High-resolution 3-D crustal shear-wave velocity model reveals structural and seismicity segmentation of the central-southern Tanlu Fault zone, eastern China

The Early-Mesozoic collision between the North and South China Cratons resulted in different tectonic and geologic structures including the Tanlu Fault zone. One way to understand the geodynamic processes beneath these structures is by studying the velocity structures and spatial distribution of earthquakes in the region. We performed ambient noise tomography for the region around the central-southern Tanlu Fault zone using the vertical component of continuous ambient noise data obtained in central East China. Empirical Green's functions were retrieved from ambient noise cross-correlation functions, from which we extracted the Rayleigh wave phase velocity dispersion curves and performed quality control on the dispersion data. The direct surface wave tomography method was used to invert all phase velocity dispersion data for the 3-D shear-wave velocity model in the crust and uppermost mantle. Our results show velocity segmentations with high-velocity structures existing beneath orogenic belts and uplifts within the first 10 km depths which correspond to the High-Pressure/Ultra-High Pressure Metamorphic rocks in the area. The existence of high-velocity structures beneath orogenic belts at shallow depths implies that the exhumation of eclogites is confined to the uppermost crust. Generally, fault zones in this area are characterized by a low velocity from 0 to 5 km and relatively high velocity in the lower part of the upper crust and the middle crust. The spatial distribution of earthquakes along Tanlu Fault reveals two seismicity zones: the southern segment and the Suqian-Tancheng segment with a low-velocity transitional zone at Suqian, which suggests that the two segments respond differently to stresses. Regions of dense earthquake distribution correspond to the transition zones of high to low shear-wave velocities, which could be as a result of instability in the upper mantle and variation in compositions of crustal materials.