[S07-6] Site effects I

[S07-6-06] Direct evaluation of site amplification factors based on observed motions of earthquakes and microtremors

^○Hiroshi Kawase¹, Fumiaki Nagashima¹, Kenichi Nakano², Yuta Mori³ (1.DPRI, Kyoto University, Uji, Japan, 2.TRI, HAZAMA ANDO CORPORATION, Tsukuba, Japan, 3.J-Power, Tokyo, Japan)

It has been used for more than three decades to characterize earthquake S-wave site amplifications from microtremor Horizontal-to-Vertical ratios (MHVR), although the skepticism has been surfaced through many studies on such a direct substitute after the proposal. To solve the controversy based on the concept of the recently proposed diffuse field assumption, we calculated MHVR as well as those from observed weak earthquake motions (EHVR) and compared fundamental characteristics of the MHVRs and EHVRs with those calculated theoretically from 1-D S-wave velocity structures. Either for MHVRs or EHVRs we found that we can reproduce their fundamental characteristics by the corresponding theories of diffuse field concept. MHVRs and EHVRs share similarities but have significant differences in their shapes, especially after the first peak frequency. This is because MHVR mainly consists of surface waves from surface sources while EHVR consists of S-waves from the bedrock. We then tried to establish a simple method to estimate velocity structures using single-station microtremor records. To that end we compared systematically EHVRs and MHVRs at 100 K-NET and KiK-net sites in Japan and calculated EHVR-to-MHVR ratios (EMRs) to get EMRs averaged over five different peak-frequency categories. We converted MHVRs to “pseudo EHVRs" from EMRs and MHVRs, which are found to have higher correlation with EHVRs than MHVRs. Then we proved that pseudo EHVRs give us similar S-wave velocity profiles than the direct use of MHVRs as substitute of EHVRs. Finally, we calculate average Vertical-to-Vertical spectral ratios (VVR) from the generalized spectral inversion (Nakano et al. 2015) and calculated the average VVRs for the same category of MHVRs. By multiplying VVRs with pseudo EHVRs we can obtain S-wave amplification of earthquakes, which can be favorably compared with actual S-wave amplification factors.