On January 19, 2020, an M_S6.4 earthquake occurred in Jiashi county. This earthquake located in the intersection of the three tectonic systems of South Tianshan, Tarim Basin and West Kunlun-Pamir. From 1997~2003 a group of strong earthquake swarms with M_S≥6.0 occurred in this area, which constitute an extremely rare Jiashi strong earthquake swarm in mainland China. Based on the digital waveforms of Xinjiang Seismic Network, the best double-couple focal mechanisms of the main shock, foreshock and some aftershocks with M_S≥3.6 were determined by CAP method, the Jiashi M_S6.4 earthquake sequence was relocated by multi-step locating method. We analyzed the characteristics of focal depth, focal mechanisms and source rupture to determine the seismogenic structure. The nodal plane parameters of the best double-couple focal mechanism by CAP method are: strike 190°, dip 32° and rake 31° for nodal plane Ⅰ, and strike 74°, dip 73° and rake 118° for nodal plane Ⅱ; the centroid depth is 12.1km. The focal mechanism of main shock is thrust type, but the M_S5.4 foreshock is a strike-slip event with a focal depth of 17.1km, and the focal mechanism parameters are: strike 83°, dip 78°, rake 173° for nodal plane I and strike 174°, dip 83°, rake 12° for nodal plane Ⅱ. The foreshock and mainshock are very close in space, but the rupture types are quite different, which shows the complexity of the seismogenic structure. The relocated sequence shows two dominant distribution directions, namely, the near EW direction and the near SN direction. Most of the aftershocks in the sequence are distributed in the EW direction, parallel to the strike of the Kepingtage nappe structure. The M_S5.4 foreshock and the M_S6.4 mainshock are both located near the dominant distribution in the near NS direction, and have a certain spatial distance from the distribution of aftershocks in the near EW direction. This feature may reflect that the mainshock and subsequent aftershocks are located on different fault zones. Combined with the geological structural background near the source area, it is inferred that the seismogenic structure of the M_S5.4 foreshock is a strike-slip fault L₀ with a high dip angle in NNW direction, and the basic information of the seismogenic fault L₀ may be: strike NNW(about 175°), the fault plane is nearly upright, and the fault depth can reach about 15km. L₀ may be a branch fault of the NNW-directed seismogenic structural system of the Jiashi earthquake swarm from 1997 to 1998. Since most of the aftershocks distributed on the east side of the Fault L₀, we judge that L₀ and related faults may have a certain control effect on the distribution of aftershocks. According to the location of the main shock, the spatial distribution of aftershocks and the occurrence characteristics of the fault in the source area, it is inferred that the seismogenic structure of the Jiashi M_S6.4 mainshock is a NS-directed gentle-dipping fracture. The main shock caused the simultaneous activity of the Kepingtage nappe structure, resulting in a dense distribution of aftershocks with a certain distance from itself.

We analyzed the variation characteristics of Poisson's ratio in crustal media from January 2009 to December 2012 at 11 fixed seismic stations(for station SCH, it is from January 2006 to December 2012)within an epicenter distance of 200km of the Xinyuan-Hejing M_S6.6 earthquake in Xinjiang on June 30, 2012 using the methods of P wave receiver functions, H-κ stacking of receiver functions, and time sliding window, and obtained the following conclusions:
(1)The crustal media's Poisson ratio of five stations in an epicenter distance less than 130km showed a significant and long-lasting decline about 2~3 years before Xinyuan-Hejing M_S6.6 earthquake. Taking the crustal Poisson ratio mean value as reference, the decrease ranges between 0.003 and 0.014, the decrease in 4 stations are more than twice the mean error. The variations of the Poisson's ratio in crust are characterized by "V" shape or "double V" shape. Earthquakes occur at the end of the formation of "V" shape. After the occurrence of earthquakes, the Poisson's ratio continues to rise. The earliest initial fall appeared in July 2009 at WUS station which has the minimum epicentral distance(77km). The Poisson ratio of the crustal media of 6 stations with epicentral distance more than 150km fluctuated up and down around the mean value, and there is no significant decline or persistent low value.
(2)We analyzed the arrival-time variations of the quasi-repetitive receiver functions Ps converted wave(t_Ps)of the 3 stations WUS, SCH and XNY and found that the travel times of Ps converted waves became smaller in the crust before the earthquake and increased after the earthquake.
(3)Through the comprehensive analysis on the descending process, decline ranges, variations process, duration of Poisson' ratio, the Ps converted waves arrival time variations, the original time of earthquake, and the number of stations, it is inferred that the cause for Poisson's ratio anomalous variations is the change of physical properties of crustal media in the process of earthquake preparation and occurrence. Since the variation characteristics of crustal media may be related to the earthquake magnitude, the size of seismogenic area, the medium properties under stations, and the focal distance, whether the medium variation characteristics exist before and after Xinyuan-Hejing M_S6.6 earthquake will need more earthquake cases analyses.
(4)The H-κ stacking of receiver functions is used to calculate the velocity ratio. Because P-wave velocity is given, this method can only be applied when the Ps converted wave velocity of Moho surface of receiver functions changes before an earthquake. With the application of receiver functions to the analysis of more earthquake cases, we can gain more insights into the variation of crustal medium parameters during the seismogenic process. This observation indicates that the receiver function method may become a new approach to detect the Poisson's ratio change of the crustal media before strong earthquake under the condition of high seismic network density.