Monitoring coseismic and post-seismic changes in site response associated with the 2011 Mw 9.0 Tohoku-Oki earthquake

It is well known that site response exhibits time-varying nonlinearity under the effect of strong ground motion. By applying the spectral ratio to the KiK-net strong-motion data, the coseismic and post-seismic changes in site response are investigated associated with the 2011 Mw 9.0 Tohoku-Oki earthquake in northern Honshu Island of Japan. It is observed that during the strongest shaking, the predominant frequency and the maximum value of the amplification factor decrease by 20-45% and 55-75%, respectively. In the following 12 years from 2011 to 2023, the predominant frequency exhibits a two-stage logarithmic linear recovery, and in this process, the maximum value of the spectral ratios at lower frequencies grows log linearly with time before the predominant frequency recovers. The observation results demonstrate the continuous change process of the site response under the major earthquake, provides a reference for the prediction of the site-response nonlinearity, and offers guidance for the seismic design of the geotechnical engineering.