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20 October 2021, Volume 43 Issue 5
Special topic on the Yunnan Yangbi MS6.4 and Qinghai Maduo MS7.4 earthquakes
ANALYSIS OF STRESS INFLUENCE OF QINGHAI MADUO MS7.4 EARTHQUAKE ON SURROUNDING FAULTS
YUE Chong, QU Chun-yan, NIU An-fu, ZHAO De-zheng, ZHAO Jing, YU Huai-zhong, WANG Ya-li
2021, 43(5):  1041-1059.  DOI: 10.3969/j.issn.0253-4967.2021.05.001
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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.

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

RESEARCH ON FAST ACQUISITION OF GNSS COSEISMIC HORIZONTAL DISPLACEMENT OF MADUO MS7.4 EARTHQUAKE IN QINGHAI PROVINCE
LI Jing-wei, CHEN Chang-yun, ZHAN Wei, WU Yan-qiang
2021, 43(5):  1073-1084.  DOI: 10.3969/j.issn.0253-4967.2021.05.003
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The May 21, 2021, Maduo MS7.4 earthquake in Qinghai Province caused serious disasters in Maduo County and its surrounding areas. The GNSS co-seismic displacement field data can play a key role in quickly determining the influence range of the earthquake and serving for the rapid investigation. After the earthquake, we immediately collected the data of 18 GNSS stations surrounding the epicenter, including 7 stations that recorded 1Hz high-frequency observation data. Various data were used to rapidly obtain the GNSS co-seismic displacements, such as, the 15-minute high-frequency data, 5 hours after earthquake and multi-day displacement data. In this paper, we used three methods to obtain the co-seismic displacement, including the dynamic difference method for 1Hz frequency data by GAMIT/GLOBK Track module, and the static difference method for the post-seismic 5-hour data and for the pre- and post-seismic multi-day data by GAMIT/GLOBK. The results are shown as follows:
(1)The dynamic difference method for 1Hz frequency data by GAMIT/GLOBK Track module has ability to quickly process the data and acquire the co-seismic displacement. When using the high-frequency data to obtain co-seismic displacement by Track module, it is suitable for the near field stations which have a large value of co-seismic deformation. However, in the far field, the accuracy of the solution is at cm level restricted by the distance of stations. In addition, the result of the Track is influenced by the stability of reference station. Although the results obtained by Track are not accurate, it can be used as a method to quickly judge the characteristics and amount of coseismic surface motion.
(2)Comparing the results obtained from the post-seismic 5-hour data and the pre- and post-seismic multi-day data, the GNSS stations’ displacements have good consistency in the magnitude, direction and influence range, especially in the near field. The difference of the results by the two methods is from 1mm to 4mm. Considering the processing accuracy of the GAMIT/GLOBK, the value of the difference is not unreasonably high. When the displacement value is small, it is difficult to obtain accurate results. In addition, the direction of the pre- and post-seismic multi-day result is consistent with that from the post-seismic 5-hour data, and the value increased. If we regard the result of the pre- and post-seismic multi-day data as the result of one day data after the earthquake which is included in the post-seismic displacement, this phenomenon coincides with the afterslip deformation, and the difference may be caused by the afterslip, especially in the near field. Although the difference exists, taking into account the timeliness and the overall consistency, we believe that using the postseismic 5-hour data to quickly obtain the co-seismic displacement is credible in an emergency.
(3)Based on the analysis of various results, it is preliminarily judged that the Maduo earthquake is dominated by left-handed strike-slip. The maximum displacement at the station QHMD, which is about 40km from the epicenter, is about 24cm to the west and 8cm to the north. The earthquake affected the area around epicenter including Maduo, Xining, Dulan, Delingha in the north, and Zebra and Ganzi areas in the south. From the comparison of the results of the static difference method for the 5 hours and multi-day data, it is believed that the post-seismic deformation taking place in the near field is significant, and continuous attention is required in the later stages.

THREE-COMPONENT GROUND MOTION SIMULATIONS BASED ON THE STOCHASTIC FINITE-FAULT METHOD FOR THE 2021 MADUO MS7.4 EARTHQUAKE, QINGHAI PROVINCE
LI Chun-guo, WANG Hong-wei, WEN Rui-zhi, QIANG Sheng-yin, REN Ye-fei
2021, 43(5):  1085-1100.  DOI: 10.3969/j.issn.0253-4967.2021.05.004
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An MS7.4 earthquake occurred in Maduo County, Qinghai Province on May 22, 2021, which is the largest magnitude event following the 2008 Wenchuan MS8.0 earthquake in China. A total of 54 aftershocks occurred up until 3 June, 2021, including 15 M4.0~4.9 earthquakes and one M5.0 earthquake. These earthquakes primarily nucleated at a depth of 8~10km, and ruptured a NNW-SSE-trending fault with a length of~170km. Based on the post-earthquake field damage survey, the seismogenic fault information, aftershock spatial distribution, as well as the strong-motion and seismic-intensity observation recordings, the Ministry of Emergency Management of the People’s Republic of China officially released the macroseismic intensity map for this MS7.4 earthquake. The maximum intensity reaches X(Chinese seismic intensity scale)in a narrow area close to the fault. Due to the strong earthquake-resistance and the low population density, the Maduo MS7.4 earthquake causes few casualties and some injuries only.
The sparse strong-motion observation stations in this region, distributed mainly to the northeast of the epicenter, collected a few number of strong-motion recordings only at far field(over 150km to epicenter)during the MS7.4 event. In order to evaluate the spatial distribution of ground motion intensity, the stochastic finite-fault method for simulating three-component(two horizontal and one vertical components)seismic ground motions were applied to reproduce the ground motions at 4 461 dummy observation positions during the Maduo MS7.4 earthquake. The inverted source rupture process from USGS and Zhang Yong of Peking University, source slip distribution model from Institute of Geology of China Earthquake Administration, and the source kinematic rupture models stochastically generated were used separately to represent the source model of this earthquake in the simulations. The average stress drops for these source models are mainly in the range of 3~4MPa. The VS30 scaling was considered to represent the amplification effects of the local site conditions. The peak ground accelerations(PGAs)and velocities(PGVs)for the simulated ground motions were first compared with those from the far-field observations and the predicted medians by the ground motion prediction equation applicable for Qinghai-Tibetan seismic zone. The PGAs and PGVs for the near-fault simulations are generally up to ~300cm/s2 and ~30cm/s, respectively. The simulated PGAs and PGVs are in good agreement with those predicted medians, and well stand for the distance decay. However, the simulated peak values are, in general, smaller than those far-field observations. Moreover, the acceleration and velocity time histories, and the 5%-damped pseudospectra accelerations (PSAs)at some typical dummy observation positions, e.g., epicenter, Maduo County, urban district of Guoluo, were provided. It was found that the amplitudes of the time histories and the PSAs at vertical component are much lower than those at both horizontal components for the simulated recordings. The amplifications on the horizontal ground motions caused by the local site condition, and the much sharp source spectral decay rate for P-wave may by the possible reasons.
Finally, the simulated three-component ground motions were applied to calculate the seismic intensities. The spatial distribution of seismic intensity was then displayed. Results indicated that: 1)the maximum seismic intensity is up to Ⅸ or Ⅷ dependent on the specific source model; 2)the principal axis of seismic intensity is well consistent with the strike of the seismogenic fault; 3)the distribution of meizoseismal area is just concentrated in a narrow area along the ruptured fault; and 4)the isoseismal lines are irregular ellipses due to the considerations on the local site conditions. Compared with the officially released macroseismic intensity map, the seismic intensities based on the simulated ground motions are generally underestimated by about 1~2 degrees. We infer that the stress drop statistically averaged used for simulations bears the responsibility. Considering the uncertainty of the seismic stress drop, the uncertainty of the seismic intensity based on the simulated ground motions is about plus or minus 1.2. At the end, we re-simulated ground motions using the source rupture model of USGS, while the value of stress drop was set to mean plus one standard deviation(about 13.59MPa). In this case, the seismic intensity based on simulated ground motions show good agreements with the macroseismic intensity map, especially for those regions with intensity equal to or greater than Ⅶ, and the maximum intensity reaches Ⅹ.

