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20 August 2021, Volume 43 Issue 4

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Research paper

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

2021, 43(4):  739-756.  DOI: 10.3969/j.issn.0253-4967.2021.04.001

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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(G_h)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 G_h value are formed in the middle and upper crust, whilst G_h 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 G_h 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.

DEEP SEISMOTECTONIC ENVIRONMENT OF THE 2021 MADOI M_S7.4 EARTHQUAKE

SONG Xiang-hui, WANG Shuai-jun, PAN Su-zhen, SONG Jia-jia

2021, 43(4):  757-770.  DOI: 10.3969/j.issn.0253-4967.2021.04.002

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On May 22, 2021, an M_S7.4 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 M_S7.4 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 M_S7.4 earthquake. To research the deep seismotectonic environments of the M_S7.4 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 M_S7.4 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 M_S7.4 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.

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

2021, 43(4):  771-789.  DOI: 10.3969/j.issn.0253-4967.2021.04.003

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A M_S6.4 earthquake occurred on May 21^th, 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 M_W6.1. 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 M_S7.0 earthquake in 1996 and the Mojiang M_S5.9 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.

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

2021, 43(4):  790-805.  DOI: 10.3969/j.issn.0253-4967.2021.04.004

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On May 22, 2021, an M_S7.4 earthquake occurred in Maduo County, Guoluo Prefecture, Qinghai Province, which is the biggest earthquake in mainland China since the 2008 Wenchuan M_S8.0 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 M_S≥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 M_S7.4 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.

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

2021, 43(4):  806-826.  DOI: 10.3969/j.issn.0253-4967.2021.04.005

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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.

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

2021, 43(4):  827-846.  DOI: 10.3969/j.issn.0253-4967.2021.04.006

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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 M_S6.4 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 M_S6.4 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.

RELOCATION AND FOCAL MECHANISM SOLUTIONS OF THE 2021 YANGBI, YUNNAN M_S6.4 EARTHQUAKE SEQUENCE

WANG Ying, ZHAO Tao, HU Jing, LIU Chun

2021, 43(4):  847-863.  DOI: 10.3969/j.issn.0253-4967.2021.04.007

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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 M_S6.4 Yangbi earthquake, the foreshock activity near the epicenter was frequent, and the maximum magnitude of foreshock is 5.6. After the M_S6.4 earthquake, another M_S5.2 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 M_S6.4 earthquake is M_W6.0, 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 M_S6.4 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.

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

2021, 43(4):  864-880.  DOI: 10.3969/j.issn.0253-4967.2021.04.008

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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 M_S6.4 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 M_S6.4 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 V_P/V_S ratio of 1.79 via fitting the P-wave and S-wave arrival-time curves with the Wadati method. The magnitude ranges from M_S0.0 to M_S6.4, 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 M_S6.4 main shock is located at 99.89°E, 25.70°N with a focal depth of 7.9km. The Yangbi M_S6.4 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 M_S6.4 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 M_S6.4 mainshock epicenter, a group of earthquakes spread along the NNE direction and extended to the Weixi-Qiaohou Fault that hosted the M_S4.1 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.

HOLOCENE ACTIVITY AND PALEOEARTHQUAKES OF THE WEIXI-QIAOHOU FAULT

CHANG Zu-feng, CHANG Hao, LI Jian-lin, MAO Ze-bin, ZANG Yang

2021, 43(4):  881-898.  DOI: 10.3969/j.issn.0253-4967.2021.04.009

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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.

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

2021, 43(4):  899-907.  DOI: 10.3969/j.issn.0253-4967.2021.04.010

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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 V_S30, 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 V_S30-based amplification terms, which depends on the amplitude and frequency of ground motion, to account for site amplification. The V_S30 data of the macroscopic site classification was estimated using the correlation between topographic gradient and V_S30. 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.

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

2021, 43(4):  908-919.  DOI: 10.3969/j.issn.0253-4967.2021.04.011

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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 M_e value is 5.9, which is smaller than M_W; 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.

SIMULATION OF STRONG GROUND MOTION FROM THE 2021 YANGBI, YUNNAN M_S6.4 EARTHQUAKE

HE Xin-juan, PAN Hua

2021, 43(4):  920-935.  DOI: 10.3969/j.issn.0253-4967.2021.04.012

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In this study, we simulated the strong ground motion from the M_S6.4 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 M_S6.5 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 M_S6.4 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 M_S6.4 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.

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

2021, 43(4):  936-957.  DOI: 10.3969/j.issn.0253-4967.2021.04.013

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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 M_S7.4 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.

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

2021, 43(4):  958-971.  DOI: 10.3969/j.issn.0253-4967.2021.04.014

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On May 21, 2021, at 21:48(Beijing time), an earthquake with magnitude M_S6.4 occurred in Yangbi County of Yunnan Province. In order to obtain the geomagnetic field change before and after the M_S6.4 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 M_S6.4 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.

