Complex Double-Event Rupture of the 2022 Mw 6.6 Luding Earthquake

On 5 September 2022, a strong earthquake (Mw 6.6) struck Luding County, Sichuan, China, causing significant casualties and property damage and marking the deadliest earthquake in mainland China in eight years. Previous studies using seismologic and/or geodetic data estimated the centroid depth of the Luding earthquake, with results varying widely from 2 to 18.4 km depth. To investigate the cause of this discrepancy, this study first performed a single point–source moment tensor inversion and found that regional and teleseismic data prefer centroid depths of 6 and 17 km, respectively. This suggests that a single point–source model may not be adequate to characterize the rupture. We then used synthetic doublet models to test the sensitivity of regional and teleseismic data to subevent depths. The results support a model that the Luding earthquake comprises two subevents: a shallow event at 6 km depth (0.7×1019 N·m) and a deeper event at 17 km depth (0.3×1019 N·m), with the deeper one rupturing 7 s after the shallow one. A relocated hypocenter depth for the Luding earthquake, based on Pn/Pg relative location, also confirms a shallow nucleation. Furthermore, a multiple point–source inversion of the teleseismic data confirms the presence of two subevents. Although the deeper subevent had a smaller moment, it significantly influenced the single point–source centroid depth inversion. Our results suggest that the large variance in published centroid depth stems from different depth sensitivities of regional and teleseismic datasets to a complex source process with multiple subevents. Therefore, the multiple point–source inversion is crucial for accurately assessing seismic hazard and understanding the rupture processes of complex moderate events such as the Luding earthquake.