Shallow shear wave velocity structure from ambient noise tomography in Hefei city and its implication for urban sedimentary envoiroment

Hefei is located in the south-east part of Hefei basin and west side of Tanlu fault zone in eastern China, with several large buried faults crossing the urban areas. To understand better of the distribution of Tanlu faults and complex sedimentary enviement in urben areas, we deployed an seismic array consisted of 58 stations with average spacing about 4 km. Cross-correlations of vertical component ambient noise data of different station pairs are computed in 1-day segments and stacked over 1 month from 30 September to 5 November 2018. Clear fundamental-mode Rayleigh waves are observed from 0. 9 s to 4. 5 s period. We then use the direct surface-wave tomographic method with period-dependent ray tracing and a wavelet-based sparsity-constrained to invert phase dispersion travel-time data simultaneously for 3-D shear-wave velocity structure. The shear-wave velocity model results from 0. 6 km to 3. 6 km depths are consistent with the known geologic features and reveal strong shallow crustal heterogeneity. The results are as follows : (1) It shows sharp velocity variation from north to south at the depths above 2 km which matches Shushan fault, also associated with gravity variations. As the boundary of different sedimentary strata in Hefei basin, it presents higher velocity to the north side of Shushan fault than the south, which related to the tectonic evolution of the eastern part of Hefei Basin; (2) Significant hight velocity is observed near the two west splay fractures of TanLu faults(Wuhe-Hefei fault and Zhuding-Shimenshan fault). The lower velocity area distributed in the east of Wuhe-Hefei fault is also consistent with the known shape of Feidong depression, infering that the depth of the central depression is more than 2 km at least; (3) A significant low velocity zone is found in the central of Hefei urban area, with the depths of 3 similar to 4 km, which is in the intersection of TanLu fault zone and the other two NWW-strike faults. The boundary of this velocity zone variouse from shellow to deep related to the strike directions of Tanlu faults zone and other faults in this area. We speculate it may be a strike slip pull apart basin controlled by Tanlu fault zone during different geologic times, which provides a seismic evidence in shallow depth that TanLu fault zone plays an important role in the sedimentary evolution of the eastern Hefei basin.