Simulation of Directional Dependency of H/V Spectral Ratios of Microtremors Casued by Lateral Heterogeneity

The authors have proposed a new theory to calculate the Horizontal-to-Vertical (H/V) spectral ratio of microtremors assuming that the wave field is completely diffuse and have attempted to apply the theory to understand the observed microtremor data. It is anticipated that 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 H/V spectral ratio of mircrotremors is not limited to horizontally flat-layered 1D structures. As observational evidence of non 1D microtremor H/V spectral ratios, we have discovered significant directional dependency 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 2D subsurface structure. For a sufficiently flat, horizontally layered structure, we can easily calculate the theoretical Green's function for that 1D model but for a laterally heterogeneous underground structure, such as the case of Uji campus, a numerical approach is needed to interpret the H/V spectral ratios. We performed numerical analyses by Spectral Element Method using point source in both the 1D and 2D models to examine the effect of the 2D basin structure on the H/V spectral ratio of microtremors. As a result, we found that the 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 a result of directional dependence as seen in the observed H/V spectral ratio of microtremors at Uji campus.