HYDROGEOCHEMICAL CHARACTERISTICS OF GROUND-WATER IN THE SURFACE RUPTURE ZONE OF MADOI MS7.4 EARTHQUAKE AND HOT SPRINGS IN THE EAST KUNLUN FAULT
LU Chang, ZHOU Xiao-cheng, LI Ying, LIU Lei, YAN Yu-cong, XU Yue-ren
2021, 43(5):  1101-1126.  DOI: 10.3969/j.issn.0253-4967.2021.05.005
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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.

ANALYSIS OF STRONG GROUND MOTION CHARACTERISTICS AND EARTHQUAKE DAMAGE FOR THE YANGBI MS6.4 EARTHQUAKE, YUNNAN
ZHANG Bin, LI Xiao-jun, RONG Mian-shui, YU Yan-xiang, WANG Yu-shi, WANG Ji-xin
2021, 43(5):  1127-1139.  DOI: 10.3969/j.issn.0253-4967.2021.05.006
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The MS6.4 earthquake occurred in Yangbi County, Dali Prefecture, Yunnan Province on May 21, 2021, the epicenter is located at the southwest boundary of Sichuan-Yunnan rhomboid block, where Weixi-Qiaohou-Weishan Fault meets Honghe Fault. According to data released by the China Earthquake Networks Center, the Yangbi MS6.4 earthquake is the only earthquake in recent years with MS>6.0 in the epicentral region. The National Strong-Motion Observation Network System(NSMONS)of China has built strong motion stations with relatively large density in Yunnan and Sichuan Provinces, NSMONS has obtained a large number of high quality strong ground motion acceleration recordings during this earthquake. In the process of earthquake, the different characteristics of strong ground motion often lead to the different characteristics of building structure damage in the epicentral region. In-depth analysis of ground motion observation characteristics is helpful to deepen the understanding of earthquake damage.
In this study, the NGA-West 2 data processing flow and reasonable and reliable high-pass filtering frequency selection criteria were used to process 29 sets of strong ground motion acceleration recordings of the earthquake, we obtained reliable peak ground acceleration(PGA), peak ground velocity(PGV), and 5%damped acceleration response spectra(SA). We drew the spatial distribution maps of PGA and PGV in the E-W, N-S, and U-D directions, compared the observed ground motion PGA, SA with the calculated values of the ground motion attenuation relationships commonly used in western China and Sichuan-Tibet region, and analyzed the amplitude and time-frequency characteristics of the observed ground motions, and a comparative analysis was performed between the spectral acceleration recorded by the near-field stations with the design spectra of the code for seismic design of buildings in China. Combined with the on-site earthquake damage investigation, the main reasons for the lighter structural damage in the meizoseismal area were analyzed.
The results show that the maximum horizontal PGA and PGV of the Yangbi MS6.4 earthquake recorded by the station both locate near the epicenter and the horizontal ground motion attenuates the slowest along the north-northwestern direction, which is basically the same as the long axis direction of isoseismals of seismic intensity map released by Yunnan Earthquake Agency. However, the vertical ground motion attenuates the slowest along the near north-south direction. The actual observation values on the soil site in the Yangbi earthquake are in good agreement with the calculated values of the horizontal ground motion attenuation relationships commonly used in western China and Sichuan-Tibet region, while the observation values on the bedrock site are smaller than the calculated values of the horizontal ground motion attenuation relationships commonly used in western China and Sichuan-Tibet region, which indicates the horizontal ground motion attenuation relationships commonly used in western China and Sichuan-Tibet region derived from the transfer method may over-predict the observation values on the bedrock site. According to the time-frequency diagram obtained by using the wavelet transform, the energy recorded in the EW and NS directions of the station 53YBX, which is the nearest station to the epicenter, is mainly concentrated in 8~15Hz, and the corresponding period range is 0.07~0.13s, while the energy recorded in the UD direction is mainly concentrated in 20Hz, and the corresponding period range is about 0.05s. When the period is smaller than 0.14s, the spectral accelerations in the EW and NS directions of the station 53YBX is significantly higher than the basic ground motion design spectra and rare ground motion design spectra of the code for seismic design of buildings in China; the remarkable period of the acceleration response spectra is 0.1s, the reaction spectrum decreases rapidly when the period is greater than 0.1s, the spectral acceleration corresponding to the superior period in the N-S direction of station 53YBX is 3.87 times the value of the rare ground motion design spectra platform. Most buildings near the epicenter are the one-story old timber frame structures which were built in the 1980s and 1990s, with a natural period of 0.1s, and the 2~3-story brick and concrete frame structures which were built in recent years. According to the analysis of above ground motion characteristics of the Yangbi MS6.4 earthquake, the most serious damage of this earthquake is the one-story old timber frame structure, while the 2~3-story brick and concrete frame structure has little damage or very light damage. This phenomenon should be related to the characteristics of the structure itself and the disrepair of the structure, the extremely rich ground motion component with a period of 0.1s, and the relatively less ground motion component with a period of more than 0.14s.

