The 2020 M_S6.4 Jiashi earthquake occurred on January 19, preceded by an M_S5.7 foreshock on January 18. These two earthquakes occurred close in space and time raising the question of the relationship between the two events. Using the observation data recorded by fixed stations and temporal stations of Xinjiang seismic network, the seismicity feature and the seismogenic fault of the Jiashi M_S6.4 earthquake sequence are studied in this paper. We relocated the Jiashi earthquake sequence from January 18 to August 31, 2020, and obtained the relocations of 1 460 earthquakes by the double-difference algorithm. The high-precision earthquake catalog reveals detailed spatial and temporal evolution of the earthquake sequence. The relocations show that the M_S6.4 earthquake is located at 39.835°N, 77.148°E, and the focal depth is 14.9km. The earthquake sequence is distributed in two dominant directions, one is NNW direction, the other is near EW direction. The length of the NNW earthquake belt is about 20km, and the length of the near EW earthquake belt is about 40km. The dip angle of the seismogenic fault of the NNW earthquake belt is steep, dipping to the west. The dip angle of the seismogenic fault of the near EW earthquake belt is steeper in the west, and gradually becomes more gentle from west to east, dipping to the south slightly. The main shock(M_S6.4) and the foreshocks including the M_S5.7 event occurred along the NNW earthquake belt. A large number of aftershocks occurred along the near EW earthquake belt, and two aftershocks above M5 occurred at the eastern side of the EW earthquake belt. The aftershocks on the south side of the main shock are rare, perhaps affected by the hard blocks of the Tarim Basin. The aftershocks distribution clearly illuminates a near EW-striking structure, likely the extension of the NNW-striking fault activated during the initial sequence. The dominant depth of the earthquake sequence is between 10km and 20km, the focal depth of aftershocks along the near EW direction is gradually shallower from west to east. We determined the focal mechanism solutions of the M_S≥5.0 earthquakes by the CAP method. The results of focal mechanism inversion show that the focal mechanism of the main shock and two aftershocks above M_S5 are mainly thrusting, and the M_S5.7 foreshock is mainly strike-slip. We also determined the moment tensor solution of the main shock using ISOLA method as a single-source. The focal mechanism solutions of the main shock obtained by the two methods are consistent. The moment tensor solution of the main shock has a large non-double couple component, which proves that the rupture process is very complex. By inversion of the main shock using ISOLA method as a multi-source, the main shock, which was reported as a single event, is instead composed of two sub-events, a strike-slip rupture and the second thrust rupture. Within 4s, a strike-slip earthquake triggered a second large rupture on a thrust fault. The first rupture is consistent with the mechanism of the M_S5.7 foreshock, and the second rupture is consistent with thrust-faulting mechanisms in the ensuing aftershock sequence. By analyzing the data of spatial distribution and focal mechanism of the earthquake sequence, it is speculated that the Jiashi M_S6.4 earthquake occurred in the middle and lower crust below the detachment layer of the Kalpin thrust tectonic zone. The occurrence of the main shock is caused by the joint action of the two faults, the NNW-striking fault with a high dip angle and the near EW-striking fault dipping south. The M_S6.4 rupture initiated the adjacent previous NNW-striking rupture of the M_S5.7 event, extending the earlier rupture both to the NNW and EW directions. The M_S6.4 earthquake is the result of the interaction between the two blocks, the south Tianshan Mountains and the Tarim Block.

In this paper, the double difference seismic tomography method is applied to the phase arrival times of 7 465 seismic events to determine the hypocenter parameters of events as well as detailed 3D velocity structure at the northern segment of Xiaojiang Fault and its surrounding area. The data was recorded by 42 stations of the Jinshajiang River network from August 2013 to November 2016. At 2~6km, V_P and V_S present low velocity anomalies along the northern segment of Xiaojiang Fault, and the V_S anomaly is especially remarkable. On both sides of the Xiaojiang Fault, there also exist obvious P and S wave low velocity areas. These low velocity areas correspond to the terrain, lithology distribution and the watershed of Jinsha River at shallower layer in the study area. Starting from 6km, a NE-directed high V_P band along Zhaotong-Ludian and Huize-Yiliang Fault is formed on the eastern side of the northern segment of Xiaojiang Fault. V_S also shows the high value in the area bounded by Lianfeng Fault, Baogunao-Xiaohe Fault and Huize-Yiliang Fault. Above 10km depth, to the west side of the Xiaojiang Fault including the Ninghui Fault, V_P shows a significant low-velocity anomaly, while to the east side it presents high velocity feature. The Xiaojiang fault zone shows a significant low V_P from north to south in the study region, and the low velocity anomaly in the northern segment is relatively significant, especially the low velocity anomaly area reaches 15km deep around Qiaojia area. Beneath the Baihetan Dam, a significant low V_P area reaching to 5km deep is found. The earthquakes around the dam formed a strip from shallow to deep on the low-velocity area side. Whereas, a stable high-velocity area is found under the Wudongde Dam. The events relocation result shows that:all the focal depths in the study area are shallower than 20km, and the predominant focal depth is within 15km. Different from the NE-trending of the major faults in the study area, the relocated seismic events are obviously distributed nearly east-west along Matang Fault and Daduo Fault and the region around Huize. The focal depths of M_S6.5 Ludian earthquake sequences are shallower than 15km, and mostly less than 10km. The aftershocks within 2a after the Ludian M6.5 earthquake form two predominant bands of about 40km and 20km along near EW and SN direction, respectively.

