As the most active and spectacular intracontinental mountain ranges in Central Asia, Tian Shan is a natural laboratory to explore and understand the geodynamic processes involved in intracontinental mountain building. The origin of Tian Shan can be traced back to the collision and accretion of several micro-continents, island arcs, and accretionary prisms initiated in the Paleozoic. This tectonic activity continued into the Mesozoic. From the Cretaceous to the early Tertiary, the mountain ranges were eroded and reduced to a flat plain. Then, in the later Tertiary, uplift was rejuvenated as a far-field consequence of the India-Eurasia collision, and continues to present day, characterized by the active seismicity in Tienshan in modern times. The geology of present-day central Tian Shan is mainly composed of intermontane basins and subparallel ranges, separated by the east-west striking Cenozoic active thrust faults stretching approximately 2 500km in length. On the southern and northern margins of the Tian Shan Range, the Tarim Basin and the Kazakh Shield act as stable blocks. Situated in the southwest foreland of Tien Shan, the Kashgar region is a seismic active area since the 20^th century, several strong earthquakes stroke the region and the surround areas, causing severe damage to the local residents. In this study, we apply the double-difference relocation technique and W-phase method to relocate the 19 January, 2020 Xinjiang Jiashi earthquake, and to determine the focal mechanisms using data provided by China Earthquake Networks Center. The relocated epicenters of the 2020 Jiashi earthquake sequence show two dominant spatial distribution directions. The major NWW-trending spatial distribution shows a Kepingtage Fault-paralleled narrow belt stretching about 34km, with most of aftershocks distributed in the northern side of the fault. The secondary spatial distribution shows a NNW-striking belt stretching about 8km. The depth profiles show a predominant epicentral depth at the range of 10~20km. The focal parameters for the 19 January, 2020 Xinjiang Jiashi M6.4 earthquake are: strike 76°, dip 81°, rake 109° for the nodal plane Ⅰ, and strike 190°, dip 21°, rake 26° for nodal plane Ⅱ, and the moment magnitude is M_W5.87. The focal parameters indicate that the earthquake event is characterized by dominant thrust with minor strike movement. Combined with the analysis of the relocated epicentral locations, focal mechanisms and geological settings, it is inferred that the seismogenic fault of the 19 January 2020 Jiashi M6.4 earthquake is the west segment of the near E-W trending Kepingtage thrust fault.

On August 8, 2017, Beijing time, an earthquake of M7.0 occurred in Jiuzhaigou County, Aba Prefecture, Sichuan Province, with the epicenter located at 33.20°N 103.82°E. The earthquake caused 25 people dead, 525 people injured, 6 people missing and 170000 people affected. Many houses were damaged to various degrees. Up to October 15, 2017, a total of 7679 aftershocks were recorded, including 2099 earthquakes of M ≥ 1.0.
The M7.0 Jiuzhaigou earthquake occurred in the northeastern boundary belt of the Bayan Har block on the Qinghai-Tibet Plateau, where many active faults are developed, including the Tazhong Fault(the eastern segment of the East Kunlun Fault), the Minjiang fault zone, the Xueshan fault zone, the Huya fault zone, the Wenxian fault zone, the Guanggaishan-Daishan Fault, the Bailongjiang Fault, the Longriuba Fault and the Longmenshan Fault. As one of the important passages for the eastward extrusion movement of the Qinghai-Tibet Plateau(Tapponnier et al., 2001), the East Kunlun fault zone has a crucial influence on the tectonic activities of the northeastern boundary belt of Bayan Kala. Meanwhile, the Coulomb stress, fault strain and other research results show that the eastern boundary of the Bayan Har block still has a high risk of strong earthquakes in the future. So the study of the M7.0 Jiuzhaigou earthquake' seismogenic faults and stress fields is of great significance for scientific understanding of the seismogenic environment and geodynamics of the eastern boundary of Bayan Har block.
In this paper, the epicenter of the main shock and its aftershocks were relocated by the double-difference relocation method and the spatial distribution of the aftershock sequence was obtained. Then we determined the focal mechanism solutions of 24 aftershocks(M ≥ 3.0)by using the CAP algorithm with the waveform records of China Digital Seismic Network. After that, we applied the sliding fitting algorithm to invert the stress field of the earthquake area based on the previous results of the mechanism solutions. Combining with the previous research results of seismogeology in this area, we discussed the seismogenic fault structure and dynamic characteristics of the M7.0 Jiuzhaigou earthquake. Our research results indicated that:1)The epicenters of the M7.0 Jiuzhaigou earthquake sequence distribute along NW-SE in a stripe pattern with a long axis of about 35km and a short axis of about 8km, and with high inclination and dipping to the southwest, the focal depths are mainly concentrated in the range of 2~25km, gradually deepening from northwest to southeast along the fault, but the dip angle does not change remarkably on the whole fault. 2)The focal mechanism solution of the M7.0 Jiuzhaigou earthquake is:strike 151°, dip 69° and rake 12° for nodal plane Ⅰ, and 245°, 78° and -158° for nodal plane Ⅱ, the main shock type is pure strike-slip and the centroid depth of the earthquake is about 5km. Most of the focal mechanism of the aftershock sequence is strike-slip type, which is consistent with the main shock's focal mechanism solution; 3)In the earthquake source area, the principal compressive stress and the principal tensile stress are both near horizontal, and the principal compressive stress is near east-west direction, while the principal tensile stress is near north-south direction. The Jiuzhaigou earthquake is a strike-slip event that occurs under the horizontal compressive stress.

We investigated frictional sliding behavior of mixture gouges of quartz with various weight proportions of biotite and their structured equivalents with same weight proportions of biotite as layers embedded in quartz gouge. Our experiments were performed under effective confining pressure of 200MPa, pore pressure of 30MPa, temperature of 100℃ and the shear displacement rate of 1.22μm/s. The results show that for structured gouges with biotite layers embedded in quartz gouge as a weak structure, the strength has a power law decreasing trend with increasing weight proportions of biotite. The fault gouges can be weakened significantly by as little as 5wt% biotite, and 30wt% biotite corresponds to a beginning point of less sensitive strength change in response to increasing biotite proportion. On the other hand, the strength of mixed gouges shows a linear decreasing trend with increasing biotite proportion. Microstructures of deformed samples show that in mixed gouges, biotite and quartz are both sheared and grain size extremely reduced, and their contributions to overall strength have a close relation with their respective contents. However, in structured gouges, the shear deformation mainly occurred in the weak biotite layers with no shears crossing the quartz gouge. These results confirm the importance of the weak fabric in its effect on frictional strength. If the weak minerals form foliations and interconnected arrangements, it will lead to weakness of fault zones.

Before the 9th July 1979 Liyang earthquake,beginning from mid June the vertical component at the Liyang station about 20km from the epicentral area decreased gradually to a negative anomaly. After decreasing to the lowest level in the last ten days of June,it increased quickly to a positive anomaly. Just before the tectonic event,the observational value was brought back to its initial value. It is consigderd that the seismic fault perhaps responsible for the shock trends WNW or NNE. The shear stress piezomagnetic modol by Stacey has been used to calculate the theoretical values of the surface Z-component anomaly for this skock.A comparision of observations with the theoretical values shows that there have bsen the strong tectonic activities along both the faults. The shock was probably accompanied by conjugated offset on the faults.