THE GRAVITY ISOSTATIC CHARACTERISTICS OF THE EPICENTER AND ITS SURROUNDING REGIONS OF THE YANGBI MS6.4, YUNNAN EARTHQUAKE
SHI Lei, LI Yong-hua, ZHANG Rui-qing
2021, 43(5):  1140-1156.  DOI: 10.3969/j.issn.0253-4967.2021.05.007
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The May 21, 2021, MS6.4 earthquake occurred in Yangbi, Yunnan, which caused 35 people died and possibly affected an area of 6 500 square kilometers. The hypocenter is located near the Weixi-Qiaohou-Weishan Fault which is the northern extension of the Red River Fault. However, the existing data show that this earthquake may not occur on the Weixi-Qiaohou-Weishan Fault. Further research is needed to explore the seismogenic fault. It is of great significance to understand the crustal structure and physical properties of the epicenter and its surrounding regions.
Affected by the Indian plate pushing on the Tibet, the southeastern margin of Tibetan plateau is characterized by intense intracontinental deformation, many deep faults and strong seismic activities. Gravity is one of the methods to study the crust structure and deep seismogenic environment. Many researches have been carried out in the study area, such as on the characteristics of gravity field, Moho depth, density structure, and isostatic anomaly. Gravity isostasy can reflect the crustal structure and its relationship with tectonic stress, which relates to seismic activities. However, the relationship between isostasy anomaly and earthquake distribution is still controversial. Some researches show that the Wenchuan earthquake locates in the extreme value region of isostatic anomaly. They believe the places where the difference between the theoretical and actual Moho depth is large are more likely to generate earthquakes. Other studies find that strong earthquakes in the western Sichuan and eastern Tibetan plateau occur mostly in the isostatic anomaly gradient regions, where the earthquake frequency is obviously higher than the extreme value regions. The crust-mantle density contrast is often taken as a constant in calculating Airy theoretical crustal thickness. The crust-mantle density contrast is inconsistent calculated with different data and methods. The gravity isostasy anomaly, which takes into account the density lateral variation, can better reflect the real structure.
In this paper, we calculate the gravity isostasy of the epicenter and surrounding regions based on the WGM2012 gravity anomaly, ETOPO1 elevation and Moho depth estimated from receiver functions in the previous study. China Earthquake Administration conducted gravity and topographic surveys on two profiles in the study area in 2011. We compared the observed Bouguer gravity and topography with the same stations extracted from WGM2012 and ETOPO1 models. The gravity anomaly standard deviation of two profiles is 4.89mGal and 8.42mGal, respectively. The standard deviation of topography is 17.86m and 28.74m, respectively. The WGM2012 and ETOPO1 data are in good correspondence with the actual measurements and can be used for the isostatic research.
We calculate the radial average logarithmic power spectrum of Bouguer gravity anomaly to estimate the reference Moho depth. The density contrast of the crust and upper mantle is obtained by apparent density estimation method in undulating interfaces. Based on the reference Moho depth and density contrast of crust and upper mantle, we get the theoretical isostatic Moho depth. We further compare the difference between the theoretical isostatic Moho depth and the actual depth obtained by the receiver function to analyze the isostatic anomalies in the whole study area, discuss the relationship between the isostatic anomalies and the distribution of moderate-strong earthquakes, and determine the isostatic state of the epicenter area of Yangbi earthquake.
Our results show that the difference between crustal mean density and uppermost mantle density varies from 0.3g/cm3 to 0.55g/cm3. The epicenter locates in the transition zone of density contrast. The change direction of transition zone is approximately perpendicular to the strike of Weixi-Qiaohou-Weishan Fault. The Sichuan Basin shows a relatively low density contrast, representing it is a stable tectonic block. The complete Bouguer gravity anomaly in the southeastern margin of Tibetan plateau has a low negative value. The crust-mantle density contrast is high(0.5~0.55g/cm3), which is speculated to be related to the extensive existence of low-velocity layers in the middle and lower crust revealed by previous tomography studies.
The theoretical isostasy Moho depth is 35~60km, reducing from northwest toward southeast. Local high value beneath the Panxi region is due to the low crust-mantle density, and the elevation is similar with adjacent areas. The theoretical Moho depth has negative correlation with crust-mantle density contrast. Wide angle reflection profile and seismic tomography studies also show there are obvious high velocity anomalies in the middle and lower crust. High velocity generally corresponds to high density, which is consistent with the low crust-mantle density contrast in Panxi region.
We obtain the gravity isostatic anomaly by subtracting the real Moho depth estimated through receiver functions H-κ stacking method from the theoretical Airy Moho depth. The Chongming MS8.0 paleo-earthquake occurred on the 4km-long isostatic anomaly gradient belt. There are 29 earthquakes of MS7.0 ~7.9, of which 16 earthquakes(55%)locate in the transition zones of positive and negative isostatic anomalies, and 5 earthquakes(17%)situate on the accompanied gradient zones with high value. Among the 42 earthquakes of MS6.5 ~6.9, 21 earthquakes(50%)and 13 earthquakes(31%)are in the transition zones of positive and negative isostatic anomalies and the accompanied gradient zones with high value, respectively. 9 earthquakes(21%)are distributed in the extreme value zones of isostatic anomaly.
The epicenter of the Yangbi MS6.4 earthquake locates on the 4km-long isostatic anomaly gradient belt. The gradient zone is basically consistent with the trend of Weixi-Qiaohou-Weishan Fault. The gradient zones of gravity isostatic anomaly are the area where anomalies change rapidly. We suggest that these regions may be in a more unstable state and liable to accumulate strain energy, thus, adjustments often begin to occur in these regions so as to achieve equilibrium in the crust.

