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, an M_W6.3 earthquake occurred in Nima, Tibet in a half-graben basin of 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 in the source location and the parameters of the seismic fault, they all show that it is a normal fault earthquake. This is inconsistent with the strike-slip character of the Yibug Caka-Riganpei Co Fault. In addition, this earthquake is another strong earthquake that occurred on Yibug Caka-Riganpei Co Fault after the Gaize M_S6.9 event on January 9, 2008. Therefore, by studying this earthquake, we can better determine the tectonic movement of the seismogenic fault, so it has great significance for understanding the seismogenic mechanism and risk of the central Qiangtang block.
The average elevation in the central Qiangtang is more than 4 800m, the environment is harsh and 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 and select SRTM data with 30m resolution(http://gdex.cr.usgs.gov/gdex/)as the reference DEM. In order to improve the orbit error in interferogram, the precise orbit data provided by ESA(https://qc.sentinel1.eo.esa.int/aux_poeorb/)is used for correction. The adaptive filtering method with filtering function based on local fringe spectrum is used to filter the interferograms, and the filtering windows are set to 128×128 and 32×32, respectively. This iterative filtering window setting from large to small can greatly improve the coherence of the interferogram. Unwrapping phase method uses minimum cost flow(MCF)technology and irregular triangular mesh(TIN). 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 -0.298m and -0.238m 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. Based on the ascending and descending deformation field, the deformation pattern of the earthquake accords with the main characteristics of normal-fault event, which is consistent with seismological results.
Secondly, taking the focal mechanism solutions published by GCMT and NEIC as initial reference values, the seismic fault parameters are determined based on the elastic half-space dislocation model and InSAR deformation field. 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 normal crust, the Poisson's ratio is set at 0.29. The results show that: 1)The coseismic slip is dominated by normal dip-slip motion, with small amount of strike-slip component, and the slip is mainly distributed at the depths of 3~12km, with the maximum slip of approximately 1.1m at the depth of 7km. The causative fault did not rupture the surface; 2)The seismic fault is the west branch fault of the Yibug Caka-Riganpei Co Fault, with a strike of ~30°, a dip of ~68°, a slip of ~-73°.
The Yibug Caka-Riganpei Co Fault is still active today, and it is generally a left-lateral strike-slip fault. The Yibug Caka Lake in the epicentral area is a pull-apart basin controlled by the strike-slip fault. The rupture pattern of the Nima earthquake is similar to that of the Gaize M_S6.9 earthquake in 2008, both of which are normal dip-slip caused by accumulation of tensile stress. This is different from the strike-slip character of Yibug Caka-Riganpei Co Fault, indicating that there is extensional stress accumulation in the Yibug Caka-Riganpei Co strike-slip fault and the central part of the Qiangtang block is under an extensional stress regime. Most shallow earthquakes in Qiangtang block occur at the junctures of active faults. Therefore, more attention should be paid to this kind of areas in the future research on seismic risk of Qiangtang block.

Key words: 2020 Nima earthquake, InSAR, coseismic slip distribution, Yibug Caka-Riganpei Co Fault