Temporal Nonlinear Site Response During Kumamoto Mw 7.0 Earthquake Inferred from Borehole Strong Motion Data

Abstract

The Mw 7.0 Kumamoto earthquake (Japan) occurred on 15 April 2016 – 16:25 UTC. Using both borehole and surface ground accelerations recorded by Japanese strong motion network KiK-net, we investigate temporal changes of nonlinear site response in the shallow crust associated with Kumamoto earthquake. We compute the spectral ratios of windowed records from a pair of surface and borehole stations, and then use the sliding-window spectral ratios to track the temporal changes in the site response of various sites at different levels of peak ground acceleration(PGA). At the site OITH11 where two distinct groups of wave trains are recorded, the resonant frequency briefly recovers in between, and then followed by an apparent logarithmic recovery. We investigate the percentage drop of peak frequency and peak spectral ratio during the main shock at different PGA levels. Our results show clear drops of resonant frequency and peak spectra ratio of ~10–40% during the main shock, and the drops generally increase with the PGA level.