic intensity, precise positioning of earthquake sequence, focal mechanism, regional tectonic stress, seismic exploration, etc. to comprehensively study the seismogenic structure of this earthquake.
    The characteristics of earthquake sequence show that the seismic structure is a high dip-angle fault spreading along the NNE direction, dipping ESE. The result of focal mechanism solutions shows that the strike of one of the two nodal planes is NNE, and the fault plane shows high dip angle. The earthquake is mainly characterized by strike-slip motion. Through the seismic exploration lines(GYL1, GYL2)laid at the epicenter area of the earthquake, a fault structure is identified, which strikes nearly NNE and dips near ESE. This fault is located between the Linze sag and the Liubao low uplift, coinciding with the distribution of the Liuling Fault, the boundary fault in the northwest of the Gaoyou Sag, so it can be judged that all the detected breakpoints belong to the Liuling Fault. The “Y-shaped” breakpoints detected by the two seismic exploration lines are characterized by high dip angle. There is a very obvious wave group disorder area at the distance of 6 500~9 000m on the GYL1 seismic exploration line. This area is about 2.5km in width displayed on the post-stack migration profile and shows an uplifting trend. The disordered uplifting of wave group is caused by intrusion of soft material into the structural breakage and weakness, squeezed by horizontal stress. The GYL2 post-stack migration profile shows obvious uplift appearing in the reflection wave group(T g)on the top of the bedrock. This arc-shaped uplift also reflects the effect of strong compression of horizontal stress.
    In order to further discuss the seismogenic structure of the Gaoyou-Baoying M S4.9 earthquake, we used the focal mechanism data to invert the modern tectonic stress field in the Northern Jiangsu-South Yellow Sea Basin where the earthquake occurred. The maximum principal stress in this area is NE-SW, while the minimum principal stress is NW-SE; both of them are nearly horizontal, and the intermediate principal stress is nearly vertical. According to Zoback's rule for dividing the types of dislocation in the direction of the force axis, the distribution of principal stresses in the Northern Jiangsu-South Yellow Sea Basin is equivalent to a strike-slip dislocation.
    To sum up, the stress characteristics reflected by the Liuling Fault are consistent with the horizontal forces on the P-axis and T-axis shown by the focal mechanism solution results, and also consistent with the horizontal state of the stress in the tectonic stress field in this region. The above characteristics indicate that the development of the Liuling Fault is affected and controlled by modern tectonic activities. At the same time, the characteristics of the strike and dip of the seismic fault reflected by the methods of seismic intensity investigation, precise earthquake positioning, focal mechanism solution and seismic exploration, etc. are consistent with each other. Therefore, the occurrence of this earthquake may be the result of continuous stress accumulation and sudden instability and rupture of the NNE-trending Liuling Fault under the long-term compression of the NE-direction principal stress.
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    HYDROGEOCHEMICAL CHARACTERISTICS OF GROUND-WATER IN THE SURFACE RUPTURE ZONE OF MADOI M S7.4 EARTHQUAKE AND HOT SPRINGS IN THE EAST KUNLUN FAULT
    LU Chang, ZHOU Xiao-cheng, LI Ying, LIU Lei, YAN Yu-cong, XU Yue-ren
    SEISMOLOGY AND EGOLOGY    2021, 43 (5): 1101-1126.   DOI: 10.3969/j.issn.0253-4967.2021.05.005
    Abstract85)   HTML8)    PDF (5963KB)(47)       Save

    Spring water is strongly related to earthquake, and groundwater within fault zone carries a large amount of information about the water-rock response and tectonic activity. Meanwhile, hydrogeochemical monitoring in the area of strong seismic activity could well obtain the precursor information related to earthquake. Therefore, it is essential to analyze the sources and characteristics of hydrogeochemistry in areas of strong earthquakes. The Bayankara Block is a rectangular active block in the east-central part of the Tibetan plateau. In recent years, the perimeter of the block is undergoing a period of moderate to strong seismic activity and has become the major area of seismicity in mainland China. However, due to the tough geological conditions surrounding the Madoi area, little has been reported on water chemistry, and the geochemical background fields have yet to be established and identified.
    On 22 May 2021, an earthquake of MS7.0 struck Madoi County, Qinghai Province, the largest magnitude earthquake in China since the 2017 Jiuzhaigou MS7.4 earthquake. After the earthquake, a near NWW-SEE surface rupture zone was formed, with a rupture area of about 70km, along which tension fissures, sand liquefaction, sand blasting and water bubbling can be seen, and there are cold springs upwelling near the surface rupture zone. One day after the earthquake, 21 water chemistry samples were taken. They are the water bubbling from the earthquake rupture zone and the hot springs near the East Kunlun fault zone, as well as 4 sandy soil samples from post-earthquake sandblasting and water bubbling sites. The ordinary and minor ionic components of spring water and stable isotopes of δD, δ18O and 87Sr/86Sr were analyzed. Percentage of oxides in sand particles was also analyzed. The sources and characteristics of spring water and sandy soils were researched, and the differences between the groundwater in surface rupture zone and the geothermal water near the East Kunlun Fault are discussed. The results show that: 1)The range of TDS of the 21 springs is 113.2~1 264.6mg/L, pH values range from 7.6 to 8.3, conductivity ranges from 200.3 to 865.7μs/cm, and temperatures range from 3 to 49℃. The spring water samples near the surface rupture zone are all from cold springs(3 to 11℃). The degree of water-rock reaction is weak. The chemistry types of spring water are Ca·Mg-HCO3, Ca·Mg·Na-HCO3, Ca-HCO3, Na·Ca·Mg-HCO3·Cl, Ca·Na·Mg-HCO3·SO4, Ca·Na·Mg-HCO3·SO4 and Ca·Na-HCO3. Calcium, magnesium and bicarbonate ions are the main ions of the spring. 2)The range of spring water average recharge elevation in the region is 0.8~2.8km. There is an abnormal hydrogen isotope value(δD=-59‰)in the spring water near the epicenter in the surface rupture zone, and Na+, Cl-, $SO_{4}^{2-}$ and other ions have high values. 3)Overall, the springs do not contain high concentrations of elements such as Ca and Sr, and most elements have EF<1, which may be related to the weak degree of water-rock reaction in the springs. Lithium in springs near the East Kunlun fault zone(maximum value of 2 014μg/L)is much greater than in springs around the surface rupture zone(6.56~43.0μg/L); and metallic trace elements of Pb, Ba, Cu, and Zn are more enriched in springs around the surface rupture zone. 4)The source of the spring water is meteoric water, and the spring water near the surface rupture zone is mixed with the surrounding water, and the results of water temperature, γNa/γCl, and elements from mantle in the East Kunlun fault zone reveal that the hot spring water circulation is deeper in the East Kunlun fault zone, with faults cutting deeply and deeper elemental recharge. The Cl- and(Na++K+)concentrations in the spring near the surface rupture zone are significantly higher than those near the East Kunlun fault zone, where the springs are more enriched in δD and δ18O.