We select the Xiluodu-Wudongde reservoir area in the downstream of Jinsha River as the research area, and use the CAP and GPAT method to obtain focal mechanisms of M_L ≥ 2.0 earthquakes from 2016 to 2017 in this region. Then, we analyze the spatial distribution characteristics of focal mechanism solutions in each local region and investigate the relationship between seismicity and regional structures. According to 414 focal mechanism solutions we get following conclusions:1)The Xiluodu dam began to impound water on May 4, 2013, and seismicity increased significantly after impoundment. We get 49 focal mechanisms in the Xiluodu dam and its adjacent area which are dominated by thrust faulting and next by strike-slip faulting, which are mainly distributed near the middle section of the Ebian-Jinyang fault zone. The distribution of nodal planes striking in NNW to NE direction is consistent with that of regional faults, and some large earthquakes are controlled by regional structures. 2)There are 39 and 24 focal mechanisms obtained in the unimpounded Baihetan and Wudongde dams and adjacent areas, and the spatial distribution of focal mechanism solutions are relatively consistent, dominated by strike-slip faulting with a small amount of thrust and normal faulting. The sinistral strike-slip earthquakes are consistent with the activity of Xiaojiang fault zone and Puduhe-Xishan Fault. The strikes of the nodal planes are distributed discretely, and many groups of faults intersect with each other in the area, suggesting that the seismogenic environment is relatively complex. 3)The seismicity in Ludian continues to be active after the Ludian M6.5 earthquake. By the end of 2017, we got 260 focal mechanism solutions in the aftershock area of the Ludian M_S6.5 earthquake of Aug 3rd, 2014, which show an "L-shape" in distribution and are dominated by thrust and strike-slip faulting. The long axis is distributed in EW direction, and the short axis is distributed in near NNW direction. The strikes of nodal planes are mainly near EW and near NE, and the nodal planes in the NW direction are less. According to characteristics of a large number of focal mechanism solutions, we deduce that there may exist a buried structure in the EW direction, the seismicity is controlled by different types of faults and the seismogenic structure is very complex. 4)The centroid depth in each region is concentrated in the range of 5~15km, indicating that the seismogenic layer in the study area is 5~15km deep in the middle and upper crust.

By using the digital teleseismic seismograms recorded by the global network(IRIS), we studied the rupture process of the M_S6.6 earthquake of Jun.30, 2012, in the border area between Xinyuan and Hejing Counties. In addition, based on the tectonic feature of the eastern segment of Kashi River Fault and regional stress state in the central segment of Tienshan region before the M_S6.6 earthquake, we discussed the possible seismogenic mechanism of the M_S6.6 earthquake. The result indicates that the lasting time of the M_S6.6 earthquake's rupture process is about 35s and the main energy release appears in the early 16s. The total rupture process consists of two sub-events, strength of the first rupture is higher than the second one. The rupture process is relatively simple and has bilateral rupture characteristics; the maximum slip amount is 45.0cm. The initial rupture point of the M_S6.6 earthquake locates on the verge of high slip amount area, the dislocation mode of main rupture area in the depth is reverse and right-lateral strike-slip type, and the one in the shallow is mainly right-lateral strike-slip type; M_S≥3.0 aftershocks are mainly distributed on the verge of high slip amount area or high gradient area of slip amount change; the feature of focal mechanisms of those aftershocks occurring within the short time after the mainshock is consistent with the slip vector distribution on the fault surface; The mid-strong earthquakes with reverse fault type taking place before the M_S6.6 earthquake had increased obviously in Xinjiang area, especially in the central segment of Tienshan area, and the azimuth and plunge of P axis of these earthquakes are consistent with the regional stress field. This suggests to a certain extent that the central segment of Tienshan area is more under the control of the tectonic stress field, which may promote the right-lateral strike-slip motion on the right-lateral wrench faulting eastern segment of the Kashi River Fault. And this may be the generating mechanism of the M_S6.6 earthquake.

