关键词: 伽师地震, 重力场变化, GNSS, 面应变, 构造活动

Abstract:

The intracontinental orogeny of Tianshan Mountains has led to strong tectonic activity in the southern Tianshan seismic belt, and strong earthquakes occur frequently at the junction of basins and mountains. On January 19, 2020, an M_S6.4 earthquake occurred in Jiashi County, Xinjiang Uygur Autonomous Region. This earthquake is of north-dipping thrust motion with a small amount of strike-slip component. The seismogenic structure of the earthquake is the Keping Fault on the southernmost edge of the Keping Tage thrust nappe in the South Tianshan Mountains. The Keping Fault has a total length of about 460km. The overall strike of the fault is near EW, dipping northwest, and the dip angle is more than 45°. It is the boundary fault between Tianshan Mountains and Tarim Basin in Southwest China. The Jiashi M_S6.4 earthquake is an important earthquake, which broke the 17 years quiet period of M_S6.0 earthquakes since 2003. It is of great value for study on the evolution of regional tectonic deformation. The process of earthquake preparation will be accompanied by changes in geophysical field. Comprehensive analysis of geophysical observation data before and after earthquakes has great practical significance for geodynamics research and identification of earthquake precursor anomaly information. The epicenter of Jiashi earthquake is located in the monitoring area of the “Kashi-Jiashi” mobile gravity observation network and the “Crustal Movement Observation Network of China” GNSS observation. In this paper, the mobile gravity and GNSS observation data from 2015 to 2021 are selected. The mobile gravity observation network uses absolute gravity point constraint to carry out the classical adjustment calculation of the whole network, and then obtains the gravity field change image; The GNSS data are solved by GAMIT/GLOBK software to obtain the movement rate of the study area, and the horizontal apparent strain field distribution is obtained by means of the partial derivative relationship between displacement and strain. By analyzing the dynamic change characteristics of gravity field, velocity field and strain field in the seismogenic area, the relationship between the change of gravity field and GNSS deformation field and the seismogenic process of Jiashi M_S6.4 earthquake is comprehensively discussed. The results show that: 1)The time-varying images of the gravity field on the time scale of one year better reflect the evolution process of the regional gravity field system near the Jiashi M_S6.4 earthquake. The “0” contour of gravity change and its four quadrant characteristics provide useful references for earthquake prediction. 2)The cumulative change of gravity field reflects that the change of regional gravity field before the Jiashi M_S6.4 earthquake was controlled by regional large faults. A series of medium and strong earthquakes in the study area occurred in the process of gravity reverse change. The characteristics of high gradient belt of gravity change are closely related to earthquake occurrence. According to the gravity cumulative change and time-varying image, it is speculated that this tectonic activity may have started in 2018. 3)Strong earthquakes are likely to occur on tectonic active fault zones with significant gravity changes. The Keping earthquake of M=6.4 in 2020 occurred in the area where the high gradient zone of gravity changes turns, which is also the transition zone of surface compressibility changes. The variation process of regional strain field corresponds to the positive and negative cumulative changes of the regional gravity field.

Key words: Jiashi earthquake, variation of gravity field, GNSS, surface strain, tectonic activity