    The hydrochemical characteristics and sources of the samples are discussed and the fluid geochemical differences between the two areas are compared, and the sources of the sand samples that emerged after the earthquake are analyzed. The paper concludes that it is of great significance for earthquake risk assessment of the East Kunlun Fault to carry out hydro-geochemical monitoring and further study of hot springs in the East Kunlun Fault in the future. The paper fills the gap of background groundwater data in the region, meanwhile, discusses the response of water chemistry after the earthquake and the characteristics and sources of water chemistry in the middle Bayan Kara block.

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    TECTONIC GEOMORPHIC FEATURES AND GEOLOGICAL SIGNIFICANCE OF THE SHIDIQUAN ANTICLINE IN THE NORTHERN MARGIN OF THE QAIDAM BASIN
    DONG Jin-yuan, LI Chuan-you, ZHENG Wen-jun, LI Tao, LI Xin-nan, REN Guang-xue, LUO Quan-xing
    SEISMOLOGY AND GEOLOGY    2021, 43 (3): 521-539.   DOI: 10.3969/j.issn.0253-4967.2021.03.004
    Abstract83)   HTML    PDF (11794KB)(261)       Save
    In the process of intense compression and shortening of the orogenic belt, a series of thrust faults and folds related to reverse faults developed in the piedmont. Determining the kinematic characteristics of these reverse faults and folds is of great significance for understanding the deformation mode of the orogenic belt. The Qilian Shan is located on the northeastern margin of the Tibetan plateau and is the front edge of the plateau expansion. The area has undergone strong tectonic activity since the Late Quaternary, with developed active structures and frequent earthquakes. There are a series of piedmont thrust faults and thrust related folds in the northern and southern margins of Qilian Shan. Compared with a large number of research results of active folds in Tian Shan area, the study of active folds in Qilian Shan is relatively weak. In the northern margin of the Qilian Shan, in addition to the study of individual active folds, most previous studies focused on the thrust faults in the northern margin of the Qilian Shan and the Hexi Corridor, and obtained the active characteristics of these faults. In the southern margin of Qilian Shan, that is, the northern margin of the Qaidam Basin, some studies have been carried out on paleoearthquakes and slip rate of the fault in the southern margin of Zongwulong Shan. However, the study on the late Quaternary folds in this area is relatively weak and there are only some sporadic works.
    Shidiquan anticline is located in the intermountain basin surrounded by Zongwulong Shan and Hongshan in the northern margin of Qaidam Basin. It forms the first row fold structure in front of Zongwulong Shan with Huaitoutala and Delingha anticline. Constraining the tectonic geomorphic features of the Shidiquan anticline is of great significance for studying the crustal shortening in the northern margin of the Qaidam Basin and the expansion of the Qilian Shan to the Qaidam Basin. In this paper, the tectonic and geomorphic characteristics of Shidiquan anticline are obtained by means of geological mapping, high-precision differential GPS topographic profile survey, geological profile survey and cosmogenic nuclide dating. Field investigation shows that Shidiquan anticline is an asymmetric fold with steep south limb and gentle north limb, and is controlled by a blind reverse fault dipping northward. The age of the alluvial fan3 obtained from cosmogenic nuclide dating is(158.32±15.54)ka. This age coincides with the Gonghe Movement, indicating that the formation of Shidiquan anticline responds to the Gonghe Movement in the northeast margin of Tibetan plateau. The uplift rate of Shidiquan anticline since 158ka is(0.06±0.01)mm/a, and the shortening rate is(0.05±0.01)mm/a. The folding effect of Shidiquan anticline indicates that the folding of the intermountain basin in the northern margin of the Qaidam Basin, similar to the thrust shortening of the piedmont fault, plays an important role in regulating the shortening of the foreland crust.
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    SIMULATION OF POST-SEISMIC EFFECTS OF THE MADUO M S7.4 EARTHQUAKE IN 2021
    TAN Hong-bo, WANG Jia-pei, YANG Guang-liang, CHEN Zheng-song, WU Gui-ju, SHEN Chong-yang, HUANG Jin-shui
    SEISMOLOGY AND EGOLOGY    2021, 43 (4): 936-957.   DOI: 10.3969/j.issn.0253-4967.2021.04.013
    Abstract82)   HTML13)    PDF (16933KB)(275)       Save

    Using the fault model issued by the USGS, and based on the dislocation theory and local crust-upper-mantle model layered by average wave velocity, the co-seismic and post-seismic deformation and gravity change caused by the 2021 Maduo MS7.4 earthquake in an elastic-viscoelastic layered half space are simulated. The simulation results indicate that: the co-seismic deformation and gravity change show that the earthquake fault is characterized by left-lateral strike-slip with normal faulting. The changes are concentrated mainly in 50km around the projection area of the fault on the surface and rapidly attenuate to both sides of the fault, with the largest deformation over 1 000mm on horizontal displacement, 750mm on the vertical displacement, and 150μGal on gravity change. The horizontal displacement in the far field(beyond 150km from the fault)is generally less than 10mm, and attenuates outward slowly. The vertical displacement and gravity change patterns show a certain negative correlation with a butterfly-shaped positive and negative symmetrical four-quadrant distribution. Their attenuation rate is obviously larger than the horizontal displacement, and the value is generally less than 2mm and 1 micro-gal. The post-seismic effects emerge gradually and increase continuously with time, similar to the coseismic effects and showing an increasing trend of inheritance obviously. The post-seismic viscoelastic relaxation effects can influence a much larger area than the co-seismic effect, and the effects during the 400 years after the earthquake in the near-field area will be less than twice of the co-seismic effects, but in the far-field it is more than 3 times. The viscoelastic relaxation effects on the horizontal displacement, vertical displacement and gravity change can reach to 100mm, 130mm and 30 micro-gal, respectively. The co-seismic extremum is mainly concentrated on both sides of the fault, while the post-earthquake viscoelastic relaxation effects are 50km from the fault, the two effects do not coincide with each other. The post-seismic horizontal displacement keeps increasing or decreasing with time, while the vertical displacement and gravity changes are relatively complex, which show an inherited increase relative to the co-seismic effects in the near-field within 5 years after the earthquake, then followed by reverse-trend adjustment, while in the far-field, they are just the opposite, with reverse-trend adjustment first, and then the inherited increase. The horizontal displacement will almost be stable after 100 years, while the viscoelastic effects on the vertical displacement and gravity changes will continue to 300 years after the earthquake. Compared with the GNSS observation results, we can find that the observed and simulated results are basically consistent in vector direction and magnitude, and the consistency is better in the far-field, which may be related to the low resolution of the fault model. The simulation results in this paper can provide a theoretical basis for explaining the seismogenic process of this earthquake using GNSS and gravity data.

