2021年5月21日漾濞 M S 6.4地震的发震断层及其破裂特征: 地震序列的重定位分析结果
摘要
According to China Earthquake Network Center (CENC), the MS 6.4 Yangbi earthquake struck northwestern Yunnan Province on 21 May, 2021 at 21:48(Beijing time). Figuring out the seismogenic fault and source rupture characteristics in time can provide a key basis for understanding the dynamic conditions in this region and estimating the risk of strong earthquakes in the future. We employed the double-difference relocation algorithm to relocate the Yangbi earthquake sequence, and obtained precise locations of 3, 863 earthquakes. In general, the result revealed a narrow 25-km-long, linear southeast seismicity trend concentrated in the 2~14 km depth range, and the orientation is 135°. The MS 6.4 mainshock located at (25.688°N, 99.877°E) after relocation, and the focal depth is 9.6 km. Based on the results of precise locations and focal mechanism solutions, the seismogenic fault might be a NW dextral strike-slip fault with southwest dip, and the dip angle tends to gradually decrease from NW to SE. The Yangbi earthquake sequence belongs to the "foreshock-mainshock-aftershock" type, revealed by the temporal and spatial evolution process of the earthquake sequence, and the fracture process mainly includes three stages: fracture nucleation stage, mainshock rupture stage, and tension rupture stage. In the first stage, small-scale fractures occurred at the relatively weak part of the seismogenic fault at the depth between 10~12 km, after two-days' nucleation, the fault entered into an unstably accelerated rupture state, resulting in the MS 5.6 foreshock. Under the joint influence of continuous loading of tectonic stress and surrounding small-scale fractures, the higher strength blocking area in the shallow part of the fault ruptured, and the MS 6.4 mainshock occurred. The tension rupture mainly occurred at the southeast end of the seimogenic fault. A horsetail splay with normal fault features was formed at the southeast end of the aftershock sequence, started by the largest after