Estimation of The Strong Motions at Mashiki Town, Kumamoto, During The 2016 Kumamoto Earthquake, Based on Microtremor Horizontal-to-Vertical Spectral Ratios

Abstract

We performed microtremor survey in Mashiki Town, Kumamoto, Japan by the single station method after the 2016 Kumamoto earthquake. We obtained microtremor data at 62 sites in the area of 1200m by 1000m. Then we calculated Microtremor Horizontal-to-Vertical Spectral Ratio (MHVR) of every site and gained 48 reliable MHVR curves. After deciding peak frequency on every MHVR curve, we made a fundamental peak frequency map of Mashiki Town. After that, we transformed MHVR to pEHVR by the MHVR-to-EHVR method. We calculated EHVR of two strong motion stations which are located at Mashiki Town, then we compared them with pEHVRs of the nearest microtremor sites. We found that pEHVRs are closer to EHVRs than MHVRs. Then, we identified one-dimensional (1D) Swave velocity structues of Mashiki Town, by using the Hybrid Heuristic Searching method. Next, we analyzed 1D dynamic soil responses of all the sites by the nonlinear equivalent linear method. We analyzed both linear and nonlinear cases for comparison and we estimated peak ground velocity (PGV) and peak ground acceleration (PGA) of all the sites. We found PGVs showed stable while PGA were significantly different from each site.