Three-dimensional crustal velocity structure around Longmenshan and Anninghe fault zones

The Anninghe and Longmenshan fault zones are the eastern boundary of the Tibetan Plateau, and they are also important strong earthquake active zones. The study of the deep structure of the fault zones and surrounding areas is of great significance to understand the dynamic process of the eastward expansion of the Tibetan Plateau and the tectonic mechanism of strong earthquake. In this paper, based on the seismic travel time data from regional seismic network and temporary seismic arrays from 2008 to 2021, the high-resolution 3D crustal P-wave velocity structure around the Longmenshan and Anninghe fault zones is obtained by double difference tomography. There are obvious low-velocity anomalies in the shallow part of the Sichuan Basin, and the low-velocity anomalies can extend to about 15 km near the foreland basin, which is related to the foreland basin receiving sediments from the eastern margin of the Tibetan Plateau. Near the Longmenshan fault zone, there are some high-velocity anomalies in the upper to middle crust parallel to the fault zone, which reveals the spatial distribution of domal complexes and thrust complexes in this area. For example, the Pengguan domal complex located in the middle of the Longmenshan fault zone extends down to a depth of about 15 km, while the adjacent Xuelongshan domal complex extends down to a depth not deeper than 5 km. There is an active microseismic zone with a focal depth deeper than 20 km between the Anninghe and Daliangshan fault zones, which is consistent with the distribution of P-wave high-velocity anomaly, therefore we speculate that the existence of high-velocity anomalies may increase the depth of the crustal brittle-ductile transition zone, and deep seismicity appeared in this region due to the strong deformation at the intersection of the Anninghe and the Daliangshan fault zones. The research results of this paper provide new information for further understanding of the deep geological structure and seismic activity mechanism in this area.