Characteristics of the seismic sequence and seismogenic environment of the M-s 6.0 Sichuan Luxian earthquake on September 16, 2021

On September 16, 2021, an M(s)6.0 earthquake occurred in the Luxian County, Luzhou City in Sichuan Province. It is another M(s)6.0 event in Sichuan Basin after the M(s)6.0 Changning earthquake on June 17, 2019. The epicenter was located in the NE-trending Huayingshan fold-thrust belt, and the intensity of the meizoseismal area were estimated to be around VIII degree. Three deaths and 159 injuries were reported after the earthquake. Until September 23, very rare aftershocks of the M(s)6.0 Luxian earthquake sequence were recorded, and the largest one was the M(s)2.8 (M(L)3.4) event at 04 :55 on September 16. The magnitude difference between the maximum aftershock and the mainshock was 3.2, showing the characteristics of isolated earthquake sequence. Based on the waveform data from permanent stations, the moment magnitude and focal mechanism solution as well as centroid depth of the M(s)6.0 Luxian event were obtained by the CAP waveform inversion method. The focal mechanism indicates this earthquake is a thrusting event, with strike/dip/rake of 286 /45/103 and 88 /46/77 for nodal planes I and II, respectively. The azimuth of P axis is 187 with plunge angle of 1, reflecting a horizontal compression mode that the principal compressive stress is featured by a near NS direction in the source area. This is significantly different with the principal compressive stress in the NW-SE direction of the regional tectonic stress field in the South China block, indicating that the earthquake may be controlled by the local stress field. The moment magnitude is M(w)5.36, and it is much lower than the surface-wave magnitude M(s)6.0. The focal centroid depth of this event is 3.5 km, close to the relocated initial rupture depth of about 5.1 km, consistent with the depth distribution characteristics of moderate and strong earthquakes in Sichuan Basin in the latest three years. All these observations imply this event occurred in the sedimentary layers of the shallow part of the upper crust with relatively small rupture size. The directions of the two nodal planes from the focal mechanism solution of the M(s)6.0 Luxian earthquake are both inconsistent with the geometric structure of the western branch fault of Huayingshan fold-thrust belt or the known nearby surface faults. Considering near NWW-trending of the spatial distribution of the relocated foreshocks and early aftershocks, we suggest that the nodal plane I in NWW direction is the coseismic fault plane with dip angle of 45. We further infer that the M(s)6.0 Luxian earthquake was caused by the dislocation of the NWW-trending blind thrust fault in the sedimentary layers, and it was under the horizontal principal compressive stress in the near NS direction.