S-wave velocity structures at Yedang Reservoir Dam inferred from amplification characteristics determined using H/V spectral ratios with background noise

Abstract

Local shear wave (S-wave) velocity structure and sedimentary cover thickness are essential parameters that control local amplification of ground motion and associated seismic hazard during earthquakes. Recently, analysis of environmental background noise from individual stations has been used to estimate horizontal-to-vertical spectral ratio (H/V) curves. In the present study, we inverted H/V curves using environmental noise immediately prior to earthquake P-wave arrival to retrieve S-wave velocity (or shear velocity, V S) profiles at four temporary seismic monitoring stations (YDF, YDS, YDU, and YDD) near the Yedang Reservoir Dam. In the first step, we used a random search algorithm to constrain the subvolume of the parameter space (S-wave velocity structure) where the minimum of the misfit was located. In the second step, we independently applied two non-linear processes (a Monte Carlo sampling algorithm and a simulated annealing algorithm) to force the inversion towards an optimal solution, using the minimum misfit model determined in the first step as an initial estimate; we then compared the results. The feasibility and effectiveness of this two-step approach were verified by inversion of H/V curves for seismic noise recorded at four seismic monitoring stations near the Yedang Reservoir Dam. Borehole and topographical data from the four stations provided a well-constrained estimation of the local shear wave profile. Comparisons of synthetic and observed H/V curves showed that combining the two inversion algorithms efficiently overcame the extreme non-linearity of the inversion problem and provided a good resolution of S-wave structures at the Yedang Reservoir Dam. The S-wave velocity profile at the YDF station, which is situated on fresh, uniform bedrock, ranged from ∼2300 to 2700 m/s, which was consistent with the borehole data. Both the YDU station (which exhibited fundamental and first-order resonance frequency harmonics) and the YDD station (which exhibited fundamental, first-, and second-order resonance harmonics) showed significant stepwise velocity profiles for the entire depth. Thus, the entire depth can be regarded as three layers, including bedrock. At the YDS station, located near the dam spillway, the uppermost layer showed the lowest V S values among all four stations (∼726–728 m/s), suggesting that the concrete dam body is well weathered or partially infiltrated by water. Because of these low V S values of the concrete dam body, the Yedang Dam may require a full safety diagnosis in the near future. Analysis of the velocity profiles of all four stations near the Yedang Dam indicated that the bedrock, composed of fresh rock (∼V S > 2700 m/s), varies greatly in depth among stations.