关键词: 尼玛地震, InSAR, 同震形变, 滑动分布, 依布茶卡—日干配错断裂

Abstract: The Qinghai-Tibet Plateau has always been one of the areas with frequent strong earthquakes in China. On July 23, 2020, a Nima Mw6.3 event occurred in the half-graben basin in the Yibug Caka-Riganpei Co fault zone in the central Qiangtang block. After the earthquake, many institutions have calculated the focal mechanism solutions based on seismic waves. Although there are differences between the focal position and the parameters of the seismic fault, they all show that it is a normal fault earthquake. This is inconsistent with the strike-slip of Yibug Caka-Riganpei Co fault. In addition, this earthquake is another strong earthquake that occurred on Yibug Caka-Riganpei Co fault after the Gaize Ms6.9 event on January 9, 2008. Therefore, studying this earthquake can better determine the tectonic movement of the seismogenic fault, as well as, it has great significance to understand the seismogenic mechanism and risk of the central Qiangtang block. Due to the average elevation in the central Qiangtang is more than 4800 m, the environment is so harsh that it is difficult to carry out the field survey of the earthquake. At the same time, GNSS sites near the epicenter are extremely sparse. Therefore, the InSAR technology, which has been successfully applied to several earthquakes in Qinghai-Tibet Plateau, and the sentinel-1 SAR data, which can be downloaded free of charge, are used to study this earthquake. Firstly, we obtain the coseismic deformation field based on GAMMA software. In the ascend and descend InSAR deformation field, we can observe that both deformation trends are basically consistent. The earthquake caused an elliptic settlement area (～12km long and ～8km wide) in the basin, and the maximum settlements in line-of-sight direction are -0.298?m and -0.238?m in the ascend and descend InSAR deformation field respectively. It can also be observed that the basin has a small amount of horizontal sliding relative to the mountains on both sides. Secondly, the seismic fault parameters are determined based on the elastic half-space dislocation model. Then, according to the linear relationship between the slip and the deformation on the fault plane, SDM method is used to invert the coseismic slip distribution on the fault. In the inversion, since the average Poisson's ratio in Qiangtang is significantly higher than that in the general crust, the Poisson's ratio is set at 0.29. The results show that:(1) the coseismic slip was dominated by normal dip motion, with few strike components, and mainly distributed at depths 3～12 km, with the maximum slip of approximately 1.1 m at 7 km. The causative fault did not rupture the surface; (2) the west branch fault of Yibug Caka-Riganpei Co fault is the seismic fault, with a strike of ～30°, a dip of ～68°, a slip of ～-73°; (3) The rupture pattern of the earthquake indicates that there is extension stress accumulation in Yibug Caka-Riganpei Co strike-slip fault and the central part of the Qiangtang block is in extension stress state.

Key words: Nima earthquake, InSAR, Coseismic deformation, Slip distribution, Yibug Caka-Riganpei Co fault