Seismogenic fault and it's rupture characteristics of the 21 May, 2021 Yangbi MS 6.4 earthquake: Analysis results from the relocation of the earthquake sequence

Abstract

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 aftershock of MS 5.2. In addition, the mainshock triggered small-scale fractures on a NEN sinistral strike-slip fault near the source area. The comprehensive study shows that the seismogenic fault of the Yangbi earthquake is not the well-known Weixi-Qiaohou fault, but the Caoping fault in the Lanping-Simao block. The Yangbi MS 6.4 mainshock is the result of the dextral strike-slip motion of the Caoping fault, which has been revived under the NWN-SES regional principal compressive stress, and the fault has obviously new fracture characteristics. This study indicates that the continuous southeastward extrusion of material from the Tibetan Plateau is leading to the reconnection and reactivation of the old faults in the junction zone between the eastern Lanping-Simao block and the Lijiang-Dali fault system, resulting in relatively frequent moderate-to-strong earthquakes in this area. Therefore, the reactivation of old faults and the generation of new faults in the southwestern boundary zone of Sichuan-Yunnan block are worthy to pay attention on the risk estimation and evaluation of regional moderate-to-strong earthquakes. We suggest that more attention should be paid to the possibility of further southern or northern migration (or expansion) of moderate-to-strong earthquakes.

Keywords

• the 2021 ms 6.4 yangbi earthquake
• the caoping fault
• newly generated seismic fault
• source rupture process
• earthquake relocation