Three-dimensional crustal Vp and Vs structures beneath the southern segment of the Tan-Lu fault revealed by active source and earthquake data

Abstract

SUMMARY The 2400-km-long Tan-Lu fault, the largest deformation zone in eastern China, plays a decisive role in the seismicity, regional tectonics and mineral deposits distributions. However, the velocity structure beneath the Tan-Lu fault, particularly in the southern segment, is poorly imaged due to the lack of local earthquakes. To obtain a high-resolution crustal structure image, we carried out an active source experiment by firing mobile airgun sources along the Yangtze River in the Anhui Province in October 2015. We manually picked 4118 P wave and 1906 Swave first arrival times from the airgun signals. We also collected 28 957 P wave and 26 257 S wave first arrival times from local earthquakes in a larger area. 3-D crustal velocity images beneath the southern segment of the Tan-Lu fault and surrounding areas are studied using traveltime tomography. Compared with the local earthquake data, the active source data provide better constraints on the upper crustal structure, which further refines the resolution of the lower-crust structure. The Vp and Vs crustal structures are consistent with the local geological settings, and earthquakes are primarily clustered near faults and are spatially correlated with low-velocity zones. Strong velocity contrasts are observed across the Tan-Lu fault zone, which is the main factor controlling local anomalies. The high Vp, Vs and Vp/Vs beneath the Qinling-Dabie orogenic belt and the Middle-Lower Yangtze River Metallogenic Belt may relate to Mesozoic lithospheric delamination and asthenospheric upwelling. These results also demonstrate that the mobile large-volume airgun sources are promising tools for 3-D crustal structure surveys.