Study of Submarine Landslides, Its Mechanism, Time, and Prediction

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Geological and geophysical data taken by Tokai Univ. and existing data sets from other institutions are analyzed to study mechanism, timing and future prediction of submarine landslide on the slope of Senoumi-Tai in the Suruga Bay, Japan. Conclusions of this study are summarized as follows.(1) Newly constructed bathymetric chart indicates presence of several topography depressions which clearly show morphological characteristics of submarine landslide on the western and northern slopes of the northern Senoumi-Tai in the Suruga bay. (2) 3.5KHz echogram records show that this feature was caused by sudden loss of surface sedimentary layer about 100m in thickness. Creeps of surface layer are also identified and interpreted as onset of future submarine slide. (3) Analysis o f large amounts of seismic reflection records results in understanding of geological structure and geological developments on western part of the bay. It is clarified that the northern Senoumi-Tai has being uplifted by i … More ntensive tectonism during middle to late Quaternary and this uplifting process is still undertaking. As a result of this uplift, old eastward flowing canyon was not able to flow anymore and new submarine canyon system started to develop along the base of western slope of the northern Senoumi-Tai and flow northward. Intensive erosion at this location and uplift of Senoumi-Tai itself created unstable slope condition and generated creeps on the western slope of the Senoumi-Tai. One of the gigantic earthquakes appears to be a trigger of the slide. Historical record suggests appearance of huge water column and generation of tsunami during Ansei earthquake in 1854. This record suggests the relation between the earthquake and the submarine land slide at the Senoumi-Tai. Debris flow are found at the northern part of the Senoumi Basin and thought to be formed by the landslide. (4) Sedimentological study defines sedimentally environments, such as sea floors characterized by continuous sedimentation, sedimentation with sudden interruption, and chaotic sedimentation, probably debris flow. (5) Gravels are dredged on the summit of the northern Senoumi and found to be derived from dominantly Abe and partly Ooi river. (6) Datings of piston core samples are examined by analysis of pollen fossils contained in sediments. Three fossil zones are identified (before 3000 years, 3000-the middle age, the middle age - present). This time stratigraphy is applied to core sample from submarine landslide in order to determine timing of the slide and suggests that the time of landslide appears to be post-middle age. (7) Relation between submarine landslide and generation of tsunami is studied by simulation on computer system. This study indicates that landslide may be able to cause tsunami under certain conditions. Less