Joint inversion of Rayleigh group and phase velocities for S-wave velocity structure of the 2021 M_S6.0 Luxian earthquake source area, China

On September 16, 2021, a M_S6.0 earthquake struck Luxian County, one of the shale gas blocks in the Southeastern Sichuan Basin, China. To understand the seismogenic environment and its mechanism, we inverted a fine three-dimensional S-wave velocity model from ambient noise tomography using data from a newly deployed dense seismic array around the epicenter, by extracting and jointly inverting the Rayleigh phase and group velocities in the period of 1.6–7.2 s. The results showed that the velocity model varied significantly beneath different geological units. The Yujiasi syncline is characterized by low velocity at depths of ~ 3.0–4.0 km, corresponding to the stable sedimentary layer in the Sichuan Basin. The eastern and western branches of the Huayingshan fault belt generally exhibit high velocities in the NE-SW direction, with a few local low-velocity zones. The Luxian M_S6.0 earthquake epicenter is located at the boundary between the high- and low-velocity zones, and the earthquake sequences expand eastward from the epicenter at depths of 3.0–5.0 km. Integrated with the velocity variations around the epicenter, distribution of aftershock sequences, and focal mechanism solution, it is speculated that the seismogenic mechanism of the main shock might be interpreted as the reactivation of pre-existing faults by hydraulic fracturing.