Journal of Natural Disaster Science

EROSION AND MASS MOVEMENTS ON MT. USU ACCELERATED BY CRUSTAL DEFORMATION THAT ACCOMPANIED ITS 1977-1982 VOLCANISM

Hiroshi KADOMURA
Professor, Graduate School of Environmental Science, Hokkaido University
Hiromu OKADA
Associate Professor, Usu Volcano Observatory, Hokkaido University
Toshiaki IMAGAWA
Postdoctoral Fellow, Japan Society for the Promotion of Science (Hokkaido University)
Ichio MORIYA
Associate Professor, Department of Geography, Kanazawa University
and
Hiroshi YAMAMOTO
Research Associate, Graduate School of Environmental Science, Hokkaido University

(Received Dec. 6, 1983 and in revised form Feb. 15, 1984)

Abstract

The 1977-1982 volcanism, both by tephra eruption and crustal deformation, of Mt. