3D Near-Surface Shear-Wave Velocity Structure from Ambient-Noise Tomography and Borehole Data in the Hefei Urban Area, China

Imaging near-surface structure with active sources, such as dynamite or vibrators, is usually difficult in urban areas because of dense population and safety issues. However, ambient-noise tomography can provide an alternative way to investigate near-surface structures without such difficulties. Here, we conduct ambient-noise tomography to investigate the near-surface shear-velocity structure in the urban area of the city of Hefei (about a 5 kmx 7 km region) in eastern China, using two weeks of continuous ambient-noise data. The direct surface-wave tomographic method with period-dependent raytracing is used to invert all surface-wave dispersion data in the period band 0.5-2 s simultaneously for 3D variations of shear-velocity structures. To constrain the top 30-m velocity structure in the inversion, we utilize the borehole data in the city of Hefei to obtain a better 3D reference wavespeed model in the top 30 m. Our tomographic results from surface to about 400 m depth suggest that the northern part of the urban region presents higher shear velocity than the southern part. This result is consistent with regional geology; that is, the southern part of the study region is closer to Chao Lake, the fifth largest freshwater lake in China, which has much thicker sediments and lower shear wavespeed. The sharp velocity variation in our obtained model matches the location of the Shushan fault, which is associated with large gravity variations. Our near-surface velocity model of the Hefei urban area provides useful information for city planning as well as for earthquake strong ground motion prediction, which may bring strong engineering interests in the future.