Aseismic Slip and Cascade Triggering Process of Foreshocks Leading to the 2021 Mw 6.1 Yangbi Earthquake

Abstract Understanding the nature of foreshock evolution is important for earthquake nucleation and hazard evaluation. Aseismic slip and cascade triggering processes are considered to be two end-member precursors in earthquake nucleation processes. However, to perceive the physical mechanisms of these precursors leading to the occurrence of large events is challenging. In this study, the relocated 2021 Yangbi earthquake sequences are observed to be aligned along the northwest–southeast direction and exhibit spatial migration fronts toward the hypocenters of large events including the mainshock. An apparent static Coulomb stress increase on the mainshock hypocenter was detected, owing to the precursors. This suggests that the foreshocks are manifestations of aseismic transients that promote the cascade triggering of both the foreshocks and the eventual mainshock. By jointly inverting both Interferometric Synthetic Aperture Radar and Global Navigation Satellite Systems data, we observe that the mainshock ruptured a blind vertical fault with a peak slip of 0.8 m. Our results demonstrate that the lateral crustal extrusion and lower crustal flow are probably the major driving mechanisms of mainshock. In addition, the potential seismic hazards on the Weixi–Weishan and Red River faults deserve further attention.