GRAVITY FIELD CHANGE BEFORE THE 2021 YANGBI MS6.4 EARTHQUAKE, YUNNAN
LIU Dong, HAO Hong-tao, WANG Qing-hua, ZHENG Qiu-yue, HUANG Jiang-pei
2021, 43(5):  1157-1170.  DOI: 10.3969/j.issn.0253-4967.2021.05.008
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Yunnan is the front area where the material of Qinghai-Tibet Plateau moves eastward and rotates southeastward. This area is subject to active crustal movement and frequent destructive earthquakes. The activity intensity of the Honghe fault zone is the strongest in the middle part of Dali and its adjacent areas. The activity sequence of the whole fault zone since the Cenozoic has also changed gradually from the north and south ends to the Dali area in the middle section. The existing research data show that, based on the current seismic gravity monitoring network covering the whole province and surrounding areas, the mobile gravity monitoring network in Yunnan and surrounding areas can identify the gravity changes related to earthquakes of magnitude 5 and above. In this paper, the relative gravity data from 2015 to 2021 in Yunnan Province is adjusted and calculated, and combined with the regional geological tectonic environment and earthquake case studies, a systematic analysis is done on the temporal and spatial distribution characteristics of the changes in the gravity field and the horizontal crustal movement of the Yangbi earthquake and its surrounding areas to obtain the relationship between the regional gravity field change and the Yangbi MS6.4 earthquake, the following results are obtained: 1)By analyzing the gravity observation data of seven periods since 2018, it is found that the gravity change is weak at the Yangbi survey point in the epicenter from 2018 to 2021, and the gravity change around the epicenter is relatively intense. the gravity change at the Yangbi measuring point at the epicenter is within 10×10-8m·s-2, and the gravity change at the Xiaguan measuring point at a distance of about 39.2km in a straight line is around 10×10-8m·s-2. Considering the observation accuracy of the instrument, the epicenter area is basically in a constant state. The variation of Yongsheng survey point in the north, Tengchong survey point in the west and Jingdong survey point in the south of the epicenter are larger, with the maximum amount exceeding 60×10-8m·s-2. 2)With the epicenter as the center, the short-term, one-year, two-year, and four-year gravitational field changes in the nearby area basically show the characteristics of four quadrants. For the short-term and multi-year gravitational changes before the earthquake, the high gradient zone of the gravity field is concentrated in Yangbi-Xiaguan area, and the zero value line of gravity change is always in this area or adjacent area. Along the north section of the Xiaojiang fault zone in the Lijiang-Xiaguan area and the east and west sides of the seismogenic fault zone in the strip-shaped area of Nanjian-Weishan Fault, the gravity transitions are drastic, and the movement of materials inside the crust is obvious. 3)The multi-year gravity changes show that the gravity change on the west side of the epicenter is negative, and that of the east side is positive. The earthquake is a normal faulting event with a strike-slip component, which is consistent with the focal mechanism solution calculated using the global network data. The change characteristics of the gravity field indicate that the preparation of earthquakes in the earthquake area is closely related to the movement of faults and material transport in the crust. 4)By analyzing the gravity change anomaly index in the one year before the earthquake, the magnitude of the anomalous gravity change is 75×10-8m·s-2. According to the gravity anomaly index formula, the magnitude of the earthquake is MS6.2, which is 0.2 less than the actual magnitude of MS6.5; According to the actual magnitude MS6.4, the time-varying distance of the earthquake is 127.4km, and the distance between the two maximum positive change measuring points of the short-term gravity field before the earthquake and the epicenter(actual time-varying distance)is about 130km. The corresponding gravity change anomaly range value(twice the time-varying distance)is 260km, which is consistent with the actual value and falls in the range of 220~350km corresponding to MS6.0 and MS7.0 earthquakes. Considering the co-seismic and post-earthquake energy release of the aftershocks of 28 MS3.0 ~4.0, 11 MS4.0 ~5.0, 3 MS5.0 ~5.6 earthquakes and combined with the calculation results of two gravity variation anomaly index parameters of magnitude and time-varying distance, it can be judged that the energy driving the activity of the underground materials in Yangbi area is completely released. The MS6.4 earthquake is the main shock of the earthquake. The research results in this paperhelp us havea more objective understanding of the seismic characteristics in Yunnan area, the temporal and spatial dynamic evolution characteristics of the gravity field before the earthquake, and the performance of precursors, which provide a useful reference for earthquake analysis and prediction in this area.

