Spatial–Temporal Distribution and Focal Mechanisms for the Shallow Earthquakes in the Rongchang-Luxian Region in Southern Sichuan Basin, China

Abstract An Ms 6.0 earthquake struck the Luxian County in southern Sichuan basin, China, on 15 September 2021, which was the largest local event thus far and caused heavy casualties and serious property losses. Using data of 107 temporary and six permanent stations from five months before to four and a half months after the time of the mainshock in the Rongchang-Luxian region, we utilize the machine learning-based workflow for phase picking, earthquake detection and relocation, and obtain precise hypocenter data for 22,806 events. We found that earthquakes increase significantly since July 2021, showing north-northeast-, northeast-, east-northeast- and east-southeast-directed seismic strips. Almost all events are located within the sedimentary layers at 1–7 km depth above the crystalline basement, and more than half of them occurred in Silurian and Ordovician formations. The Ms 6.0 Luxian earthquake sequence occurred in an area where no clear seismicity was observed before the mainshock. On map view, the aftershock zone shows a convex polygon with an overall strike of 112°. The focal depths of aftershocks southwest to the mainshock are distributed in a narrower range of 4–7 km and extend to a wider range of 2–7 km on the northeast side. It is speculated that the seismogenic fault is a blind thrust fault dipping to the southwest at 2–7 km depth. The focal mechanisms of 13 ML 3.0+ earthquakes show either pure reverse faulting or reverse faulting dominated mechanisms with a strike-slip component. The centroid depths fall in the range of 1.95–6.75 km. The maximum and middle principal stress axes are nearly horizontal; the azimuth of the maximum horizontal stress is about 120° and the stress shape ratio is 0.78. These results provide an important reference for reunderstanding the local seismic risk and for detailed studies on the seismological aspects about the unusual Luxian earthquake in the future.