Sounzan Landslide

Location

Hakone-mach, Kanagawa Prefecture, (35*14'N; 139*02'E).

Date of Slide

July 26, 1953

Size of Slide

Length: 900 m; Width: 380 m; Area: 3.5 ha; Volume: 8.0x105m3.

Damages

Debris slided from a landslide turned into debris flow, flowing at speed of 7 m/sec. and buried a temple, roadway including 13 deaths and 15 injury.

Geology and Mechanism of Failure

The Hakone Volcano was active during the Late to Middle Quaternary about 0.4 ma ago, and during that process a large scale caldera was formed. About 40,000 years ago a central crater hill (Kamiyama) was formed within the crater. The crater hill is composed of volcanic eruptive materials of andesite and tuff breccia and the Sounzan landslide originated at the head position of the eroded canyon. In this region, the surficial materials up to 25 m from the ground surface had been altered to weak clay containing montmorillonite, kaorinite and alunite by high geothermal and volcanic gas. The cause of the slide has been attributed to the landslide area is covered with unstable talus deposits which had fell from the steep slopes above, and the active erosion along the canyon decreased the stability of the slopes above. The immediate triggering mechanism of the 1953 landslide was suspected to be the torrential rain which measured 432 mm in 10 days prior to the event.

Mitigation Measures

Down stream areas of the Sounzan Landslide is one of the most representative and important hot springs resort in Japan, therefore the disaster prevention has been very important. Consequently, erosion and sedimentation control and landslide mitigation works have been implemented since slide occurrence. In order to control the sediment discharge and to prevent erosion, 25 check dams, drainage channels and training levees were constructed along Nazawa between 1954 and 1984. Furthermore, numerous venting boreholes to relieve the volcanic gas have been drilled to control the hydrothermal alteration which is one of the causes of the sliding. In order to protect the resort area, six steel check dams and associated training levees have been constructed between 1986 and 1993 for the expected debris flow caused by the future expansion of landslide. As results of the mitigation measures, no noticeable landslide has occurred since the 1953 disaster.