11:15 AM - 11:30 AM
[HQR05-09] Unconsolidated mudstone gravels in embankment and debris-flow deposits at the site of a landslide in the Izusan area, Atami City, Shizuoka Prefecture, Japan
Keywords:landslide in the Izusan area, Atami City, embankment , debris-flow deposits , unconsolidated mudstone gravels
On July 3, 2021, a landslide occurred at the headwaters of the Aizome River in the Izu-san district of Atami City, resulting in a debris flow that caused 28 fatalities and damaged 64 homes. My collaborators and I conducted investigations to elucidate the cause of the landslide. As a result: (1) The collapsed black-colored embankment included sediment deposited from the 1950s to early March 2011, containing chart rock fragments with late Paleozoic to Mesozoic radiolarian fossils, unconsolidated mudstone gravels derived from uppermost Pliocene to Lower Pleistocene marine strata, shells of the freshwater bivalve Corbicula sp. that lived after 1950, and shells of modern to mid-Holocene marine mollusc shells. (2) Chart rock fragments in the black-colored embankment may have been supplied from the mouth of the Tama River.
This study examined the physical properties, such as density, of soft mudstone clasts contained in embankments and debris flow deposits, and conducted species identification of bivalve fossils found in the clast. Additionally, we measured the density of volcanic rock clasts derived from the bedrock. As a result, the following findings were obtained:
1. The dry and wet densities of unconsolidated mudstone gravels range from approximately 1.4 to 1.9 g/cm³ and 1.7 to 2.2 g/cm³, respectively, with a water content of 20.6% to 45.2%. In contrast, the dry density of volcanic rock clasts ranges from 2.1 to 3.2 g/cm³. Therefore, the dry density of unconsolidated mudstone gravels is lower than that of volcanic rock clasts, and while the wet density partially overlaps, it remains lower than the dry density of volcanic rock clasts.
2. The bivalve fossils found in the unconsolidated mudstone gravel within the debris flow deposits were identified as Portlandia sp. There are no previous reports of this genus from the marine Pliocene-Pleistocene formations in the Oiso Hills, but it has been reported from the Ōtsuka Formation, which is exposed in Yōbara, Sagamihara City, and Mutsukura, Aikawa Town, Kanagawa Prefecture. Therefore, it is highly likely that the source of the unconsolidated mudstone gravels is the Ōtsuka Formation distributed in Sagamihara City, Kanagawa Prefecture.
This study examined the physical properties, such as density, of soft mudstone clasts contained in embankments and debris flow deposits, and conducted species identification of bivalve fossils found in the clast. Additionally, we measured the density of volcanic rock clasts derived from the bedrock. As a result, the following findings were obtained:
1. The dry and wet densities of unconsolidated mudstone gravels range from approximately 1.4 to 1.9 g/cm³ and 1.7 to 2.2 g/cm³, respectively, with a water content of 20.6% to 45.2%. In contrast, the dry density of volcanic rock clasts ranges from 2.1 to 3.2 g/cm³. Therefore, the dry density of unconsolidated mudstone gravels is lower than that of volcanic rock clasts, and while the wet density partially overlaps, it remains lower than the dry density of volcanic rock clasts.
2. The bivalve fossils found in the unconsolidated mudstone gravel within the debris flow deposits were identified as Portlandia sp. There are no previous reports of this genus from the marine Pliocene-Pleistocene formations in the Oiso Hills, but it has been reported from the Ōtsuka Formation, which is exposed in Yōbara, Sagamihara City, and Mutsukura, Aikawa Town, Kanagawa Prefecture. Therefore, it is highly likely that the source of the unconsolidated mudstone gravels is the Ōtsuka Formation distributed in Sagamihara City, Kanagawa Prefecture.