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.
It is reported the breakage of boulders/cobbles in conglomeratic deposits occur along tectonic lines (e.g. Futamura 2016). The boulders/cobbles in such environments are found to be broken either in extension, sheared or both (Fig. 1). There is no doubt that these boulders/cobbles are broken due to shock waves induced during the large seismic events. The most important aspect is how to identify the stress state and seismic shock waves causing the breakage of boulders/cobbles. 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.
The authors devised a special drop-weight equipment to carry out experiments on the boulders sampled from Horai district, Kusanagi and Miho district of Shizuoka City, which are constituted Nihon-daira formation, which is also related to the deposits in Seno-umi area in Suruga Bay. The behavior of samples under shock waves induced by the drop of cylindrical steel object from a certain height is observed. During the experiments, shock forces and accelerations were both measured simultaneously.
