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[HDS11-01] Spread-type landslide in the Shimogo-area, Fukushima Prefecture, Northeast Japan: Characteristics of geomorphology and geology
Keywords:landslide, Spread-type landslide, Caldera deposits, siltstone
A landslide with geomorphic features of a spread-type landslide is observed in the Edamatsu area, Shimogo Town, Minamiaizu District, Fukushima Prefecture, Northeast Japan (Oyagi, 2003). The landslide slope is composed of lacustrine deposits that accumulated during the Tonohetsuri caldera forming stage in the Early Pleistocene and andesite of the post-caldera stage (Yamamoto, 1999). The landslide area is a gently slope that reaches from a linear scarp (separating scarp) with a specific height of about 200 m to the Tsurunuma River. Several linear depressions extending parallel to the separating scarp can be observed within the landslide mass, and the width of the landslide mass is approximately twice the distance from the scarp to the river.
The stratum that forms the landslide mass is andesite, which is a Mt. Futamatayama volcanics, lacustrine deposits consisting of clastic rocks are distributed below the andesite. The basement of the andesite on the non-landslide slope is 600 m to 610 m above sea level, but the crushed andesite on the landslide slope is distributed up to the river bank at 470 m above sea level.
In the riverbed of the Tsurunuma River, deformed siltstone is distributed at the toe of the landslide mass consisting of fractured andesite. The deformed siltstone is a lacustrine deposit, which contains some interbedded sandstone blocks, and andesite blocks and riverbed gravels. The deformed siltstone is mainly deformed by plastic flow, although some shear deformation is observed, suggesting that the deformed siltstone distributed in the riverbed acted as a lubricant for landslide activity. The same type of deformed siltstone can be found in the northwest side of the landslide. In the northwest side of the landslide terrain, the landslide area is separated by a linear topography, and deformed silt is distributed in the stream that flows along the edge of the landslide terrain. In the southeastern extension of the separating scarp, deformed siltstone is distributed in the undeformed lacustrine deposits. The distribution of these deformed siltstones suggests that fractured materials generated in the lacustrine deposits are filling the open fractures, suggesting a spread-type landslide in which geological bodies separate and spread on the flowing materials and the flowing materials fill the open fractures.
The stratum that forms the landslide mass is andesite, which is a Mt. Futamatayama volcanics, lacustrine deposits consisting of clastic rocks are distributed below the andesite. The basement of the andesite on the non-landslide slope is 600 m to 610 m above sea level, but the crushed andesite on the landslide slope is distributed up to the river bank at 470 m above sea level.
In the riverbed of the Tsurunuma River, deformed siltstone is distributed at the toe of the landslide mass consisting of fractured andesite. The deformed siltstone is a lacustrine deposit, which contains some interbedded sandstone blocks, and andesite blocks and riverbed gravels. The deformed siltstone is mainly deformed by plastic flow, although some shear deformation is observed, suggesting that the deformed siltstone distributed in the riverbed acted as a lubricant for landslide activity. The same type of deformed siltstone can be found in the northwest side of the landslide. In the northwest side of the landslide terrain, the landslide area is separated by a linear topography, and deformed silt is distributed in the stream that flows along the edge of the landslide terrain. In the southeastern extension of the separating scarp, deformed siltstone is distributed in the undeformed lacustrine deposits. The distribution of these deformed siltstones suggests that fractured materials generated in the lacustrine deposits are filling the open fractures, suggesting a spread-type landslide in which geological bodies separate and spread on the flowing materials and the flowing materials fill the open fractures.