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[S07-7-03] Temporal nonlinear site response during Kumamoto Mw7.0 earthquake inferred from borehole strong motion data
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 main shock and the triggered Mj 5.7 event. 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). Our preliminary results show clear drop of resonant frequency of up to 60% during the main shock at 8 sites with PGA from 200 to 1000 Gal. We investigate the percentage drop of peak frequency and peak spectral ratio during the main shock at different PGA levels.