The seismogenic mechanism of the Luding MS6.8 earthquake revealed from preseismic S-wave velocity structure and b-value distribution of the epicenter area

    On 5 September 2022, an M_s6.8 earthquake occurred in Luding County, Garze Prefecture, Sichuan Province, which broke a "quiet period" of large earthquakes in the southeast section of the Xianshuihe fault and caused a major natural disaster. The seismogenic structure, seismicity and stress state in the epicenter area of the Luding earthquake plays an important role in understanding the seismogenic mechanism of strong earthquake. In this paper, based on the seismic waveform recorded from 50 short-period seismic stations deployed in the Luding area before the earthquake and the seismic travel time data collected from the regional seismic networks, we investigated high-resolution S-wave velocity structure, spatial earthquake distribution and b-value variation images of shallow crust in the Luding area before the earthquake by ambient noise tomography, double-difference relocation and improved b-value imaging method, respectively. The results show that the mainshock rupture of the Luding earthquake initiated from an asperity with high-velocity anomaly and high stress characteristics in the Moxi segment of the Xianshuihe fault. On the west side of the mainshock, we revealed a hidden normal fault resulted from the largest M5.0 aftershock, which is concomitant branch fault within Xianshuihe Fault system. The mainshock ruptured  both a dominant asperity  and another smaller southeastern asperity with high-velocity, and caused clustered aftershocks there. These results indicate that the high-velocity "rivet" structures cross fault and high stress accumulation before the earthquake in the source area controlled the occurrences of the Luding mainshock as well as strong aftershocks in general. Identifying these special "rivet" structures through high-resolution structure imaging as well as high stress situation through b-value imaging can effectively evaluate the seismogenic capacity of faults, which is of great significance to seismic hazard assessment in key areas.