A STUDY ON THE REGIONAL WAVE VELOCITY CHANGES BEFORE THE MS6.4 YANGBI EARTHQUAKE USING AMBIENT NOISE
YANG Jian-wen, JIN Ming-pei, YE Beng, GAO Qiong, CHEN Jia, ZHANG Hua-ying, DENG Jia-mei
2021, 43(5):  1171-1187.  DOI: 10.3969/j.issn.0253-4967.2021.05.009
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At 21:48:34 on May 21, 2021, a MS6.4 earthquake occurred in Yangbi County(25.67°N, 99.87°E), Dali Prefecture, Yunnan Province, with a focal depth of 8km. The epicenter is located in Cangshanxi Town, Yangbi County. The earthquake occurred on a secondary fault zone on the west side of the Weixi-Qiaohou-Weishan Fault. The focal mechanism is right-lateral strike-slip with a small amount of normal faulting, and the earthquake sequence is the foreshock-mainshock-aftershock type. The Yangbi MS6.4 earthquake is the largest earthquake in Yunnan Province after the Jinggu MS6.6 earthquake on October 7, 2014, and the largest earthquake in the northwestern Yunnan after the Yongsheng MS6.0 earthquake on October 27, 2001.
The preparation and occurrence of earthquakes are related to changes of regional stress field. The accurate measurement of changes in seismic wave velocity to monitor the changes in crustal stress over time is an effective way to make physical earthquake predictions. Studies have shown that before a strong earthquake, the physical properties of the medium in the crust will change, and this change can be reflected by changes in seismic wave velocity. Therefore, according to the temporal and spatial change characteristics of the seismic velocity in the crust, the physical properties of the crustal medium can be known. Ambient noise has a higher time sampling rate than natural seismic sources, is much more economical than artificial seismic sources, and has the advantages of repeatability, economy and environmental protection, so it is very suitable for tracking changes in the physical properties of the internal structure of the earth's crust.
In the previous work, we explored the application of seismic data and ambient noise cross-correlation technology to earthquake tracking, analysis and prediction. Based on the continuous waveform data of the Yunnan seismic network, the Rayleigh wave relative travel time offset time series of the station pairs are generated regularly and applied to the daily earthquake situation analysis and judgment. The occurrence of the Yangbi MS6.4 earthquake is a good test for applying the results of wave velocity measurement from ambient noise to the earthquake analysis and prediction practice. Therefore, based on the continuous broadband vertical component waveform data recorded by the station pairs near the epicenter of the Yangbi MS6.4 earthquake, and using the ambient noise wave velocity measurement method, the relative travel time offsets of the Rayleigh wave between the stations are obtained. A retrospective study of the regional wave velocity changes before the Yangbi MS6.4 earthquake was performed to gain deeper insights into the mechanism of earthquake preparation. This study also provides a good reference sample for earthquake cases to measure the regional underground medium wave velocity changes and capture the characteristics of wave velocity anomalies before strong earthquakes in Yunnan area by using ambient noise technology.
Based on the continuous broadband vertical component waveform data recorded by 7 stations near the epicenter of the Yangbi MS6.4 earthquake(4 stations of Yunnan network and 3 of Xiaguan network)from January 1, 2019 to May 21, 2021, the empirical Green's functions between 17 station pairs were extracted from the background noise cross-correlations. In the frequency range of 0.1~0.5Hz, the relative travel time offsets of the direct Rayleigh waves in the very day's empirical Green's function and the reference empirical Green's function of the station pair are measured directly using the cross-correlation time delay calculation method. On this basis, based on the Rayleigh wave relative travel time offset time series of 17 station pairs, with ±1.5 times the standard deviation as the anomaly threshold, the abnormal changes in the regional wave velocity around the epicenter of the Yangbi earthquake five months before the Yangbi earthquake were analyzed. In addition, based on the YUL--TUS, HDQ--BAS, EYA--CHT and YUL--CHT station pairs, the methods of “single station pair” and “multi-station pair combined average” are used to analyze the time-varying characteristics of the relative travel time offset near the epicenter. Conclusions are drawn as follows:
(1)From the perspective of the overall change of the abnormal station pairs, in the five months before the Yangbi MS6.4 earthquake, the distribution of abnormal station pairs in relative travel time offsets of the Rayleigh wave experienced a change process from scattered distribution in the outer periphery to concentration to the epicenter, and during the whole change process, the abnormal station pairs mainly show negative anomalies(the relative travel time deviation exceeds -1.5 times the standard deviation line).
(2)The analysis of the relative travel time offset of Rayleigh waves based on the combined stations of YUL-TUS, HDQ-BAS, EYA-CHT and YUL-CHT near the epicenter shows that about 5 months(158 days)before the Yangbi MS6.4 earthquake, the wave velocity of the underground medium near the epicenter showed an obvious accelerating trend, the amount of relative travel time offset change was -0.35%.
(3)The abnormal station pairs before the earthquake were concentrated near the epicenter, and dominated by negative anomalies. The wave velocity of the underground medium near the epicenter before the earthquake showed a significant acceleration trend, which was an obvious phenomenon observed before the Yangbi earthquake. Whether the above-mentioned change characteristics have to exist before strong earthquake requires further examination with more earthquake cases.
(4)Based on seismic waveform data, this paper uses ambient noise cross-correlation and cross-correlation time delay calculation methods to obtain the relative travel time offset of Rayleigh waves of station pairs and analyze the changes in wave velocity near the epicenter. The physical meaning is clear. The wave velocity measurement by ambient noise does not depend on a specific seismic source, and the data results have a high time sampling rate and can be updated every day. The method of measuring wave velocity from ambient noise is expected to become a new method and new approach for earthquake prediction.

Research paper
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
2021, 43(5):  1188-1207.  DOI: 10.3969/j.issn.0253-4967.2021.05.010
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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.

