Shock test on rounded rock fragments in Suruga Bay sediments and its implications on past mega-earthquakes

摘要

The breakage of boulders/cobbles in conglomeratic deposits are reported to occur along tectonic lines. The authors have been undertaking both in-situ investigations and laboratory experiments to understand the fundamental mechanism and causes of the breakage of the boulders/cobbles. Some sampling and observations are done in the close vicinity of Horai district, which is located along the famous Median Tectonic Line of Japan. In addition, some sampling of broken boulders/cobbles was done in Seno-umi in Suruga Bay, Shizuoka by Bousei-maru investigation ship of Tokai University. A drop-weight equipment was developed to carry out experiments on the boulders/cobbles sampled from Horai district, Kusanagi and Miho district of Shizuoka City, which constitute Nihon-daira formation. The authors report the outcomes of this experimental study in relation to mega earthquakes in the past. Furthermore, some numerical studies on ideal cases were carried out in order to see the stress state causing the breakage of the boulders.