Abstract
The coast from the Ainuma to Toyohama, southern Hokkaido, is characterized by the following three landslides; Toyohama Slide on Oct. 17, 1962; Orito Slide on Dec. 20, 1985, and Tachimachi-misaki Slide on Aug. 9, 1991. Our research observations and analyses have revealed that the Orito Slide and Tachimachimisaki Slide are primary dip-slipping deep seated slides due to geological and geomorphic structures, such as gentle-dipping of hard siltstones intercalating clayey tuffs acting as slip-surfaces. While, the Toyohama Slide is secondary slipping from the debris toe of an ancient slide. The Tachimachi-misaki Slide and Toyohama Slide were also caused by long-term slope instabilization. Finally, the three slides were probably triggered by precursory and considerable durational precipitations, which possibly upheaved groundwater level in the sliding slopes.