MULTI-SCALE ANALYSIS OF THE GRAVITY FIELD IN THE SOUTHEASTERN QINGHAI-TIBET PLATEAU AND ITS TECTONIC IMPLICATIONS
FANG Dong, HU Min-zhang, HAO Hong-tao
2021, 43(5):  1208-1232.  DOI: 10.3969/j.issn.0253-4967.2021.05.011
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Since the Cenozoic, the Qinghai-Tibet plateau is uplifting sharply due to the India-Eurasian violent collision. Its crust has thickened accompanied with north-south shortening and east-west extrusion. The eastward-moving material beneath the southeastern margin of Qinghai-Tibet plateau is stopped by hard blocks such as Alax, Ordos and South China block. Under the interaction, a complex structural belt of north-south direction is formed, called the North-south structural belt. And it is also known as the“North-South Seismic Belt” because of dense distribution and high intensity of earthquakes. Therefore, studies of the crustal structure and mass movement characteristics have scientific significance to reveal the mechanism of tectonic activity and earthquake incubation beneath the southeastern Qinghai-Tibet plateau. It will help improve capabilities of regional earthquake preparedness and disaster mitigation.
Firstly, the simulation test of simple geological body is carried out to verify the effectiveness of wavelet multi-scale analysis method in gravity field separation and the stability of the application program. When designing a simple geological body model, both the superposition effect of geological bodies at different depths and the decomposition effect of the test model are considered. The approximate field source depth of the separated regional field and local field is calculated by radial logarithmic power spectrum, and the calculation results are consistent with the design depth. It is determined that the wavelet base is “bior3.5” and the decomposition order is 5. The test results show that the “bior3.5” wavelet basis can effectively separate the regional and local anomalies in the geological body of the combined model, and obtain an ideal separation effect.
Then, based on the global gravity field model WGM2012 data, the Bouguer gravity anomaly data in the southeastern Qinghai-Tibetan plateau is decomposed in the 5 orders by using multi-scale wavelet analysis, and the radial logarithmic power spectrum is used to analyze the decomposition results, and a subset of the Bouguer gravity anomaly at different depths of this area is obtained. Based on this, this paper discusses the regional crustal structure, mass movement and seismogenic environment. The results show that the small-scale gravity anomaly of the 2nd and 3rd order mainly reflects the deformation information of the middle and upper crust, and the approximate field source depth of spectrum estimation is 3.5km and 12.6km. The second-order wavelet details are mainly distributed in strips with positive and negative alternations, while the third-order wavelet details are mainly displayed in tongue shape and trap shape. Compared with the second-order wavelet, the scale of the third-order details is larger and the range is clearer, but the location areas of the positive and negative anomaly distribution of the two are basically the same. The small-scale gravity anomaly indicates that the strong earthquakes mainly occur in the high gravity gradient zone and the boundary of the active block in this area. A comparative analysis of gravity anomalies at various scales reveals that seismogenic environment is not only controlled by the structure of the upper and middle crustal fault blocks but also related to the changes in the density of the lower crust. The lower crust at the epicenter appears as a low anomaly zone. The low-density, low-velocity, and plasticity of the deep medium conditions of the lower crust may have caused the stress of the lower crust in this area not to accumulate and“escape” to the upper crust easily to trigger strong earthquakes. This dynamic process of interaction between shallow and deep crust may be an important condition for earthquakes in the study area. The variation of Bouguer gravity anomaly in the second-order and third-order detail maps is consistent with the geological structure observed on the surface, making this area one of the areas with the most intense Meso-Cenozoic crustal deformation and seismicity; the meso-scale gravity anomaly of the 4th order mainly reflects the deformation information of the middle and lower crust, and the approximate field source depth of spectrum estimation is 26.2km, showing the existence of a low Bouguer gravity anomaly trap in the Songpan-Garze block. It is consistent with the observation results that there is a thicker low-velocity and low-resistance layer in the crust of Bayan Har block. It may be related to the large thickness of the lithosphere, the higher temperature of the lower crust and the melting of parts of the middle and lower crust at high temperatures. In the Panzhihua area, there is a high gravity anomaly trap, which may be caused by the mass residues in the middle and lower crust by the deep high-density material ascent during the Panxi ancient rift period. As one of the mass eastward migration channels of the Qinghai-Tibet Plateau, part of the materials in the Sanjiang area(Nujiang River, Lancang River and Jinsha River)gushes upward along the fault zone and accumulates in the middle and lower crust, resulting in a low Bouguer gravity anomaly area in the middle and lower crust of the area; the large-scale gravity anomaly of the 5th order mainly reflects the deformation information of the lower crust, and the approximate field source depth of spectrum estimation is 48.8km. It embodies the characteristics of rigid block. Myanmar microplate passes through western Yunnan in the direction of SEE, and the block shows still a high positive gravity anomaly, reflecting that these blocks have high density and strong rigidity and are not likely to fracture when the Indian plate pushes Eurasia northward, which plays a key role in the formation of the eastern Himalayan tectonic syntax. Large-scale gravity anomaly shows a regional negative anomaly in the Chuan-Dian block, which provides indirect evidence about the existence of “lower crustal flow” in the southeastern Qinghai-Tibet Plateau. Clamped by the Sichuan Basin and the Yunnan-Burma block, the flowing direction and accumulation of low-density mass can be clearly revealed. The low-density mass in the lower crust flows in both directions from north or south. A small part of the mass flows to the north through the Xianshuihe fault zone. Most of the mass flows to the south and is blocked by the southern Yunnan block. One flow goes to Panzhihua-Pu’er. One branch flows in the direction of Dongchuan-Qujing, and has a tendency to flow in the direction of Anshun-Guiyang, causing low-density mass to accumulate in the Lijiang-Daocheng-Panzhihua-Dongchuan-Kunming area.

STUDY ON FORESHOCK SEQUENCES OF THE GLOBA L GREAT SHALLOW-FOCUS EARTHQUAKES WITH MW≥8.0
XUE Yan, XIE Meng-yu, LIU Jie, ZHUANG Jian-cang
2021, 43(5):  1233-1249.  DOI: 10.3969/j.issn.0253-4967.2021.05.012
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Of all of the short-term and imminent precursors, foreshocks are recognized as one of the most effective indicators for earthquake prediction. This paper studies the statistical characteristics of foreshock activity of 29 global great shallow-focus earthquakes with MW≥8.0 from 1976 to 2017. Among these earthquakes, there are 23 thrusting ones.
Results show that there are 8 great earthquakes with foreshocks, accounting for 27.6% of the total. In addition, the 8 great earthquakes with foreshocks are all thrusting events, accounting for 34.8% of the total thrusting earthquakes. The maximum magnitude of foreshock is greater than 5.0. The epicenters of foreshock sequences are densely concentrated in space and near the main shock. The epicenter distance between the largest foreshock and the main shock is 10~53km, the magnitude difference ΔM between the maximum foreshock and the main shock ranges from 1.1 to 2.8, and the origin time difference Δt(the interval between the origin time of the largest foreshock and that of the main shock)is from 2 hours to 15 days for 87.5% of the earthquakes except the Kuril Islands MW8.3 earthquake, which occurred on November 15, 2006, while its maximum foreshock of MW6.6 occurred 45 days before the MW8.3 main shock.
Compared with the background seismicity, the foreshock sequence has the characteristic of high frequency. Of the eight earthquakes, the seismicity of five foreshock sequences accelerated within 15 days before the main shock, for the other three the seismicity began to increase during 35~45 days prior to the main shock, and the frequency increased again one day and six days before the main shock.
The focal mechanisms of foreshocks are similar to that of the main shock. While the focal mechanism of aftershock sequences is complicated and diverse. Such consistency of focal mechanisms with main shocks did not exist in aftershock sequences.
The parameters, α-value, p-value and b-value of foreshocks and aftershocks were calculated by the ETAS(Epidemic Type Aftershock Sequence)model, and b-value was got by the maximum likelihood method. The sample size N and the lowest calculated magnitude Mj can affect the calculation results of earthquake sequence parameters. Considering the reliability of results, it is assumed in this paper that when the number of foreshocks with MjMc(Mc indicates the minimum magnitude of completeness)is more than or equal to 30, the parameters, α-value, P-value and b-value, of the foreshock and aftershock sequences can be calculated. And the lowest calculated magnitude Mj of foreshocks and aftershocks is taken as the same or very close. Among the eight foreshock-mainshock-aftershock sequences, four can be compared to calculate the parameters of foreshocks and aftershocks. It should be noted that we take the aftershock activity within one month after the main earthquake as the aftershock sequence.
The calculated foreshock and aftershock sequence parameters show that there are no common features between the foreshock and aftershock for the α-value, meaning the ability to generate higher magnitude aftershocks, and the p-value, indicating the decay of sequences. While the b-value, indicating the stress state, shows a distinct characteristic that the b-value of foreshocks is obviously lower than that of aftershocks. Compared with regional background b-value, the b-value of foreshocks is 10%to 24%lower than the regional background b-value. And the b-value difference between foreshocks and regional seismicity is 2.2~7.1 times of the standard deviation of the regional background b-value. But the b-value of aftershocks is higher than or close to the regional background b-value.
In order to discuss the stability of b-value of foreshocks, we selected two foreshock sequences with sufficient data and studied the variation of b-value with sample size. Results show that the b-value is relatively low at the beginning, and then increases gradually with increase of sample size. When the calculated samples N≥70, the b-value is basically stable.

