Shallow Crustal Structure of the Tanlu Fault Zone Near Chao Lake in Eastern China by Direct Surface Wave Tomography from Local Dense Array Ambient Noise Analysis

Ambient noise tomography (ANT) has been used successfully to image shallow earth structure. Here we perform ANT on a local dense seismic array around the Tanlu fault zone (TFZ) to the southeast of Hefei City, Anhui Province in eastern China. The array consists of 53 stations with average spacing close to 5 km. Cross-correlations of vertical-component ambient noise data of different station pairs are computed in 1-h segments and stacked over 1 month from 17 March to 26 April 2017. Clear fundamental-mode Rayleigh waves are observed between 0.2 and 5 s period. We then use the direct surface-wave tomographic method with period-dependent ray tracing to invert group and phase dispersion travel-time data simultaneously for three-dimensional (3D) shear-wave velocity (V_s) structure. The V_s model shows clear correlation with the known geologic features. The TFZ is associated with a high-velocity anomaly zone in the shallow crust, corresponding to metamorphic rocks due to magma intrusion. Low-velocity anomaly zones are mainly located to the west of the TFZ, caused by thick sedimentary layers in the Hefei Basin. Our study shows that, with ambient noise data recorded on a dense array and an advanced surface-wave inversion method, we can image detailed structure around the fault zone.