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    AN EXPERIMENTAL STUDY ON THE TRANSIENT CREEP OF GRANITE
    NIU Lu, ZHOU Yong-sheng, YAO Wen-ming, MA Xi, HE Chang-rong
    SEISMOLOGY AND GEOLOGY    2021, 43 (1): 20-35.   DOI: 10.3969/j.issn.0253-4967.2021.01.003
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    Many of the large earthquakes in the continental crust nucleate at the bottom of the seismogenic zone in depths between 10 and 20km which is related to the broad so-called ‘brittle-to-plastic or brittle-to-ductile’ transition region. From the field studies and seismic data, we could know that the dominant deformation mechanism at the base of seismogenic zone is likely to be semi-brittle flow of fault rocks. The physical and chemical processes acting in the ‘brittle-to-plastic’ transition are of great interest for a better understanding of fault rheology, tectonic deformation of the continental lithosphere and the generation of strong earthquakes. So it’s of great significance to know more about this transition. Despite the importance of semi-brittle flow, only few experimental studies are relevant to semi-brittle flow in natural rocks. In order to study the semi-brittle deformation and rheological characteristics of granite, we performed a series of transient creep experiments on fine-grained granite collected from the representative rock of Pengguan Complex in Wenchuan earthquake fault area using a solid-medium triaxial deformation apparatus(a modified Griggs rig). The conditions of the experiments are under the temperatures of 190~490℃and the confining pressures of 250~750MPa with a strain rate of 5×10 -4s -1. The temperature and pressure simulate the in-situ conditions of the Wenchuan earthquake fault zone at the corresponding depths of 10~30km. We observe the microstructures of the experimentally deformed samples under the scanning electron microscope(SEM). The mechanical data, microstructures and deformation mechanism analysis demonstrate that deformation of the samples with experimental conditions could be covered by three regimes: 1)Brittle fracture to semi-brittle flow regime. We could see the strain and stress curves of the samples characterizing with strain hardening behavior and without definite yield point under low temperatures and pressures, which correspond to the depths of 10~15km; 2)Brittle-ductile transition regime. The strain and stress curves of the samples tend to be in a steady state with definite yield point under temperature and pressure at the depths of 15~20km. The main deformation mechanism is cataclasis, and dynamic recrystallization and dislocation creep are activated; and 3)Ductile flow regime which is at depths of 20~30km. The strength of granite increases with depth and reaches to the ultimate at the depth of 15~20km, and then decreases with depth at 20~30km. Based on the analysis of strength of granite, microstructures and deformation mechanism, we conclude that the granitic samples deformed with the characteristics of transient creep, and the strength of Longmenshan fault zone reaches maximum at the depths of 15~20km where it is in the brittle-to-plastic regime. Based on the Mohr circle analysis, the rupture limit at depths of 15~20km is close to the limit of friction, and at the same time, this depth range is also consistent with the focal depth of Wenchuan earthquake. Therefore, it implicates that the deformation and strength of Pengguan complex granitic rocks should control the nucleation and generation of the Wenchuan earthquake.
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    A TYPICAL THRUST RUPTURE EVENT OCCURRING IN THE FORELAND BASIN OF THE SOUTHERN TIANSHAN: THE 2020 XINJIANG JIASHI M S6.4 EARTHQUAKE
    ZHANG Wen-ting, JI Ling-yun, ZHU Liang-yu, JIANG Feng-yun, XU Xiao-xue
    SEISMOLOGY AND GEOLOGY    2021, 43 (2): 394-409.   DOI: 10.3969/j.issn.0253-4967.2021.02.009
    Abstract79)   HTML    PDF (6361KB)(79)       Save
    A M S6.4 earthquake occurred on January 19 th, 2020 at Jiashi, Xinjiang, this earthquake is another strong earthquake since the Jiashi M S6~7 earthquake swarm events from 1997 to 2003, and the epicenter was located near the Kalpin nappe in the western part of southern Tianshan. The Kaplin nappe is located in front of southern Tianshan Mountains, which is a thin skinned thrust belt composed of a series of nearly NE-SW thrust nappes under the strong and sustained regenerative orogeny in the Tianshan area. There are some differences in focal positions and fault parameters given by different institutions, therefore in this paper, high resolution InSAR coseismic deformation fields were obtained based on the ascending and descending tracks of Sentinel-1 SAR images to obtain the focal mechanism. The 30m resolution SRTM DEM data is chosen as the external DEM to eliminate the phases caused by topography, the robust Goldstein filtering is applied for phase smoothing, and the Delaunay minimum cost flow method is used for phase unwrapping. The variation range of interference fringes shows that the east-west span of the earthquake deformation field is about 40km, and that of the north-south direction is about 20km, the displacement results show that the maximum uplift displacement is 5.9cm and the maximum subsidence is 3.7cm along the LOS direction of the ascending data, the maximum uplift displacement is 6.4cm and the maximum subsidence is 2cm along the LOS direction of the descending data. And then the InSAR-derived deformation fields are used to obtain the seismogenic mechanism of this earthquake, and to improve the computational efficiency, the quadtree segmentation method is used to desample the original high-resolution InSAR observations before inversion. The coseismic slip distribution of the causative fault was inversed using a uniform sliding inversion method based on a Bayesian approach, and then the fine slip distribution of the fault plane of Jiashi earthquake was inversed using the distributed slip inversion method based on the constrained least squares. It should be noted that the fault plane is set as the shovel shape according to the geometric relationship between the seismogenic fault parameters inverted by uniform sliding and the exposed position of the Kapling Fault on the surface during the distributed slip inversion. According to the difference between the observed and simulated values, it can be seen that the residual error of the inversion model is small, indicating the reliability of the inversion result. The final result shows that the epicenter is located at 39.9°N, 77.28°E and the strike and dip angle of the seismogenic fault is 276° and 10.7°, respectively, the maximum dip slip and strike slip of fault plane is about 0.29m and 0.03m, respectively, which are located at the depth of about 5km underground. The cumulative coseismic moment is 1.73×10 18N·m from InSAR inversion, which is equal to the moment magnitude of M W6.1 and the Kalpin Fault is supposed to be the causative fault. Then, regional GPS-derived surface strain rate, tectonic dynamic background, and regional deep and shallow structures were comprehensively analyzed. The results show that the Jiashi M S6.4 earthquake is a typical thrust event that occurred in the thrust nappe of the southern Tianshan. The 2020 Jiashi event and the 1997—2003 Jiashi M6~7 earthquakes swarm are the results of rupture of many faults with different scales and properties. And these events are all controlled by the thrust nappe of southern Tianshan.
