Earthquake Research Institute, University of Tokyo
Abstract
Geo-slicer sampling of riverbed strata, boring sampling of riverside strata, single-channel seismic reflection survey, and electric survey were carried out to understand the causes of ground fissures and ruptures with a vertical dislocation of 25cm found by Shimazaki and others beneath the Old-Edo River in Tokyo in 1997. We concluded that fissures and ruptures were caused by gravity slumping due to liquefaction taking place near a paleo-valley, which was found by the reflection survey to run across the Old-Edo River. In the 1997 survey, at least two liquefaction and slumping events, probably corresponding to two large earthquakes, were identified from a Geoslicer sample down to about 8m in depth. Their ages were reported as after 1800yBP and 2900－3700yBP, respectively. Dating results for new samples obtained in this study are consistent with previous estimates
