Multimodal direct fitting of SPAC coefficients using amplitude response

Abstract

We usually analyse microtremors without any considerations of higher modes of surface waves. However, recent studies have demonstrated that the use of higher modes would improve estimated velocity models. In this study, we apply multi-mode analysis with amplitude response to the direct fitting method in SPAC method. The proposed method has the advantage that we don?t need to identify observed modes, which can avoid mode misidentification. The proposed method was applied to synthesized microtremors. Estimated SPAC coefficients were good agreed with theoretical ones proposed in this study, even if in the frequency where two modes have about the same power fractions. The S-wave velocity was then successfully inverted by the direct fitting of SPAC coefficients. The proposed method also works well in the analysis of field data acquired in Tsukuba City, Japan. In the inversion performed in this study, we do not use prior information about velocity structure. It is concluded that the multi-mode analysis proposed here is very robust in the multimodal analysis in SPAC method.