A METHOD FOR RAPID ASSESSMENT OF DEATH TOLL FROM EARTHQUAKE-INDUCED LANDSLIDE BASED ON GIS AND THE LOGISTIC MODEL
BAI Xian-fu, NIE Gao-zhong, YE Liao-yuan, DAI Yu-qian, YU Qing-kun
2021, 43(5):  1250-1268.  DOI: 10.3969/j.issn.0253-4967.2021.05.013
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The scientific assessment is one of the cornerstones for improving the earthquake disaster loss assessment system and the accuracy of the assessment of casualties caused by seismogeological hazards. However, this question has long been plaguing scientists engaged in the earthquake disaster risk assessment. So far there is no better solution yet. To solve this problem, the authors develop a new method for rapid assessment of the death toll caused by the earthquake-induced landslide based on GIS techniques and the logistic regression model. The method mainly consists of three procedures: 1)On GIS platform, the study area is divided into 1km×1km grid cells, and the number of the people in each cell is assigned to the cell as its population attribute; 2)based on the cell’s earthquake risk attribute, the landslide death rate in each cell is calculated by using the logistic regression model, and 3)by adding the death number in each cell, the total death toll in the study area are got.
To test our new model, we select four earthquake events for case study: the 2014 Ludian, Yunnan MS6.5 earthquake; the 2008 Wenchuan, Sichuan MS8.0 earthquake, and the 2012 Yiliang, Yunnan MS5.6 & 5.7 double earthquakes. Here, the Ludian earthquake case is for testing the effectiveness of our new model. The other two cases serve for testing the practicability of our model for the future earthquake disasters in other places in China.
Our logistic model of the landslide death-rate in the kilometer grid shows that the absolute values of the coefficients of the 2 impact factors are relatively large. These 2 factors, which indicate respectively the highest landslide-hazard grade and the lowest one, are playing critical roles in deciding the death toll from earthquake-induced landslide.
In each kilometer grid, when the impact factors of the landslide-hazard grade are 5, 4, and 3, their corresponding coefficients are respectively 0.040 77, 0.031 30, and 0.013 65. If the number of the sub-grids corresponding to these 3 impact factors rise, the death rate will accordingly rise. The more the sub-grids with high impact factors, the higher the death-rate will be. When the impact factors of the landslide-hazard grade are 2 and 1, their corresponding coefficients are respectively -0.016 66 and -0.09652. If the number of the sub-grids corresponding to these 2 impact factors rise, the death rate will accordingly fall. The more the sub-grids with low impact factors, the lower the death-rate will be.
For the Ludian earthquake, we estimate 233 deaths according to our model, with an error rate of 6.80%compared with 250 dead and missing people in reality, and Kappa coefficient is 0.912, indicating the feasibility of our model. In the 2008 Wenchuan earthquake event, a total of about 20 000 people died and went missing due to earthquake-induced landslides, while our evaluation is 18732, with an error rate about 6.5%. For the Yiliang double earthquake event, we get 48 deaths by our model, 11 less than the actual number. The error rate is 18.64%, and Kappa coefficient is 0.899. The results suggest that on the premise of the allowable error, our method is practical and can be applied to assessing the landslide death toll for other earthquake events in future.

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
2021, 43(5):  1269-1291.  DOI: 10.3969/j.issn.0253-4967.2021.05.014
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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.

THREE-DIMENSIONAL SEISMIC VELOCITY STRUCTURE OF THE MIDDLE PART OF NORTH TIANSHAN MOUNTAINS, XINJIANG BASED ON SEISMIC RELOCATION AND LOCAL SEISMIC TOMOGRAPHY
ZHANG Zhi-bin, LIANG Xiao-feng, ZHOU Bei-bei, LIU Dai-qin, TANG Ming-shuai
2021, 43(5):  1292-1310.  DOI: 10.3969/j.issn.0253-4967.2021.05.015
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As one of the largest and most active intraplate orogenic belts in the world, Tianshan has experienced significant crustal shortening and uplift since the Neogene. Its crustal deformation and structure have been deeply concerned by researchers. The predecessors carried out many studies on the deep structure of the Tianshan area, but limited by the distribution of stations, the relatively fine three-dimensional crustal velocity structure is still lacking. The middle section of North Tianshan is the most concentrated area of population and economy in Xinjiang. There are a series of active faults in the piedmont, causing frequent occurrence of strong earthquake in the region. Due to the lack of targeted research on the deep structure of the region, it is difficult to understand the mechanism of earthquakes in the region. In this study, we used the observations of 14 fixed stations and 4 temporary stations set up in the middle section of north Tianshan Mountains in Xinjiang to obtain the three-dimensional velocity structure of the crustal by using local seismic tomography and relocated the local seismic events in the area using the three-dimensional velocity structure. The seismic events used in the inversion were recorded by at least 5 stations, and the azimuth coverage was greater than 180°. Finally, 629 seismic events were obtained, including 5 238 P-wave traveltimes and 2 144 S-wave traveltimes. The time-distance curve indicates that there are some seismic events with deeper focal depths in the study area. After relocation, the residual is stable at about 0.52, the theoretical deviation of the source position is about 0.5km, the root mean square of the residual is reduced from 0.8 seconds to 0.5 seconds, and the positioning accuracy is obviously improved. The inversion of the three-dimensional velocity structure shows that the velocity structure in the middle section of the Tianshan Mountains in Xinjiang shows obvious longitudinal inhomogeneity. At a depth of 5km, the shallow layer shows high-velocity zone beneath Tianshan and low-velocity zone on the side of the Junggar Basin, which is characterized by a high-velocity anomaly along the Tianshan Mountains belt. At a depth of 10km, the study area is dominated by high-velocity anomalies. Among them, with Hutubi as the boundary, the east part is characterized by high-velocity anomalies, and the west is the low-velocity anomalies, showing an obvious velocity interface. At a depth of 15km, the east of Hutubi shows a relatively low-velocity anomaly, and the west shows a high-velocity anomaly, indicating that there is a large gradient of P-wave velocity in this range. At a depth of 20km, the area shows low-velocity anomalies, but on the southern Junggar fault zone, it shows high-velocity anomalies. In the middle and lower crust, the area is dominated by low-velocity anomalies near the southern margin of Junggar and the Bogda arc fault. The low-velocity zone may be caused by a large ductile shear system in the area. S-wave is limited by observation conditions, and the upper and middle crust exhibit characteristics such as relatively low wave velocity and low wave velocity ratio. The relocation results show that in the basin-mountain junction, especially near the Bogda arc structure, the middle and lower crust earthquakes occur frequently. The deeper depth of the source indicates that the area has a lower geothermal gradient, and the velocity structure in the area is lower than normal, and structural deformation is strong. There is a seismogenic zone extending about 10km underground from the surface to the middle and lower crust around the Hutubi gas storage. The seismic activity of this area is importantly related to the gas injection and pumping of the gas storage. In addition, the relocation distribution characteristics of the aftershocks of Hutubi earthquake show that the Hutubi earthquake occurred on the southern marginal fault zone of Junggar. The distribution of aftershocks indicates that the fault may be south-dipping, and the dip angle is about 50°. Aftershocks are basically located above the main shock, and the initial rupture point is at the bottom, causing rupture from bottom to top. At the same time, the source is located in a high-velocity anomaly area, which creates conditions for the preparation and occurrence earthquake.