Because of the special geographical location and complex tectonic background of the Chuan-Dian rhombic block and its adjacent areas, researches on the tectonic stress and fault mechanics of the region have been a concern for scientists, recently. Focal mechanism solutions and focal depths of 75 M≥3.5 moderate earthquakes occurring in the Chuan-Dian rhombic block and its adjacent regions from Aug. 1st, 2007 to Apr.21th, 2013 are obtained by using CAP method. Combining the results with the historical focal mechanism solutions from Harvard CMT, the characteristics of the spatial distribution of the focal mechanisms and focal depths are investigated, and their tectonodynamic background is discussed. Main conclusions are drawn as follows. 1)The spatial distributions of focal mechanisms differ among different faults and sub-blocks inside the block and those outside the block, which indicate the heterogeneity of the regional stress filed. 2)Spatial distribution of focal mechanisms is consistent with the tectonic background, suggesting that the recent faulting and earthquake mechanisms of the Chuan-Dian rhombic block and its adjacent regions are controlled mainly by the faulting of respective fault zones and the interaction between sub-blocks. 3)The special features of focal mechanisms, P axis and T axis distribution in the Lijiang-Xiaojinhe Fault zone further confirm the absorption and shielding effect of the fault on the plateau's escaping masses. 4)The predominant focal depth in the Chuan-Dian rhombic block and its adjacent region is 5 to 15km, suggesting that brittle seismic layer beneath the Chuan-Dian rhombic block is in the upper crust.

In this paper,the shear wave splitting features in Longtan reservoir area are studied by adopting the traditional cross-correlation coefficient method and polarization analysis,using the data recorded by the seismic network founded by a project under the National Science and Technology Pillar Program from April 2009 to April 2010.We found that most of polarization directions at seismic stations are consistent with the direction of the overall regional stress field,but local structures and faults may control or influence the fast shear-wave polarization direction.The time-delay normalized to source-station path is between 10 to 25ms/km,and among them,the time-delay is about 10ms/km at the stations LIL and XIL,which are farther away from the dam.The water depth is relatively shallow and seismic activity relatively weak after water storage,indicating the effect of reservoir water penetration or loading on the state of cracks in the reservoir area.We also found that the delay time changes consistently with the water level at stations DPD and GAL.It may be related to crack expansion and water penetration caused by the reservoir impoundment.

The FOCMEC method was adopted to resolve focal mechanisms for earthquakes occurring from Sep.2006 to the end of 2008 in the range of Longtan reservoir by using the first P polarization and SH/P amplitude ratio from earthquake waveforms recorded by the Longtan reservoir seismic network. Then,the stress field of the reservoir area was inversed.According to the results,most of the M_L≥2.0 earthquakes are of thrust faulting since the impoundment in Oct.2006.The stress fields in both the cluster A and B are oriented in NWW-SEE direction and almost horizontal,indicating that the horizontal compressive stress is still the dominant stress after the impoundment,and the dip angle of the maximum principal compressive stress is more horizontal than that before impoundment.However,the middle and minimum stress axes are somewhat inconsistent in the two clusters,which indicates the local disturbance of stress field.The genesis of reservoir induced earthquake is supposed as the coupling of pore pressure action with the fluid lubricating and weakening action under the loading,whereas the increase of shear stress due to water loading is not the causation of reservoir induced earthquake.

Based on digital seismograms recorded by Zipingpu reservoir digital seismic network and YZP station of Chendu Digital Tele-seismic Network from August 2004 to May 2008 before the M8.0 Wenchuan earthquake,the paper calculates the inelastic attenuation,geometrical spreading and Q of the Zipingpu reservoir region using Atkinson's method,and site response using Moya's method.Seismic source parameters of 287 earthquakes with M≥1.6 were calculated and the relations between them discussed.Result shows:The relation between inelastic attenuation Q andfis Q(f)=47.8×f^0.91;the site response has correlation with f;and the dependence between seismic moment,corner frequency and earthquake magnitude of small earthquakes is not obvious in the Zipingpu reservoir region,which maybe has something to do with reservoir-induced earthquake.