Detection of urban hidden faults using group-velocity ambient noise tomography beneath Zhenjiang area, China.

The Mufushan-Jiaoshan fault (MJF) is a hidden active fault located on the north side of the Ningzhen Mountain Range and developed along the Yangtze River in Zhenjiang area, China. In this paper, the structure of MJF is detected and studied using group-velocity ambient noise tomography. In the study area (18 km × 25 km), 47 short-period seismic stations were deployed with the average station spacing of about 3 km and 24 days (from 27 February to 22 March 2019) of continuous ambient-noise recordings were collected. And 510 group velocity dispersion curves in the period band 0.5-5 s were extracted using the vertical component data. And then the three-dimensional shear-wave velocity structure was inverted using group dispersion data by the direct surface-wave tomographic method. Our results are consistent with the geological background of the study area, showing that in the depth range of 0.6-1.5 km, the north side of MJF presents a relatively high velocity, and the south side presents a distribution pattern of high and low velocity. While in the depth range of 1.5-2.0 km, the shear-wave velocity (V) model is relatively simple with relatively low velocity on the north side and relatively high velocity on the south side. And the gradient zone of V may be the location of the main fracture surface of MJF. The good correspondence between the V model and the fault structure indicates that the ambient noise tomography method can be used as an effective method for detecting hidden faults in urban environments.