PRE-SEISMIC POLARIZATION CHARACTERISTICS OF MAGNETIC DISTURBANCE OF THE 2015 MW7.8 NEPAL EARTHQUAKE
GUAN Yi-liang, DONG Xiao-na, YIN Yu-zhen, FENG Li-li, YIN Hai-tao
2021, 43(5):  1311-1325.  DOI: 10.3969/j.issn.0253-4967.2021.05.016
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On April 25, 2015, a strong earthquake with magnitude 7.8(MW)and focal depth of 8.2km(according to USGS)occurred in Nepal(28.15°N, 84.71°E), which caused heavy casualties and property losses to Nepal, China, and some neighboring countries. The focal mechanism solution shows that the earthquake is a low-angle thrust earthquake, resulting from the collision and following long-term compression between the Indian Plate and Eurasian Plate. In this paper, we analyze the magnetic disturbance characteristics before the Nepal MW7.8 earthquake using polarization method based on the geomagnetic second data of 48 stations in mainland China.
Polarization method uses the ratio of the vertical component(Z)to the horizontal component(H)of the geomagnetic field, i.e. Z/H. This method can suppress external interference and thus extract seismo-magnetic anomaly information. Previous geophysical studies have shown that the perturbation in geomagnetic field during the earthquake preparation has a far greater impact on Z than H. The high polarization anomaly may contain some magnetic field information related to earthquake preparation. The calculation of geomagnetic second data includes three main parts: spectrum analysis, polarization calculation and abnormal signal extraction. The dominant frequency band(0.01~0.2Hz)is selected for subsequent calculation and analysis.
(1)By analyzing the relationship between the polarization and the perturbation in the geomagnetic field through spectrum analysis, we find that the polarization value at each stage is obviously negatively correlated with the geomagnetic K index, indicating that the high polarization anomaly is almost not related to the geomagnetic activity and therefore can be used to analyze the pre-seismic anomaly. In order to ensure the reliability of the calculation results, it is recommended that the data length should exceed 1 year, based on the annual variation characteristics of polarization value.
(2)The polarization value of the Lhasa station(epicentral distance of 628km)and the Shiquanhe station(epicentral distance of 652km)had risen for 3 days before the Nepal earthquake, and the anomaly amplitude of the Lhasa station, which is closer to the epicenter, is significantly higher than that of the Shiquanhe station. We analyze the polarization value of stations within 1 500km from the epicenter of the Nepal earthquake, which reveals a synchronous increase of polarization value starting about 98 days before the earthquake. Considering the regional synchronization of the perturbation in the geomagnetic field, a daily correlation analysis method is proposed to analyze the polarization of stations within 1 500km from the epicenter. We find that there is a significant increase in polarization correlation during earthquake preparation, and the earthquake occurred at the synchronization transition phase. It is suggested that the synchronization may be attributed to the additional effect of the source field associated with earthquake on the regional geomagnetic field. Certainly, this method requires higher data quality, and some certain interference factors need to be eliminated to reduce the influence of individual data on the overall results.
(3)The pre-seismic magnetic disturbance changes of each station are different in anomaly amplitude, which is associated with the spatial position, tectonic setting, and signal source of the abnormality. Subsequently, spatial analysis based on the relative variation of polarization value is necessary. The results show that continuous polarization anomalies exceeding 3 days before the earthquake occurred in more than 20% of stations, the spatial scope and abnormal amplitude experience a change trend from increasing to reducing, however, the spatial distribution of anomalies which has obvious regional characteristics always revolves around the epicenter. The time-space changing process of polarization anomalies really reflects the dynamic changes of the regional geomagnetic field, which is the result of external influence with strong dynamic characteristics. Plate movement is the main driving force of the perturbation in the regional geomagnetic field, while a large amount of melting fluid substances provide good channel for preparation and propagation of geomagnetic field. Thus, the generation and distribution of polarization anomalies are closely related to the geodynamic evolution of geological structures. Stress accumulation caused by geological activities is the main reason for the perturbation in the geomagnetic field.
(4)The study suggests that multiple synchronous polarization abnormality and turning in daily correlation have important indications for strong earthquakes, which will provide a new approach for monitoring and prediction. However, the quantitative relationship between anomaly amplitude and epicentral distance is not obvious, which is affected by tectonic environment of station, seismogenic background, complexity of changes of spatial geomagnetic field and fewer seismic examples. Therefore, in order to obtain more evidence and improve reliability, more seismic examples and theoretical analysis is necessary.

Application of new technique
OUTLIER DETECTION METHOD AND APPLICATION OF SATELLITE GRAVITY DATA TRAINED BY LONG SHORT-TERM MEMORY NETWORK
YANG Yu, WU Yun-long, YAO Yun-sheng, SHAN Wei-feng
2021, 43(5):  1326-1338.  DOI: 10.3969/j.issn.0253-4967.2021.05.017
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