关键词: 伽师地震, 应力触发, 黏弹性松弛

Abstract: Jiashi seismic region of Xinjiang is located at the junction of South Tianshan, Tarim Basin and Pamir. From January 1997 to March 2003, a series of strong earthquakes occurred in Jiashi area, Xinjiang. In particular, since January 21, 1997, 7 earthquakes with M_S>6 have occurred in the area in just 4 months, which is extremely rare in China and in the world. According to the theory of seismic stress triggering, earthquakes are interrelated with each other. When an earthquake occurs, it will regulate the stress state of its surrounding active faults, thus triggering or inhibiting the seismic risk of surrounding potential faults. Also, the viscoelastic relaxation theory believes that the rheological action of the hot lower crust and mantle will transfer the coseismic stress field changes of the lower crust and upper mantle to the upper crust seismogenic layer in a period ranging from years to hundreds of years after the earthquake. This in turn affects the mechanical properties of the fault.
We invert the focal characteristics of the two Jiashi earthquakes in 1998 and 2003, reconstruct the fault rupture model, and calculate the interaction stress between the two earthquakes and the M_W6.0 earthquake in 2020 based on the viscoelastic relaxation theory to study the triggering relationship among the three earthquakes. We use a viscoelastic layered semi-infinite space model(PSGRN/PSCMP program)to estimate the changes of fault stress caused by coseismic and post seismic deformation. At the same time, we study the influence of different model parameters, including viscosity coefficient, receiving fault parameters and earth medium model parameters, on the calculation results.
The results show that: 1)The 1998 earthquake is a typical sinistral strike-slip earthquake, with fault strike of 57°, dip angle of 81° and the focal depth of 11.5km. The 2003 earthquake is a thrust strike-slip type earthquake with focal depth of 15.2km, the seismogenic fault is a northward low dip angle reverse fault and the main rupture direction of the earthquake is SE, the strike angle is 293° and dip angle is 20°. According to the calculation of fault dip angle, the deepest rupture caused by two seismogenic faults is less than 20km; 2)The 1998 earthquake led to the increase of stress in the western section of the 2003 earthquake fault and the unloading of stress in the eastern section, which increased the stress in the vicinity of the epicenter of the 2003 earthquake by 0.01~1MPa, showing an obvious triggering effect. The first two earthquakes resulted in the increase of stress in the eastern section of the 2020 earthquake fault, but the increase of stress at the epicenter was not more than 0.006MPa, which did not have an obvious triggering effect. This earthquake was mainly caused by other factors, possibly including the tectonic loading or viscoelastic triggering of previous big earthquakes; 3)Considering that there are many factors affecting the distribution of Coulomb stress on the fault, we calculated the stress distribution with different parameters, and the results show that the change of parameters has no obvious effect on the results. In the mutual triggering relationship between the earthquakes, the effect of viscoelastic relaxation after earthquake is not obvious, and the effect of coseismic stress step change is dominant. This may be due to the fact that the fractures caused by the two earthquakes are concentrated in the shallow part, and the coseismic stress in the deep part is relatively small, so the viscoelastic relaxation effect is not obvious. Our stress results are in good agreement with the aftershock distribution obtained by previous relocation methods. The results show that after a long-term stress evolution, the stress loading of the eastern segment of the 2003 earthquake fault is still more than 0.01MPa, so the seismic risk monitoring of this segment should be strengthened. Our results have certain significance for understanding the mechanism and regular pattern of earthquake occurrence in Jiashi area.

Key words: Jiashi earthquake, stress triggering, viscoelastic relaxation