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 amplification factor of the site response decrease 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 the amplification factor exhibits a one-stage logarithmic linear recovery process. 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.