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    PALEOSEISMOLOGIC STUDY ON THE SHIMIAN FAULT IN THE NORTHERN SECTION OF THE DALIANGSHAN FAULT ZONE
    FENG Jia-hui, CHEN Li-chun, WANG Hu, LIU Jiao, HAN Ming-ming, LI Yan-bao, GAO Shuai-po, LU Li-li
    SEISMOLOGY AND GEOLOGY    2021, 43 (1): 53-71.   DOI: 10.3969/j.issn.0253-4967.2021.01.004
    Abstract78)   HTML    PDF (15854KB)(208)       Save
    The Daliangshan fault zone(DF)constitutes an important part of the large-scale strike-slip Xianshuihe-Xiaojiang fault system(XXFS). Affected by the channel flow of the middle-lower crust in the western Sichuan region, the XXFS is strongly active, and large earthquakes occur frequently. On average, there is an earthquake of magnitude 7 or more every 34 years. However, the DF, as an important part of the middle segment of the XXFS, has only recorded several earthquakes with magnitude 5-6, and no earthquakes with magnitude over 6 have been recorded. The reason for the lack of strong earthquake records may be related to the lack of historical records in remote mountainous areas, but the main reason may be attributed to the active behavior of the faults. He et al.(2008)hold that the DF is a new fault, resulting from straightening of the middle section of the XXFS, and its activity gradually changes from weak to strong, and will probably replace the Anninghe-Zemuhe Fault. However, this view lacks evidence of strong earthquakes. In recent years, some scholars have studied the paleoearthquakes on the DF, and found the signs of strong earthquake activity, and considered that the fault has the seismogenic capacity of earthquakes with magnitude more than 7. These studies are mainly concentrated in the middle and southern segments of the DF. Although there are scattered activity data and individual trench profiles, direct evidence of Holocene activity and paleoearthquake data are very scarce in the northern part of DF. On the basis of the previous studies, combined with our detailed field geomorphological surveys, we excavated a set of two trenches at Lianhe village in Shimian Fault to reveal the direct evidence of fault activity in Holocene. From paleoseismic analysis and radiocarbon samples accelerated mass spectrometry(AMS)dating, four paleoseismic events are identified, which are E1 between 20925—16850BC, E2 between 15265—1785BC, E3 between 360—1475AD, and E4 between 1655—1815AD. The results of the latest two events should be relatively reliable, and the latest event may be related to the Moxi earthquake of magnitude 7 3/4 on June 1, 1786 or the Dalu earthquake of magnitude ≥7 on June 10, 1786. Among the four events revealed, three are since the Holocene, and the recurrence interval of the latest two events is about 800 years. Compared with other active faults at the triple junction, the recurrence interval is slightly longer than that at the northern segment of the Anninghe fault zone, but close to that at the Moxi segment of the Xianshuihe fault zone. Compared with the western segment of Xianshuihe Fault and the northern segment of Anninghe Fault, the Shimian Fault also has a higher seismic risk, which needs further attention.
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    ANALYSIS OF STRESS INFLUENCE OF QINGHAI MADUO M S7.4 EARTHQUAKE ON SURROUNDING FAULTS
    YUE Chong, QU Chun-yan, NIU An-fu, ZHAO De-zheng, ZHAO Jing, YU Huai-zhong, WANG Ya-li
    SEISMOLOGY AND EGOLOGY    2021, 43 (5): 1041-1059.   DOI: 10.3969/j.issn.0253-4967.2021.05.001
    Abstract78)   HTML6)    PDF (8146KB)(89)       Save

    The seismogenic fault of the Maduo MS7.4 earthquake in Qinghai Province on May 22, 2021 is not on the conventionally north boundary of the Bayan Har Block, but a secondary fault named Kunlunshankou-Jiangcuo Fault inside the Bayan Har Block which is nearly parallel to the East Kunlun Fault, with a distance of about 70km. As a result, the study on the stress effect of the Maduo earthquake on surrounding faults is urgent, especially on the main boundary faults of the Bayan Har Block, such as the East Kunlun Fault. In this paper, the lithospheric structure of the study area is stratified by using the USTClitho1.0 results of the unified seismic velocity model of the lithosphere in Chinese mainland. The co-seismic slip model of the Maduo earthquake is inversed by the results of InSAR deformation field and precise aftershock location. The model reveals that the coseismic slip of this earthquake is mainly sinistral strike-slip, the fault strike is 276 degrees, the dip angle is 80 degrees, the average rake angle is 4 degrees, the maximum slip is about 5.1m, and the main slip area is mainly concentrated on the depth of 0~15km. By considering the Burgers rheological model which is more consistent with the actual deformation process of lithosphere, the paper calculates the co-seismic Coulomb stresses and viscoelastic Coulomb stresses in the source area and peripheral faults induced by the Maduo earthquake by using PSGRN/PSCMP program.
