Rapid Landslides in Hokkaido, Japan

Abstract

I review rapid deep-seated landslides which took place in Hokkaido. Such landslides are giant rockfalls and rapid rockslides, as follows. The Ainuma-Toyohama coast facing Japan Sea, southern Hokkaido, is characterized by the following three landslides ; Toyohama Slide on Oct. 17, 1962 ; Orito Slide on December 20, 1985, and Tachimachi-misaki Slide on August 9, 1991. The Orito Slide and Tachimachi-misaki Slide are primary dip-slipping slides. While, the Toyohama Slide was secondary slipping from the toe of an ancient slide, killing 14 passengers with a bus. Kodomari Slide occurred on April 17, 1978, and blocked the main road for a month. Tenninkyou Rock Slide occurred on October 16, 1980, and crushed a rock shade, and the debris went over the Chubetsu River injuring 4 persons by attacking a hotel. Sounkyou Rockfall took place on June 9, 1987. Many long collumns of welded tuff fell down and the debris went over the Ishikari River, killing three people including truck drivers. Okushiri Port Slide took place on July 12, 1993, induced by Hokkaido Nanseioki Earthquake (M=7.8), southern Hokkaido. This is a rock slide which crushed a hotel and 29 people were killed. Toyohama Tunnel was crushed by giant rockfalls, killing 20 people who were just running by a bus and car on February 10, 1996.In conclusion, for the landslides mentioned above, I emphasize the importance of vertical and horizontal discontinuity, intercalation of clayey tuffs, cooling joints and caprock structure in view point of geology. I also point out that such landslides were triggered by a certain preceding precipitation, increase of under-ground-water pressure and earthquakes.