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.

The seismogenic structure of the Lushan earthquake has remained in suspensed until now. Several faults or tectonics, including basal slipping zone, unknown blind thrust fault and piedmont buried fault, etc, are all considered as the possible seismogenic structure. This paper tries to make some new insights into this unsolved problem. Firstly, based on the data collected from the dynamic seismic stations located on the southern segment of the Longmenshan fault deployed by the Institute of Earthquake Science from 2008 to 2009 and the result of the aftershock relocation and the location of the known faults on the surface, we analyze and interpret the deep structures. Secondly, based on the terrace deformation across the main earthquake zone obtained from the dirrerential GPS meaturement of topography along the Qingyijiang River, combining with the geological interpretation of the high resolution remote sensing image and the regional geological data, we analyze the surface tectonic deformation. Furthermore, we combined the data of the deep structure and the surface deformation above to construct tectonic deformation model and research the seismogenic structure of the Lushan earthquake. Preliminarily, we think that the deformation model of the Lushan earthquake is different from that of the northern thrust segment ruptured in the Wenchuan earthquake due to the dip angle of the fault plane. On the southern segment, the main deformation is the compression of the footwall due to the nearly vertical fault plane of the frontal fault, and the new active thrust faults formed in the footwall. While on the northern segment, the main deformation is the thrusting of the hanging wall due to the less steep fault plane of the central fault. An active anticline formed on the hanging wall of the new active thrust fault, and the terrace surface on this anticline have deformed evidently since the Quaterary, and the latest activity of this anticline caused the Lushan earthquake, so the newly formed active thrust fault is probably the seismogenic structure of the Lushan earthquake. Huge displacement or tectonic deformation has been accumulated on the fault segment curved towards southeast from the Daxi country to the Taiping town during a long time, and the release of the strain and the tectonic movement all concentrate on this fault segment. The Lushan earthquake is just one event during the whole process of tectonic evolution, and the newly formed active thrust faults in the footwall may still cause similar earthquake in the future.

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.