In May 1997, a large-scale landslide (called the Sumikawa Landslide) occurred behind the Sumikawa hot spring located on the northeast side of Yakeyama volcano. A steam explosion and mudflow occurred in connection with the landslide. The landslide is about 650 m long, 150 to 350 m wide and 50 to 70 m deep in the main part. This landslide can be divided into three sliding blocks. The landslide occurred in the Hachimantai hydrothermal alteration zone.The bedrock of the landslide area is composed of tuff to tuff breccia (Tobe Formation) which was formed in the Miocene Tertiary Period.Quaternary strata composed of tuff (Sumikawa tuff) and andesite lava (Yakeyama volcanic products) cover these tertiary strata.A slip surface is formed on the altered Sumikawa tuff that contains abundant montmorillonite. Borehole temperature tends to increase near the slip surface. Below the slip surface, the borehole temperature reached 70 to 100 degrees.The groundwater of the landslide area runs through the fissured andesite lava which covers the altered tuff and indicates pH 7.7-8.4. Before the landslide, hot springs of Sumikawa area indicated pH 2.1.A primary cause of the landslide is assumed to be the existence of the clay-like tuff created by the hydrothermal alteration and the concave shape of the bed rock.