Evaluations of the Effects of the Basin Edge in H/V Spectral Ratios of Microtremors Based on Diffuse Field Interpretation

Abstract eng:
Based on the diffuse field assumption, the Horizontal-to-Vertical spectral ratios of microtremors (MHVR) can be derived from the ratio of the imaginary part of Green’s functions of the horizontal response at the point of horizontal harmonic loading and those of the vertical component at the same point for sites with flat-layered subsurface structure as well as for 2D and 3-D basin structures. We have shown that, by using a numerical method such as a 3-D spectral element method to calculate the Green’s functions of the response at the same point of harmonic loading on the surface of a 2-D basin structure it is possible to explain the directional dependency that can be seen in the observed MHVRs at Uji campus of Kyoto University, qualitatively. In this study, we observed microtremors in Reno, Nevada, USA, in order to observe directional dependent MHVRs and evaluate the effect of the basin edge from the observed MHVRs, as we conducted at the Uji campus. In Uji, the basin is on the footwall of a reverse fault, while in Reno it is on the hanging wall of a normal fault. In both locations, NS compnent is the fault-parallel and EW component is the fault normal direction. We focus on the effect of the basin edge to the MHVRs and study the relationship between the basin edge shape and the difference between NS/UD and EW/UD in the observed MHVRs. At Uji, the condition of the lateral heterogeneity close to the basin edge changes the characteristics of the MHVRs. Peaks in MHVR change to higher frequency due to the change of the depth of the basin as the site gets closer to the basin edge and the amplitude of the peaks also decreases as the site gets closer to the basin edge. This can be modeled with a flat-layered velocity structure for each observation point. We can fit the peak frequency of the observed MHVRs with theoretical ones by modifying the layer thicknesses, but the amplitudes and directional dependence of theoretical MHVRs do not match as the observations. From these results, we can see that the condition of the lateral heterogeneity close to the basin edge changes the characteristics of the MHVRs. We are comparing the MHVRs of observed microtremors in Reno to the results in Uji, and will try to determine if it is possible to identify directional effects and the basin edge shape in Reno as well based on MHVRs there.

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