    The results show that, besides the fracture surface of the seismogenic fault, there are three positive co-seismic Coulomb stress change areas on the west and east ends of the seismogenic fault, of which the stress loading area on the west end is oriented toward the northwest of the seismogenic fault, and the other two stress loading areas on the east end are toward the north and east of the seismogenic fault. The positive section of co-seismic Coulomb stress change of the peripheral faults is consistent with the distribution of the source area. The co-seismic Coulomb stress change induced by Maduo earthquake is bigger than 0.01MPa on the near source section of East Kunlun Fault, the east section of Kunzhong Fault, the northwest segment of Gande-Nanyuan Fault and the middle segment of Wudaoliang-Changshagongma Fault. The maximum co-seismic Coulomb stress changes at the depth of 12.5km reach 0.165MPa, 0.022MPa, 0.102MPa and 0.012MPa, respectively, which proves that the Maduo MS7.4 earthquake has a strong seismic triggering effect on the above faults. By comparison, the impact of Maduo MS7.4 on co-seismic Coulomb stress change is also positive in the middle section of Longriba Fault, the south section of Xianshuihe Fault and the north section of Longmenshan Fault, but the magnitude is relatively smaller(less than 0.01MPa), in which the co-seismic Coulomb stress change in the middle section of Longriba Fault increases by thousands of Pa, while the co-seismic Coulomb stress change in the south section of Xianshuihe Fault and the north section of Longmenshan Fault increases by only tens to hundreds of Pa.
    For the fault sections with co-seismic Coulomb stress change bigger than 0.01MPa mentioned above, their viscoelastic Coulomb stress changes during 50 years are calculated. The results show that the viscoelastic relaxation of lithosphere after the Maduo earthquake further increases the viscoelastic Coulomb stress changes on the above faults, especially the East Kunlun Fault, where the cumulative Coulomb stress will be increased by 0.038MPa after 50 years. The seismic triggering effect of Maduo earthquake on the above faults will continue to increase over time and more attention should be paid to the seismic risk of the above faults in the future.

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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
    Abstract76)   HTML    PDF (11430KB)(183)       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.
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    ANALYSIS ON FORMATION MECHANISM OF TYPICAL VARIATIONS OF GEO-RESISTIVITY AT BAOCHANG STATION
    DAI Yong, GAO Li-xin, YANG Yan-ming, WEI Jian-min, Ge-gen
    SEISMOLOGY AND GEOLOGY    2021, 43 (3): 647-662.   DOI: 10.3969/j.issn.0253-4967.2021.03.011
    Abstract72)   HTML    PDF (3248KB)(103)       Save
    Fixed-electrode quasi-Schlumberger arrays are mainly used in geo-electric observation of China earthquake networks. The distance between power supply poles is generally about 1km. The detection depth is estimated to be within 0.705km by conventional geophysical and electrical methods in homogeneous medium. The resistivity at seismic station for precursor information monitoring reflects the overall electrical characteristics within the detection range below the polar distribution area, which is also known as apparent resistivity or geo-resistivity. Due to the small distance between power supply poles, small detection depth and great influence from shallow layer, there are usually annual, diurnal and step variations in geo-resistivity curves. Because of the above variations, the characteristics of abnormal variations before earthquakes are usually not obvious, or even annihilated. In this paper, taking Baochang station as an example, the causes of long-term, annual, diurnal and step variations are analyzed by inversion and numerical simulation. Baochang station is located in Baochang Town, Taipusi Banner, Xilin Gol League, Inner Mongolia. Its geographical coordinates are 41.9°N and 115.3°E. The regional geological structure is the eastern segment of Inner Mongolia axis, the fourth-order structural unit. The nearest fault structure is the Chifeng-Kaiyuan Fault, which is the northern boundary fault of North China fault-block region. The resistivity of geo-electric survey area at Baochang station basically presents horizontal distribution characteristics, and the type of electric sounding curve is KH. The inversion results show that the vertical profile of the survey area is divided into four layers: the first layer is frozen soil layer with depth from 0m to 1m, the second layer is sand gravel layer with depth from 1m to 6.5m, the third layer is aquifer with depth from 6.5m to 71.5m, and the fourth layer is quartz porphyry layer with depth greater than 71.5m. When power supply electrode distance AB is 560m and measuring electrode distance MN is 80m, the one dimensional influence coefficients of NS and EW direction in the third layer are all over 0.9, which is one order of magnitude larger than those in the other three layers. This indicates that the variation of resistivity in the range of 7m to 71m can effectively reflect the variation of geo-resistivity. Since 1993, the geo-resistivity at Baochang station has been declining for a long time in NS and EW direction, and the variation rate shows obvious anisotropic characteristics, which is mainly the result of the continuous effect of regional stress on the resistivity of the third layer. There is a normal annual variation pattern of “high in winter and spring, low in summer and autumn” in both directions of geo-resistivity at Baochang station, resulting mainly from the seasonal variation of temperature and rainfall on the resistivity of the first layer. The normal diurnal variation of geo-resistivity at Baochang station is characterized by “high in the morning, low in the afternoon and night”, which is mainly caused by the influence of temperature on surface resistivity. Similar diurnal variation also exists in the hourly value curves of geo-resistivity at the stations of Xiaomiao, Ganzi, Wujiahe, Qingguang and Baodixintai. The geo-resistivity step variation of Baochang station has the characteristics of “low frequency in winter and spring, high frequency in summer and autumn”, and most of them coincide with rainfall, pumping, embedding steel strand, etc. The results of experiments and numerical simulation show that the above factors are the main interference sources of the geo-resistivity step variations.
