Landslide Structure and Regional Perspective on Sumikawa Landslide at Hachimantai Volcanic Area, Northern Honsyu Japan

Abstract

The 1997 Sumikawa landslide which occurred on May 11 1997 was reactivation of the combined unit of two ancient secondary landslides of the Sumikawa left bank ancient large-scale landslide developed on the east piedmont of Yakeyama volcano edifice. The main part of the moved body of the 1997 landslide was composed of the Yakeyama andesite lava flows which had flowed at a “buried” ancient valley in Middle Pleistocene. The moved body can be separated into two moved units. The type of movement was combination of two block slides with somewhat rotational component and different directions of movement in 20 degrees.The slide surface can be found along the ancient valley between the landslide moved body which is composed of the Yakeyama andesite lava flows and the underlying formations of the Early Pleistocene pre-Yakeyama caldera-fill deposits of silt and pyroclastics and the Miocene pyroclastic rocks. This suggests that the slide surface seems to be inherited from the ancient valley.Geological profile of Yakeyama volcano suggests lateral plastic flow of the caldera-fill deposits by the weight of the Yakeyama volcano edifice. This lateral plastic flow could have possibility of producing the circular distribution of many large-scale ancient landslides along the estimated limb of pre-Yakeyama caldera and then along the periphery of the Yakeyama volcano edifice. The Sumikawa left-bank ancient large-scale landslide is considered to have related to formation of the two ancient secondary landslides and the 1997 Sumikawa landslide.Another circular distribution of gigantic ancient landslides in the eastern part of Hachimantai volcanic area suggests an underlying large caldera of 11-12 km in diameter, similar to the pre-Yakeyama caldera.