Three-dimensional velocity structure of the MS 6.0 Luxian earthquake source region and adjacent areas based on a dense seismic array

On September 16, 2021, an M_S 6.0 earthquake struck Luxian County in the Sichuan basin. To investigate the regional velocity structure and its relationship with seismic activity, we gathered seismic phase data from permanent stations for events occurring between January 2009 and April 2021 as well as data from a dense mobile seismic array that operated from April 2021 to July 2023. Utilizing Double-Difference tomography, we have determined well-constrained earthquake relocations and have derived a detailed 3D velocity structure. Many of the earthquakes exhibit linear clustering patterns, with an average depth of 4.3 km and a NE-SW orientation. Approximately 96 % of the seismic events occurred within a depth range of 0–7 km. The early aftershock sequence of the Luxian event also displayed a linear trend, with a length of 6 km but with an ESE orientation. The mainshock occurred at a depth of 6.2 km, located at the northwestern end of the aftershock sequence. The aftershock sequence along with other linear seismic clusters, predominantly occurred within regions characterized by high seismic velocities and low Poisson's ratios, both within the sedimentary cover above the crystalline basement. The heterogeneity of the velocity structure likely plays a significant role in controlling the occurrence of moderate-to-strong earthquakes in the deeper parts of the study region, which deepens the existing understanding from previous research: pre-existing faults, their scales, and their slip-tendencies under the present-day regional and reservoir-scale stress fields are also controlling factors for induced earthquakes, especially larger ones. We have identified five areas where moderate-to-strong earthquakes are speculated to have a higher likelihood of occurrence. These findings hold considerable importance for local seismic hazard assessments.