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    THE LATEST ACTIVITY OF SUDIAN FAULT ON THE BORDER BETWEEN CHINA AND MYANMAR AND ITS TECTONIC SIGNIFICANCE
    CHANG Zu-feng, CHANG Hao, MAO Ze-bin, LUO Lin, WANG Qi
    SEISMOLOGY AND GEOLOGY    2021, 43 (3): 559-575.   DOI: 10.3969/j.issn.0253-4967.2021.03.006
    Abstract71)   HTML    PDF (13466KB)(53)       Save
    The Sudian Fault extends in nearly NS direction and crosses the border between China and Myanmar, with a length of about 100km. Historically, many earthquakes have occurred along the fault. However, restricted by traffic, climate and other factors, there has been little research on the late Quaternary activity of the fault for a long time. On the basis of results of field geological and geomorphological investigation, trenching and geochronology, the movement characteristics of the fault in late Quaternary, the latest active age and sliding rate are analyzed in this paper. The Neotectonic activity of the Sudian Fault is obvious. Beaded Quaternary basins in areas of Sudian, Mengdian, Huangcaoba and Longzhong have developed along the fault. Many boiling springs and gas springs are distributed linearly in the area of Humeng in the south section of the fault. The fault controls the Lama River and Zhanda River obviously. Fault landforms are mainly characterized by clear fault scarps, straight linear ridges and fault valleys. Mengdian pull apart basin is developed in the middle segment of Sudian Fault. In the Zuojiapo area of the western margin of the basin, there is a clear linear ridge about 1.7km long and a parallel fault valley which is close to the west side of the linear ridge. Trench excavation was carried out in this fault valley(24.97°N, 97.93°E). Zuojiapo trench reveals that three faults have developed in Quaternary deposits. At the position of 2~3m(from west to east)on the S wall of the trench, a fault dislocated all the strata(unit②~unit⑥)below the modern loam layer(unit①). These strata are obviously offset and some of them are cut off. The 14C age of the displaced unit ④(tested by BETA laboratory, USA)is(7 680±30)a, two 14C ages of the displaced unit ③are(6 970±30)a and(5 860±30)a, and the 14C age of the displaced unit ②is(1 260±30)a. The fault developed at 21m in the east section of S-wall of the trench has offset the lower bedrock(unit⑧), the middle gravel layer(unit⑤and unit⑦), the upper dark gray gravelly clay layer(unit④)and the peat interlayer(unit④'). In the peat interlayer(unit④'), there is obvious structural deflection deformation, and its 14C age is(350±30)a. There is another fault developed at 26~27m in the east section of S-wall of this trench, which cuts off the light yellow and light gray gravelly clay(unit ②), gray black gravelly clay(unit③), gray white sandy gravel(unit⑤), yellow gravelly silty clay(unit ⑥), yellow clay gravel(unit ⑦)and hornblende schist and quartz schist of Gaoligongshan group(unit ⑧). The fault shows obvious normal fault property, and the maximum offset is 1.3m. A 10cm wide schistosity zone is developed and gravels are arranged along the fault plane. The 14C ages of the faulted upper stratum(unit ③)are(1 100±30)a and(870±30)a. The N-wall also reveals the existence of faults, corresponding to the S-wall of the trench. These faults and dislocated strata fully indicate that the fault was active during the Holocene. According to field investigation, the Sudian Fault is mainly characterized by horizontal dextral strike-slip movement. For example, in Mengnong tea field, obvious synchronous dextral displacement occurred in three gullies along the fault. From south to north, the displacements of the three gullies are 40m, 42m and 45m, respectively. Shutter ridge landform is developed at the gully mouth. In the lower part of the northernmost gully, there is a pluvial fan, and the 14C age of the bottom of the pluvial fan is(13 560±40)a, which is less than the formation age of the gully, but roughly represents the formation age of the gully, indicating that the Sudian Fault is mainly characterized by horizontal dextral strike-slip movement. In Sudian area, the Mengga River is right-laterally offset 1 050~1 100m by the fault. At 1.7km north of Sudian, a diluvial fan is right-laterally offset 18~22m. There are fault scarps with a height of 1~1.5m developed on the alluvial fan, Quaternary faults and bedrock fault scarps with a height of about 8m developed on its extension line. The three points of the scarps, Quaternary faults and bedrock scarps are in a straight line, which absolutely shows the reliability of the dislocation of the alluvial fan. An organic carbon sample is obtained 1.8m below the alluvial fan, and its 14C test age is (6 210±30)a. This age should be close to the formation age of the pluvial fan, indicating that the fault underwent obvious horizontal dextral strike-slip movement during the Holocene. In the Sadung Basin, Myanmar, a river is offset about 380m right-laterally, forming a hairpin bending landform. Due to the continuous collision between the Indian plate and the Eurasian plate, the Indosinian block in the southeastern margin of the Tibet Plateau around the Eastern Himalayan Syntaxis escaped southerly, and the western Yunnan became the most intense part of the south extrusion. During the southerly escapement of the Indosinian block, the right-lateral strike-slip movement of Sudian Fault and other faults striking near SN plays a role in adjusting and absorbing the block strain. Under the action of current NNE tectonic stress field, the intersection of the dextral strike-slip Sudian Fault striking NS and the sinistral strike-slip Dayingjiang Fault striking NE is the key part of tectonic stress concentration, which will be the seismic risk area to be focused in the future. The research result of late Quaternary activity of the fault is of great practical significance for the correct understanding and reasonable assessment of the medium to long-term strong earthquake risk in this area, and for the mitigation and prevention of the earthquake disaster in the border area.
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    STUDIES ON NEW ACTIVITY OF LINTAN-DANGCHANG FAULT, WEST QINLING
    ZHANG Bo, TIAN Qin-jian, WANG Ai-guo, LI Wen-qiao, XU Yue-ren, GAO Ze-min
    SEISMOLOGY AND GEOLOGY    2021, 43 (1): 72-91.   DOI: 10.3969/j.issn.0253-4967.2021.01.005
    Abstract71)   HTML    PDF (24979KB)(307)       Save
    Located in the intervening zone between Tibetan plateau and surrounding blocks, the Lintan-Dangchang Fault(LDF)is characterized by north-protruding arc-shape, complex structures and intense fault activity. Quantitative studies on its new activity play a key role in searching the seismogenic mechanism, building regional tectonic model and understanding the tectonic interaction between Tibetan plateau and surrounding blocks. The LDF has strong neotectonic activities, and moderate-strong earthquakes occur frequently(three M6~7 earthquakes occurred in the past 500 years, including the July 22nd, 2013, Minxian-Zhangxian M S6.6 earthquake), but the new activity of the fault is poorly known, the geological and geomorphological evidence of the Holocene activity has not been reported yet. Based on remote sensing interpretation and macro-landform analysis, this paper studies the long-term performance of LDF. Based on the study of fault activity, unmanned aircraft vehicle photogrammetry and differential GPS, radiocarbon dating, etc., the latest activity of LDF is quantitatively studied. Then the research results, historical strong earthquakes and small earthquake distribution are comprehensively analyzed for studying the seismogenic mechanism and constructing regional tectonic models. The results are as follows: Firstly, the fault geometry is complex and there are many branch faults. According to the convergence degree of the fault trace and the fault-controlled macroscopic topography, the LDF is divided into three segments: the west, the middle and the east. The west segment contains two fault branches(the south and the north)and the south Hezuo Fault. The south branch of the west segment mainly dominates the Jicang Neogene Basin, and the south Hezuo Fault controls the south boundary of Hezuo Basin. The middle segment has more convergent and stable trace, consisting of the main fault and south Hezuo Fault, and these faults separate the main planation surface of the Tibetan plateau and Lintan Basin surface geologically and geomorphologically. The fault traces in the east segment are sparsely distributed, and the terrain is characterized by hundreds of meters of uplifts. The branch faults include the main fault, Hetuo Fault, Muzhailing Fault and Bolinkou Fault, each controlling differential topography. Secondly, the motion property of the LDF is mainly left-lateral strike-slip, with a relative smaller portion of vertical slip. The left-lateral strike-slip offset the Taohe River and its tributaries, gullies and ridges synchronously, and the maximum left-lateral displacement of the tributary of Taohe River can reach 3km. Meanwhile, the pull-apart basins and push-up ridges associated with the left-lateral fault slip are also developed in the fault zone. The performance of vertical slip includes tilting of the main planation surface, vertical offsets of the boundary and interior of Neogene basin and hundred meter-scale differential topography. The vertical offset of the Neogene is 300~500m. Thirdly, one fault profile was newly discovered in Gongqia Village, revealing a complete sequence of pre-earthquake-coseismic-postseismic deposition, and this event was constrained by the radiocarbon ages of pre-earthquake and post-earthquake deposition. The event was constrained to be 2090~7745aBP(confidence 2 σ), which for the first time confirmed the Holocene activity of the fault. Fourthly, a gully with two terraces at least on the west side of Zhuangzi Village in the east segment of the main fault retains a typical faulted landform. The T2/T1 terrace riser of the gully has a left-handed dislocation of 6.3~11.8m, and the scarp height on terrace T2 is 0.4~0.7m, the radiocarbon age of the terrace T2 is7170~7310aBP, so the derived left-lateral strike-slip rate since the early Holocene in the east segment of the main fault is 0.86~1.65mm/a, and the vertical slip rate is 0.05~0.10mm/a. The derived slip rates are in line with the regional tectonic model proposed by the predecessors, so the LDF plays an important role in the internal deformation of the West Qinling. The clockwise rotation of the middle to east segments of the LDF acts as an obstacle to the left-lateral strike-slip motion, which inevitably leads to the redistribution and rapid release of stress, so earthquakes in the middle-east segment of the LDF are unusually frequent.
