Integrated seismotectonic model of the 2013 Mw6.6 and 2022 Mw5.8 earthquake sequences in the southwestern section of the longmenshan fault zone, China, and its implication

Abstract

Summary The 2008 Mw7.9 Wenchuan earthquake ruptured the middle and northeastern segments of the Longmenshan Fault Zone (LMSFZ), and the 2013 Mw6.6 Lushan earthquake ruptured a 50km-long fault in the southwestern segment. Subsequently, an Mw5.8 earthquake occurred approximately 10 km distant from the Mw6.6 Lushan earthquake. Therefore, the potential risk for larger earthquakes (>Mw6.6)on the southwestern section must be considered. This study collects the latest seismological and GPS data to construct an integrated seismotectonic model for the two neighboring earthquake sequences. The model integrates the fault planes involved, the mainshock rupture processes, the mainshock-caused Coulomb stress perturbation, the aftershock distribution and the 3-D velocity structure of the source region, providing information for seismic risk evaluation. We find that three fault planes were involved, two of which were related to the mainshocks, and the third was generated by the aftershocks following the first mainshock. The mainshocks were caused by nearly pure thrust faulting on the two planes with dip angles of approximately 45° and almost opposite dipping directions, thereby forming a conjugate angle of around 90°. The third plane was located between the two mainshocks, approximately parallel to the second mainshock's fault plane. Each of the mainshocks primarily ruptured a single asperity, displaying simple time history. The Coulomb stress change of the first mainshock facilitated the generation of the second mainshock and the third fault plane, and the second mainshock increased the stress on the first mainshock's fault plane. The aftershocks were distributed within stratified materials by spatially varying interfaces and characterized by high Vp and Vs velocity and a low Vp/Vs ratio. The atypical dip angles of approximately 45° for thrust faults and the conjugate angle of approximately 90° are indicative of high stress state. The single asperity rupture implies simple stress accumulation. The mainshock-caused Coulomb stress change did not reduce the seismic risk in the source region. The varying interfaces are interpreted as a consequence of long-term horizontal compression. All of these characteristics suggest that the two earthquake sequences were generated by the breakage of three immature faults under strong compression by background stress, and the high stress state remains within the southwestern LMSFZ.