Abstract
The distinctive bathymetric feature exists in the Suruga Bay, Japan. It has been called as Senoumi (Stone flower sea) from old times. Senoumi is a 30 km wide and 20 km long concave feature. Its origin has not been explained yet; however, the feature might be a combined consequence of intensive tectonic activity in the plate border, landslides, and a submarine flow coming from the Oi River. If the Senoumi was caused by a landslide, the latter would be larger than any on-land landslide in Japan. The downshelf “exit” from this feature is much narrower than its central part. This is not usual shape of landslides, but it is similar to the liquefied landslides such as those in quick clays which mobilize great strength reduction after failure. To study Senoumi as a landslide, the shear behaviors of the following three soil samples were investigated by the cyclic and seismic undrained stress control ring shear tests. One sample is volcanic ash taken from the base of landslide deposits (mass transport deposits), from 130 to 190 m deep layer below the submarine floor which was drilled and cored by the Integrated Ocean Drilling Program Expedition 333. Another two samples are the Neogene silty–sand and silt taken from the Omaezaki hill adjacent to the Senoumi, because the shear zone might have been formed in Neogene layers extending from on-land to the continental shelf. The largest strength reduction from peak to steady-state shear resistance in the undrained cyclic loading test was found in volcanic ash. The strength reduction in Neogene silty–sand was smaller than volcanic ash, while the Neogene silt mobilized the least post-failure strength reduction. An integrated model simulating the initiation and motion of earthquake-induced rapid landslides (landslide simulation (LS)-RAPID, Sassa et al. Landslides 7–3:219–236, 2010) was applied to this study. The steady-state shear resistance and other geotechnical parameters measured by the undrained ring shear tests and the greatest strong motion record in the 2011 off-the-Pacific Coast of Tohoku earthquake (M w 9.0), also known as “2011 Tohoku Earthquake” at the observation point MYG004 (2,933 gal) were input to this model. As the result, it was found that landslides would be triggered by 0.30–1.0 times of MYG004 in volcanic ash, 0.4–1.0 times of MYG004 in Neogene silty–sand and Neogene silt, though the depth and area of triggered landslides were different in soils and intensity of shaking. Feature, created by LS-RAPID using the parameters of volcanic ash, was most similar to the Senoumi in depth and extent. The result obtained from this study includes a hypothesis to be proved, but presents the strong need to investigate the risk of the large-scale submarine landslides which could enhance tsunami wave and possibly enlarge the submarine landslide retrogressively into the adjacent coastal plain by the upcoming mega earthquake in the Nankai Trough.
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Acknowledgments
We acknowledge the SATREPS of the Government of Japan and two projects of the International Programme on Landslides which is a joint program of ICL and ICL supporting organizations, IPL-157 Dynamics of sub-aerial and submarine megaslides (leader, Kyoji Sassa) and IPL-161 Risk identification and land-use planning for disaster mitigation of landslides and floods in Croatia (leader, Hideaki Marui). R. Urgeles of the Institut de Ciències del Mar, Spain for information on marine landslides and T. Furumura of the Center for Integrated Disaster Information Research, the University of Tokyo for consultation on earthquake shaking are appreciated. The authors thank the research and technical cooperation from Y. Kaneda, M. Yasunaga as well as the ship and personnel of the Drilling Vessel Chikyu of the Japan Agency for Marine-Earth Science and Technology. Thanks go to Croatian young researchers of Rijeka University to support experiments during their stay in Japan. This research used samples and data provided by the Integrated Ocean Drilling Program (IODP), and used the combined bathymetric and topographic mesh data of the Geological Survey of Japan.
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Sassa, K., He, B., Miyagi, T. et al. A hypothesis of the Senoumi submarine megaslide in Suruga Bay in Japan—based on the undrained dynamic-loading ring shear tests and computer simulation. Landslides 9, 439–455 (2012). https://doi.org/10.1007/s10346-012-0356-2
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DOI: https://doi.org/10.1007/s10346-012-0356-2