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    STUDY ON THE 3D CRUSTAL VELOCITY STRUCTURE OF BODY-WAVE IN GONGHE AREA
    LUO Ren-yu, CHEN Ji-feng, YIN Xin-xin, LI Shao-hua
    SEISMOLOGY AND GEOLOGY    2021, 43 (1): 232-248.   DOI: 10.3969/j.issn.0253-4967.2021.01.014
    Abstract71)   HTML    PDF (11689KB)(277)       Save
    A M W6.4( M S7.0)earthquake occurred in Gonghe, Qinghai on 26 April 1990. The Gonghe area is located on the northeastern margin of the Qinghai-Tibet Plateau. The geological tectonic movement in this area is mainly affected by the uplift of the Qinghai-Tibet Plateau. There has been no earthquakes larger than moderate strength in the Gonghe Basin since the historical records, and there are no large-scale active faults on the surface of the epicenter area, so the earthquake has aroused great concern. No major earthquakes have occurred in the Gonghe area since 1995, but the data of small earthquakes is very rich, which ensures the completion of the research. The TomoDD method combines the double-difference relocation method with seismic tomography, and solves two problems at the same time, one is the problem of fine positioning of the earthquake, and the other is the calculation of the 3D velocity structure of the earth’s crust. In this paper, we collected 63872 P and S wave arrival time data in Gonghe and surrounding area recorded by Qinghai, Gansu seismic networks and temporary seismic array from January 2009 to January 2019. The 3D crustal velocity structure and source position parameters of the region are inversed. The relationship between the geological structure setting of the main shock and the velocity structure and seismicity of the region was analyzed. The results show that the crustal velocity structure in the Gonghe area shows lateral inhomogeneity. The Gonghe mainshock is located in the low-velocity anomaly directly below the Gonghe Basin, close to the high-low-velocity anomaly boundary. There is an obvious high-speed anomaly in the southwest of the mainshock, which thrusts from underground to near-surface in the northeast direction. It is estimated that the Wayuxiang-Lagan concealed fault is located at 35.95°N, the dip of the fault is about 45° at the deep part. It is inferred that the occurrence of the Gonghe main shock is caused by the sliding of the Wayuxiangka-Lagan Fault whose strike is NWW and dip is SSW under the action of horizontal tectonic stress. The high-velocity anomaly is about 5~40km deep underground in the northeast direction of the Riyueshan Fault, and a large number of small earthquakes occurred around the high- and low-velocity transition zone. It is presumed that under the action of the near-horizontal NE-directed tectonic stress, the high- and low-velocity zones were further interacted to generate faults and ground folds, and a large number of small earthquakes occurred during the fusion process.
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    STUDY ON THE CRUSTAL STRUCTURE OF THE CENTRAL AND WESTERN PART OF THE NORTH CHINA CRATON
    PAN Ji-shun, LI Peng-hui, DUAN Yong-hong, ZHAO Yan-na, PENG Yi-cong, SUN Kai-xuan
    SEISMOLOGY AND EGOLOGY    2021, 43 (5): 1269-1291.   DOI: 10.3969/j.issn.0253-4967.2021.05.014
    Abstract70)   HTML10)    PDF (9041KB)(46)       Save

    The North China Craton is the oldest craton in China and also the main tectonic unit of the Chinese mainland. The geological marks from Archean to Mesozoic era are complete and have attracted scientists all over the world. It has been the natural experimental site for the study of continental formation and evolution. A series of complex tectonic movement and evolution processes occurred in the North China Craton since Mesozoic. A series of rift basins were formed due to the thinning of lithosphere in its eastern part, so its crust structre is complicated. But the lithosphere is thick in its western part, so the crust structure of the Ordos block is simple. Shanxi rift zone is located between the eastern block of North China Craton and the western Ordos block. The crust and lithosphere structure of Shanxi rift zone is changed from stable craton structure in the west to severely damaged craton structure in the east, showing obvious transition characteristics. Therefore, it is of great significance to study the structural characteristics of the Shanxi rift zone and its two sides so as to reveal the failure dynamics of the North China Craton. Based on the teleseismic waveform data recorded by 150 mobile seismic stations in the central and western part of the North China Craton(107°E~117°E; 34°N~41°N)in the recent three years, the crustal velocity structure images of the study area are obtained by using the H-κ stacking method of P-wave receiver function and the common conversion point(CCP)superposition method. Our research results show that the crustal thickness in the Ordos block is between 37km and 47km, the Moho surface is relatively flat. The crust thickness of Shanxi rift zone is between 34km and 46km. Under the depression of Linfen Basin, Moho surface shows obvious uplift, and the uplift amount is between 4km and 10km. It is inferred that the formation of Shanxi rift zone is closely related to the movement of mantle materials. Compared with the existing Bouguer gravity anomaly data in this area, the distribution characteristics of crustal thickness in the study area are consistent with the distribution characteristics of positive and negative Bouguer gravity anomalies in the eastern and western Taihang uplift, respectively. The calculation results of crustal thickness and wave velocity ratio in different tectonic units in this region show that the wave velocity ratio in the three tectonic units decreases with the increase of crustal thickness. On the whole, the study area is divided into east and west areas with 111.5°E as the boundary. The Poisson's ratio of Ordos area to the west is lower than that of Shanxi rift zone to the east of 111.5°E, which reflects that the eastern part of Ordos block has the characteristics of stable ancient block and the crustal structure is relatively simple; however, the upwelling of upper mantle material under the Shanxi rift zone leads to higher Poisson's ratio than the mountainous areas on both sides. As far as the Shanxi rift zone is concerned, it is divided into north and south regions with 38°N as the boundary. The crust to the north of 38°N is characterized by low velocity due to partial melting, while the area south of 38°N still maintains a relatively stable crust and presents high-velocity characteristics. The difference of crustal structure and material composition between the north and the south of Shanxi rift zone may be related to the uneven subsidence of Shanxi rift zone, and more data are needed for further comprehensive study on the related dynamic process.

