The seismogenic fault of the Maduo M_S7.4 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 M_S7.4 earthquake has a strong seismic triggering effect on the above faults. By comparison, the impact of Maduo M_S7.4 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.

On August 8, 2017, a strong earthquake of M7.0 occurred in Jiuzhaigou County, Aba Prefecture, northern Sichuan. The earthquake occurred on a branch fault at the southern end of the eastern section of the East Kunlun fault zone. In the northwest of the aftershock area is the Maqu-Maqin seismic gap, which is in a locking state under high stress. Destructive earthquakes are frequent along the southeast direction of the aftershocks area. In Songpan-Pingwu area, only 50~80km away from the Jiuzhaigou earthquake, two M7.2 earthquakes and one M6.7 earthquake occurred from August 16 to 23, 1976. Therefore, the Jiuzhaigou earthquake was an earthquake that occurred at the transition part between the historical earthquake fracture gap and the neotectonic active area. Compared with other M7.0 earthquakes, there are few moderate-strong aftershocks following this Jiuzhaigou earthquake, and the maximum magnitude of aftershocks is much smaller than the main shock. There is no surface rupture zone discovered corresponding to the M7.0 earthquake. In order to understand the feature of source structure and the tectonic environment of the source region, we calculate the parameters of the initial earthquake catalogue by Loc3D based on the digital waveform data recorded by Sichuan seismic network and seismic phase data collected by the China Earthquake Networks Center. Smaller events in the sequence are relocated using double-difference algorithm; source mechanism solutions and centroid depths of 29 earthquakes with M_L≥3.4 are obtained by CAP method. Moreover, the source spectrum of 186 earthquakes with 2.0≤M_L≤5.5 is restored and the spatial distribution of source stress drop along faults is obtained. According to the relocations and focal mechanism results, the Jiuzhaigou M7.0 earthquake is a high-angle left-lateral strike-slip event. The earthquake sequence mainly extends along the NW-SE direction, with the dominant focal depth of 4~18km. There are few shallow earthquakes and few earthquakes with depth greater than 20km. The relocation results show that the distribution of aftershocks is bounded by the M7.0 main shock, which shows obvious segmental characteristics in space, and the aftershock area is divided into NW segment and SE segment. The NW segment is about 16km long and 12km wide, with scattered and less earthquakes, the dominant focal depth is 4~12km, the source stress drop is large, and the type of focal mechanism is complicated. The SE segment is about 20km long and 8km wide, with concentrated earthquakes, the dominant depth is 4~12km, most moderate-strong earthquakes occurred in the depth between 11~14km. Aftershock activity extends eastward from the start point of the M7.0 main earthquake. The middle-late-stage aftershocks are released intensively on this segment, most of them are strike-slip earthquakes. The stress drop of the aftershock sequence gradually decreases with time. Principal stress axis distribution also shows segmentation characteristics. On the NW segment, the dominant azimuth of P axis is about 91.39°, the average elevation angle is about 20.80°, the dominant azimuth of T axis is NE-SW, and the average elevation angle is about 58.44°. On the SE segment, the dominant azimuth of P axis is about 103.66°, the average elevation angle is about 19.03°, the dominant azimuth of T axis is NNE-SSW, and the average elevation angle is about 15.44°. According to the fault profile inferred from the focal mechanism solution, the main controlling structure in the source area is in NW-SE direction, which may be a concealed fault or the north extension of Huya Fault. The northwest end of the fault is limited to the horsetail structure at the east end of the East Kunlun Fault, and the SE extension requires clear seismic geological evidence. The dip angle of the NW segment of the seismogenic fault is about 65°, which may be a reverse fault striking NNW and dipping NE. According to the basic characteristics of inverse fault ruptures, the rupture often extends short along the strike, the rupture length is often disproportionate to the magnitude of the earthquake, and it is not easy to form a rupture zone on the surface. The dip angle of the SE segment of the seismogenic fault is about 82°, which may be a strike-slip fault that strikes NW and dips SW. The fault plane solution shows significant change on the north and south sides of the main earthquake, and turns gradually from compressional thrust to strike-slip movement, with a certain degree of rotation.

