Journal of Natural Disaster Science

DRY DEBRIS FLOW OF PYROCLASTIC FALL DEPOSITS TRIGGERED BY THE 1978 IZU-OSHIMA-KINKAI EARTHQUAKE : THE "COLLAPSING" LANDSLIDE AT NANAMAWARI, MITAKA-IRIYA, SOUTHERN IZU PENINSULA

(Received 23 July, 1982 and in revised from 14 December, 1982)

Abstract

A peculiar "collapsing" landslide occurred at Nanamawari, Mitaka-iriya, on the Izu Peninsula during the 1978 Izu-Oshima-Kinkai earthquake. Field investigations showed the causes and phenomena connected with this slide. The landslide began on a slope of 25 degree with the sliding of scoria, soil and part of the paleosol. The sliding surface was formed within the upper part of the paleosol, probably in a weathered ash bed, several meters below the original surface of the slope.

During of transportation, the sliding materials disintergrated into dry debris and flowed onto the fiat surface across the river, dashed against the opposite slope then rebounded and settled in a lobe-shaped deposit with lateral ridges and a distal mound.

The landslide continued less than 30 seconds, but its maximum velocity was more than 11.7 m/s. The basic causes of the landslide were insufficient lateral support by strata on the lower slope and a weak halloysite-rich paleosol in which the sliding surface formed. The landslide at Nanamawari is considered a special type of slide found in Japan.

Key words

landslide, dry debris flow, earthquake, pyroclastic fall deposits, paleosol