GRAVITY VARIATION CHARACTERISTICS IN YUNNAN AND ITS SURROUNDING AREAS FROM 2010 TO 2020

ZHANG Xin-lin, WANG Jian, HU Min-zhang, WANG Jia-pei, LI Zhong-ya, ZHANG Yi

2021, 43(4):  972-983.  DOI: 10.3969/j.issn.0253-4967.2021.04.015

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Based on the absolute gravity observation data of 10 gravity datum stations of the Crustal Movement Observation Network of China(CMONOC)in Yunnan and adjacent areas during 2010-2020, the gravity datum and its dynamic variation of each gravity datum station are obtained. The gravity variation trend of 9 stations at three different time scales and time periods shows that the gravity variation increased first and then decreased, and the turning point occurred around 2014, while Kunming Station is just the opposite. The Ludian M_S6.5, Yingjiang M_S6.1 and Jinggu M_S6.6 events occurred successively in 2014 when the gravity change increased to the turning point. Thereafter, the gravity change trend decreased until the occurrence of the Yangbi M_S6.4 earthquake in 2021. The results show that the variation of the gravity field in Yunnan and adjacent area is large and fast, and the transition period of increasing to decreasing is short and the variation trend is consistent. The gravity field change mechanism may be dominated by that the Qinghai-Tibetan plateau moves to the northeast caused by the push of Indian Plate, then is blocked by the Sichuan Basin, and the crustal material under the Qinghai-Tibetan plateau moves forward to Yunnan and its adjacent regions. The repeated observations of the absolute gravity survey network covering the whole country provide abundant and reliable data for obtaining the time-variable gravity field which is related to the crustal movement. Many scholars have done a lot of research on the results of the absolute gravity dynamic changes in the Chinese mainland, but the region is mainly focused on the Qinghai-Tibetan plateau, while the results of the absolute gravity dynamic change in other regions are still rarely revealed.

(1)The absolute gravity observation data of 10 gravity datum stations in Yunnan and Panzhihua from 2010 to 2020 show that except Kunming observation station, the gravity change trend increased first and then decreased, and the turning point occurred around 2014. Kunming station is the only station in the cave among the 10 observation stations, and its gravity change may be affected by the water content of the mountain. Five gravity observation campaigns at each observation station were carried out in different months of different years. Due to the lack of hydrological data at each observation station, the seasonal gravity change was not considered. Therefore, eliminating its influence is one of the important jobs in our future study.

(2)Earthquakes of M_S6.1, M_S6.5 and Ms6.6 occurred on May 31, August 3 and October 7, 2014 in Yingjiang, Ludian and Jinggu, Yunnan Province, respectively. The epicentral distances of all the gravity datum stations to the three events are more than 100km, so the coseismic gravity changes caused by events can be ignored.

(3)The Ludian M_S6.5, Yingjiang M_S6.1 and Jinggu M_S6.6 earthquakes occurred successively during the turning point period, and the gravity changes show a decreasing trend until the occurrence of the Yangbi M_S6.4 event in 2021. The mechanism of gravity field change of Yunnan and the adjacent areas may be as follows: The Qinghai-Tibetan plateau is pushed to the northeast by the Indian plate, then blocked by the Sichuan Basin, and the crustal material under the Qinghai-Tibetan plateau moves forward to Yunnan and its adjacent regions.

(4)Yunnan and the adjacent areas are characterized by complex tectonic environment and rapid seismic energy accumulation and release, which puts forward new demands for absolute gravity measurement mode, adding absolute gravity measurement stations and shortening observation period, so as to enhance the ability of more absolute gravity measurement to serve for monitoring the seismic activity and regional geological tectonic activity in Chinese mainland.

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

2021, 43(4):  984-998.  DOI: 10.3969/j.issn.0253-4967.2021.04.016

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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 M_S7.4 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 M_S7.0 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 M_S7.0 and Maduo M_S7.4 earthquake which occurred also on the block boundary and inside the block, and calculating the coseismic gravity change at Zhongdian station by the Maduo M_S7.4 after removing the effect of Yangbi M_S6.4 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 M_S7.4 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 M_S7.4 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 M_S6.4, the co-seismic gravity change induced by Maduo M_S7.4 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 M_S7.0 earthquake, the co-seismic gravity change of about(9.1±0.22)×10^-8m·s^-2 by Maduo M_S7.4 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 M_S7.4 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.

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

2021, 43(4):  999-1012.  DOI: 10.3969/j.issn.0253-4967.2021.04.017

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The Yangbi M_S6.4 earthquake in Yunnan Province and Maduo M_S7.4 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)M_S≥7.0 Earthquakes in the study area since 2002, such as Wenchuan M_S8.0 earthquake, Yushu M_S7.1 earthquake, Lushan M_S7.0 earthquake, Jiuzhaigou M_S7.0 earthquake, Maduo M_S7.4 earthquake and Nepal M_S8.1 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.

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

2021, 43(4):  1013-1029.  DOI: 10.3969/j.issn.0253-4967.2021.04.018

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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.

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

2021, 43(4):  1030-1039.  DOI: 10.3969/j.issn.0253-4967.2021.04.019

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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 M_S6.4 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.