关键词: 定日M_S6.8地震, 同震形变场, 震源滑动模型, 同震库仑应力

Abstract:

According to the official determination of China Seismic Network, at 09:05 on January 7, 2025, an M_S6.8 earthquake(hereinafter referred to as Dingri earthquake)occurred in Dingri County(28.50°N, 87.45°E), Shigatse City, Xizang, with a focal depth of 10km. The earthquake occurred in the southern part of the Qinghai-Xizang Plateau, which is located in the intersection area of the Shenzha-Dingjie rift and the south of Xizang detachment system. The Dengmecuo fault(about 11km)is the closest to the earthquake, and the focal mechanism is tensile rupture. The earthquake had high magnitude, high intensity and shallow source, and the towns and villages in the epicenter area were relatively concentrated. In addition, the landform type of the epicenter and the surrounding area is a river alluvial plain, and the soil is soft, which amplifies the earthquake damage effect. Due to the comprehensive superposition of various factors, the earthquake caused severecasualties and building damage.

The Dingri earthquake is a shallow-source normal-fault earthquake. The ground vibration and building(structure)damage caused by the release process of seismic radiation energy are higher than other earthquakes of the same magnitude, and the surface rupture characteristics are more significant. Therefore, the in-depth study of the Dingri earthquake, the acquisition of the co-seismic deformation field and the source sliding model, and the understanding of the earthquake's seismogenic mechanism and dynamic process can provide scientific and technological support for seismic damage assessment and secondary disaster analysis. In addition, based on the fault slip model, the Coulomb stress change in the surrounding area caused by co-seismic dislocation can be calculated, which is of great significance for the scientific evaluation of the future seismic risk and potential seismic disaster risk in the adjacent area.

The Dingri earthquake occurred at a high altitude area, with an average elevation of about 4471m within 10km near the epicenter. The harsh natural conditions and the surrounding GNSS and strong seismic stations are scarce. Therefore, SAR images have become an important data source for obtaining the coseismic deformation of the earthquake and inversion of fault slip distribution. In this paper, based on the ascending and descending SAR image data before and after the Dingri earthquake taken by the Sentinel-1A satellite of the European Space Agency, the co-seismic deformation field of the Dingri earthquake was obtained by D-InSAR technology. On this basis, the source sliding model of the earthquake was jointly inverted based on the coseismic deformation data of the ascending and descending orbits, and the Coulomb stress variation characteristics of the surrounding area caused by the co-seismic dislocation were calculated. The deformation characteristics of the Dingri earthquake, the source rupture mechanism and the stress adjustment effect on the adjacent area are analyzed and discussed. Form the following understanding:

(1)The results of the coseismic deformation field of the Dingri earthquake obtained based on the D-InSAR technology ' two-track method ' show that the long axis of the coseismic deformation field of the ascending and descending orbits is nearly NS-trending. The coseismic deformation is characterized by two obvious deformation areas in the east and west and a butterfly-like stripe pattern. The LOS deformation of the ascending and descending orbits is between -0.58~0.33m and -0.80~0.66m, respectively.

(2)Based on the coseismic deformation data of ascending and descending orbits, the moment magnitude of the Dingri earthquake obtained by joint inversion is M_W7.06 by using the SDM layered model method. The rupture process of the earthquake shows a unilateral rupture characteristic from the initial rupture point to the north along the fault. The fault dislocation is a standard fault mechanism with a little strike-slip component. The length of the main rupture zone of the seismogenic fault is about 55km, and the slip distribution is concentrated in the depth range of 0~15km underground. The maximum slip is 4.25m, which occurs at a depth of 8.6km underground. The main rupture zone of the earthquake has reached the surface, located about 35~53km north of the epicenter along the strike, and the potential surface rupture length is about 18km.

(3)The results of the change in coseismic Coulomb stress show that the Dingri earthquake led to a decrease in coseismic Coulomb stress on both sides of the seismogenic fault. The Coulomb stress at the north and south ends of the fault rupture section and its surrounding areas increases significantly, and the loading amount is much larger than the earthquake-triggering threshold of 0.01MPa. There is a possibility of further felt aftershocks in these areas in the future.

Key words: Dingri M_S6.8 earthquake, coseismic deformation field, source sliding model, coseismic Coulomb stress.