Horizontal‐to‐vertical (H/V) spectral ratios of microtremors (HVRM) have been traditionally interpreted as representing either the S‐wave amplification directly or the Rayleigh‐wave ellipticity for a horizontally layered structure. However, based on the diffuse field theory, we have derived an alternative theoretical basis that HVRM corresponds to the square root of the ratio between the imaginary part of the horizontal Green’s function and that of the vertical one. Under that condition, the 1D horizontal layering assumption is not needed to interpret HVRM. As observational evidence of such non‐1D HVRM, we discovered significant directional dependency at a site on the Uji campus, Kyoto University, Japan. The observed microtremor north–south/vertical spectral ratios are quite stable and have only one peak around 0.5 Hz. On the other hand, the east–west/vertical spectral ratios are smaller in amplitude and have higher peak frequencies and sometimes two separated peaks. The directional dependency of observed HVRM is aligned to the axis of the 2D basin structure. We performed numerical analyses by spectral element method using a unit load on the surface to examine the effect of the 2D basin structure on the imaginary parts of the Green’s functions. We found that the 2D basin structure clearly changes the characteristics of the H/V spectral ratios in both perpendicular and parallel directions relative to the basin axis. Thus, we succeeded in theoretically simulating the qualitative difference between the H/V spectral ratios for two orthogonal horizontal components of the HVRM observed on the Uji campus.
Online Material: Snapshots and animations of wave propagation.