High-resolution 3D Crustal S-wave Velocity in the shallow crust of the Tanlu Fault Zone in Hefei, Anhui, eastern China and its implication for urban sedimentary envoiroment

Abstract:

Tanlu fault zone is a largest intraplate strike-slip NNE-trending faults zone in eastern China. It consists of four main buried fractures in the southern part, crossing the eastern Hefei sedimentary basin and Chao Lake in Anhui province. In this study, we use dense-array ambient noise to construct a 3-D shear wave velocity model of shallow crust in Hefei urban and Chao Lake. Approximately one month of continuous data based on 227 three-component short-period QS5A portable stations, were recorded with a sampling rate of 100Hz. We then obtained nearly 6700 phase velocity measurements at periods of 0.5-8.5s and used the direct inversion method of surface wave travel times based on ray tracing to invert shear wave velocity structure in the depths of 0-8km. The 3-D model reveals important geological structural features of the study areas. Significant low velocity anomalies are found in Hefei urban and the eastern Chao Lake, whereas high velocities with deep roots are observed beneath the Tanlu fault zone and Yinping Mountain. This pattern suggests that the distinct features are associated with multistage tectonism along the Tanlu faults. We propose that the Tanlu fault area was uplifted to some extent after experiencing an extensional regime with magmatic upwelling derived from great depth, which controls the pattern of eastern Hefei sedimentary basin and subsidence of eastern Chao Lake; second, the southeast-dipping low-velocity anomalies beneath eastern Chao Lake were formed first because the Palezoic sediments currently exposed on Yinping Mountain were compressed and folded during the Indosinian epoch; third, in Hefei urban area, several distinct low velocity areas are identified at the shallow depth of 1km. This indicates that there are at least two sedimentary centers in the eastern Hefei basin: one is in the eastern Hefei urban area, and another is near the north margin of Dabie orogen, which both characterized by the maximum sedimentary thickness larger than 6km. Our results reinforce the idea that the Hefei basin experienced a complex sedimentary evolution effected by both Dabei orogen and Tanlu fault since Mesozoic. These resulting images of shallow crust structure serves as a basis model for the future works in deep resource exploration and earthquake strong ground motion simulation in sedimentary urban area.