Three-dimensional velocity structure and tectonic characteristics of earthquake area in Yibin

Abstract

In this study, on the basis of absolute first-arrival times of 84756 P- and S-waves from 6085 earthquakes recorded at 56 fixed stations in Yibin and surrounding areas in China from January 2009 to January 2019, focal parameters and three-dimensional (3D) body-wave high-resolution velocity structures at depths of 0–30 km were retrieved by double-difference tomography. Results show that there is a good correspondence between the spatial distribution of the relocated earthquakes and velocity structures, which were concentrated mainly in the high-velocity-anomaly region or edge of high-velocity region. Velocity structure of P- and S-waves in the Yibin area clearly shows lateral inhomogeneity. The distribution characteristics of the P- and S-waves near the surface are closely related to the geomorphology and geologic structure. The low-velocity anomaly appears at the depth of 15–25 km, which is affected by the lower crust current. The Junlian-Gongxian and Gongxian-Changning earthquake areas, which are the two most earthquake-prone areas in the Yibin region, clearly differ in earthquake distribution and tectonic characteristics. We analyzed the structural characteristics of the Junlian-Gongxian and Gongxian-Changning earthquake areas on the basis of the 3D body-wave velocity structures in the Yibin region. We found that although most seismicity in the Yibin area is caused by fluid injection, the spatial position of seismicity is controlled by the velocity structures of the middle and upper crust and local geologic structure. Fine-scale 3D velocity structures in the Yibin area provide important local reference information for further understanding the crustal medium, seismogenic structure, and seismicity.