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    STUDY ON THE CHARACTERISTICS OF CO-SEISMIC GRAVITY CHANGE INDUCED BY MADUO M S7.4 EARTHQUAKE BASED ON OBSERVATIONS OF CONTINUOUS GRAVITY STATIONS
    WEI Jin, HAO Hong-tao, HAN Yu-fei, HU Min-zhang, JIANG Ying, LIU Zi-wei
    SEISMOLOGY AND EGOLOGY    2021, 43 (4): 984-998.   DOI: 10.3969/j.issn.0253-4967.2021.04.016
    Abstract70)   HTML12)    PDF (5703KB)(62)       Save

    The Bayan Kara block is a secondary block in the Qinghai-Tibet Plateau interior with the strongest seismic activity occurring in the recent years. Unlike the 8 earthquakes above M7.0 occurring on the block boundary in the past 20 years, the Maduo MS7.4 earthquake occurred inside the block, thus providing a new research perspective for the composition of coseismic gravity change observations. At the same time, China Earthquake Administration set up 5 gPhone continuous gravity stations, which operated normally before and after the earthquake, about 800km away from the epicenter, near the northeast edge of the Qinghai Tibet Plateau and the adjacent areas. Among them, the Maqin gravity station is inside the block and the Songpan gravity station is on the block boundary. The location of this earthquake and the distribution characteristics of the gravity stations provide natural experimental sites and samples for studying the coseismic gravity change in the stations at different locations in and around the block. In order to check the dislocation theory based on surface deformation observation, accumulate coseismic gravity change data by strong earthquakes, and analyze the features of the coseismic gravity change by surface gravity observation from different perspectives, the gravity earth tide and barometric observation data measured by gPhone gravimeters and sampled at 1Hz in these 5 continuous gravity stations from May 10 to 25, 2021 are collected. In this paper, firstly, exponential and step function methods are used to extract the coseismic gravity change in these 5 gravity stations, so as to analyze whether the post-seismic gravity signals contain relaxation signal. The relaxation time observed by gravimeter is very short compared to the near-field results of Lushan MS7.0 earthquake, only about 11 minutes. And the step method result is more consistent with the model one. Comparing with the coseismic dislocation theory by Okubo and Sun model, it is found that the difference between the observation results of the two methods and the extreme value simulated by the two models is nearly 1×10-8m·s-2. Moreover, the observed and simulated results have a good consistency in terms of direction. However, there is about an order of magnitude difference between the observation and simulation in the gravity station position. By discussing the calculation accuracy of barometric admittance, mean tidal factor and co-seismic gravity change of 7-days data before and after the earthquake at the 5 stations, analyzing the relationship between the coseismic gravity change and vertical displacement in the same site of GNSS and gravity station at Maqin and Songpan, comparing the observations and simulations of the coseismic gravity changes in the two gravity stations(Maqin and Songpan)located at the block boundary and inside the block induced by the Jiuzhaigou MS7.0 and Maduo MS7.4 earthquake which occurred also on the block boundary and inside the block, and calculating the coseismic gravity change at Zhongdian station by the Maduo MS7.4 after removing the effect of Yangbi MS6.4 earthquake, it is considered that, these gPhone gravimeters analyzed in the paper can capture more than 0.5×10-8m·s-2 coseismic gravity change, and the Maqin station, which is about 175km away from the epicenter, observed about [(2.9±0.70)~(4.0±0.70)]×10-8m·s-2 coseismic gravity change generated by Maduo MS7.4 earthquake. Based on the daily solution of GNSS vertical displacement, the co-seismic vertical deformation at Maqin and Songpan stations is all about-(4±5)mm. Taking the average gravity gradient value, -320×10-8m·s-2/m, the displacement-induced gravity change by Maduo MS7.4 earthquake is calculated, which is(1.2±1.5)×10-8m·s-2. It is proved that part of the contribution of the co-seismic positive gravity variation at Maqin and Songpan stations comes from the variation of vertical deformation in this area. By eliminating the co-seismic effect from the Yangbi MS6.4, the co-seismic gravity change induced by Maduo MS7.4 earthquake in Zhongdian station is about(1.09±0.76)×10-8m·s-2. Compared with the observed and simulated co-seismic gravity change induced by Jiuzhaigou MS7.0 earthquake, the co-seismic gravity change of about(9.1±0.22)×10-8m·s-2 by Maduo MS7.4 earthquake recorded at Songpan station should include the effect of other factors. This may be related to the seismic and tectonic background, as Songpan station is just at the east boundary of Bayan Kara block. The coseismic gravity change in Linzhi station is negative, which is consistent with the simulation results of dislocation theory. Based on the observation results of this paper, it is considered that the Maduo MS7.4 earthquake can produce about(0.5~4)×10-8m·s-2 co-seismic gravity change in the far field range of 175~800km. The coseismic gravity variation signal observed by the gravity station is not only related to crustal deformation and epicentral distance, but also to the seismotectonic background of the block where the gravity station locates. The results can provide a reference for determining the coseismic gravity change caused by medium strong earthquakes in the future.

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