Numerical Simulation of Horizontal-to-Vertical Spectral Ratios of Microtremors with Directional Dependence Caused by Lateral Heterogeneity

As observational evidence of directional dependence of Horizontal-to-Vertical (H/V) spectral ratios of microtremors (or ambient noise), we have obtained data at a site in Uji campus, Kyoto University, Japan, where the bedrock depth varies from east to west from 250m to 420m within the distance of 1 km. This directional dependence can be considered to be the result of this 2D subsurface structure. We have a new theory to calculate the H/V spectral ratio of microtremors (HVRM) assuming that the wave field is diffuse and this new theory can be applied to understand the subsurface velocity structure as well as the condition of lateral heterogeneity, since the new theory to calculate the HVRM is not limited to horizontally flat-layered 1D structures. From numerical simulation of HVRM we found that a simple 2D basin structure clearly changes the characteristics of the H/V spectral ratio in both perpendicular and parallel directions relative to the basin edge and we have succeeded to simulate qualitatively the difference between the two orthogonal horizontal components as seen in the observed HVRM at Uji campus.