In order to analyze 3-dimensional movement and deformation characteristics and seismic risk of the Xianshuihe fault zone, we inverted for dynamic fault locking and slip deficit rate of the fault using the GPS horizontal velocity field of 1999-2007 and 2013-2017 in Sichuan-Yunnan region, and calculated annual vertical change rate to analyze the vertical deformation characteristics of the fault using the cross-fault leveling data during 1980-2017 locating on the Xianshuihe fault. The GPS inversion results indicate that in 1999-2007, the southeastern segment of the fault is tightly locked, the middle segment is less locked, and the northwestern segment is basically in creeping state. In 2013-2017, the southeastern segment of the fault is obviously weekly locked, in which only a patch between Daofu-Bamei is locked, and the northwestern segment is still mostly in creeping state, in which only a patch at southeastern Luhuo is slightly locked from surface to 10km depth. The cross-fault leveling data show that annual vertical change rate of the Zhuwo, Gelou, Xuxu and Goupu sites on the northwestern segment is larger, which means vertical movement is relatively active, and annual vertical change rate of the Longdengba, Laoqianning, and Zheduotang sites on the southeastern segment is small, which means the fault is locked, and the vertical movement changes little before and after the Wenchuan earthquake. Combining with the 3-dimensional movement and deformation, seismic activity and Coulomb stress on the Xianshuihe Fault, we consider the seismic risk of the southeastern segment is larger, and the Wenchuan earthquake reduced the far-field sinistral movement and the fault slip deficit rate, which may reduce the stress and strain accumulation rate and relieve the seismic risk of the southeastern segment.

GPS campaign observations can monitor dynamic characteristic of crustal deformation near the fault zone effectively. Dynamic characteristic of crustal deformation is the manifestation of the dynamic action of the faults in deep and shallow structures. The locking and movement state of faults in deep and shallow structures can be an objective characterization of strain accumulation in seismogenic fault. So we can use dynamic GPS observations to invert fault locking and fault slip deficit rate by some models, and then judge the mid- to long-term seismic potential of the faults. Research about the faults around the Daliangshan sub-block is relatively poor, and the moderate-strong earthquakes increased significantly around the sub-block over the past decade, which makes fault locking and seismic potential around the sub-block be the problem to be urgently studied. Therefore, by using the GPS horizontal velocity field of 1999-2007, 2009-2013 and the negative dislocation model of DEFNODE, we inverted for spatial fault locking and fault slip deficit rate in the Daliangshan sub-block which contains three major fault zones, named Daliangshan, Mabian-Yanjin and Huize-Yiliang, before and after the Wenchuan earthquake. We analyzed the seismic potential characteristic of the three faults combining with the seismic gap and the spatial distribution of b value. The results show that the locking state of the three faults was basically same before and after the Wenchuan earthquake, which indicates that the earthquake probably has a very weak influence on the faults. The inversion results of two periods show that the southern segment of Daliangshan, Mabian-Yanjin and Huize-Yiliang Faults are basically completely locked except the southwestern segment of Huize-Yiliang Fault. The slip deficit rates of the three faults are not huge before and after the earthquake. Daliangshan Fault is mainly of a sinistral strike-slip deficit, Mabian-Yanjin Fault has a little amount of sinistral strike-slip and compressional deficit which was slightly enhanced after the earthquake, and Huize-Yiliang Fault is characteristic of compressional deficit with a small amount of dextral strike-slip deficit, which reduced to about zero after the earthquake. Combining with some other results, we conclude that current seismic potential for strong or major earthquakes exists on the three faults.

Borehole strain monitoring is an important geodetic means with a wide range of use in geodynamics research. One of the main reasons for the slow development of this kind of observation is that the establishment of a borehole strain monitoring site is costly and the success rate is not very high. Some sites fail due to the unfavorable borehole conditions,that is,rocks at the depth where the sensor is embedded are not intact but fractured. Sometimes even if the rocks were found not as good as required,the instrument had to be installed because of the expansive cost in drilling the hole. To solve the problem,it is necessary to prospect the rock condition of the site before drilling. Fractured rocks usually contain ground water in the fractures,which lower the rocks' resistivity. Resistivity imaging survey can be applied to the investigation of underground condition and give local distribution of resistivity with relatively high resolution. Three experiments have been carried out in Shanxi Province,in which single profiling is done at Shanghuangzhuang,cross profilings at Dongmafang and at Jiaokou,respectively. Three boreholes at Shanghuangzhuang and one at Dongmafang and at Jiaokou each were drilled for comparison of different types of instruments. Results of rocks strength experiments and instrument installations for the five boreholes agree well with results of the surveys. It suggests that resistivity imaging survey is an effective method to predict the underground condition of rocks. Instrument installation should avoid low-resistivity zones indicated by the profiling to prevent putting the sensor into fractured rocks.

On the basis of data from the field invesigation alongwith geophysical exploration and focal mechanism solutions,the authors have illustrated the tectonic framework of the crust and the characteristics of the tectonic stress field that had remained in the northern part of recent Henan area since Late Tertiary.A model has been developed for this crust segment that has been in a process of being deformed and the model has been put to test in many a way.The potential risk regions have been discussed for moderate and strong shocks.