Generating Factor of Strong Ground Motions for a period of 2 to 3 seconds Observed in the Western Part of Tokyo Bay, during the 2011 off the Pacific Coast of Tohoku Earthquake

Abstract

To understand the generating factor of strong ground motions for a period of 2 to 3 seconds observed in the western part of Tokyo Bay during the 2011 off the Pacific coast of Tohoku earthquake, we estimated shallow and deep S-wave velocity structures at 6 sites near the seismic stations by array microtremors observations. As a result, the existence of soft-soil sediments for Vs 150m/s to a depth of more than 50m and Vs 400m/s to a depth of 200m was confirmed in the JKPM (Jonanjima Seaside park) and UK1222 (Ukishimacho park) stations locating in reclaimed lands. Also, site amplification factors at 6 sites calculated using the estimated S-wave velocity structures were in good agreements with those observed using the seismic data of the main shock. In this study, therefore, we concluded that the strong ground motions for a period of 2 to 3 seconds, which were observed in the western part of Tokyo Bay during the 2011 off the Pacific coast of Tohoku earthquake, were due to the site amplifications of S-waves excited by deep underground structures to a depth of more than 200m.