Three-dimensional crustal velocity structure and activity characteristics of the Madoi Ms7.4 earthquake in 2021

In this paper, using natural earthquake P-wave arrival time data recorded by the seismic network in the surrounding area of Madoi, the three-dimensional fine P-wave crustal velocity structure at depths above 60 km in the epicenter of the Madoi Ms7.4 earthquake was inverted using the double-difference seismic tomography method. On the basis of the relocation of the source of the aftershock sequence, we summarized the strip-shaped distribution characteristics along the strike of the Jiangcuo fault, revealing the significant heterogeneity of the crustal velocity structure in the source area. Research has found that most of the Madoi Ms7.4 aftershocks were located in the weak area of the high-speed anomaly in the upper crust. The focal depth changed with the velocity structure, showing obvious fluctuation and segmentation characteristics. There was a good correspondence between the spatial distribution and the velocity structure. The high-velocity bodies of the upper crust in the hypocenter area provided a medium environment for earthquake rupture, the low-velocity bodies of the middle crust formed the deep material, and the migration channel and the undulating shape of the high-speed body in the lower crust corroborated the strong pushing action in the region. The results confirmed that under the continuous promotion of tectonic stress in the Madoi area, the high-speed body of the Jiangcuo fault blocked the migration of weak materials in the middle crust. When the stress accumulation exceeded the limit, the Madoi Ms7.4 earthquake occurred. Meanwhile, the nonuniform velocity structure near the fault plane determined the location of the main shock and the spatiotemporal distribution of the aftershock sequence.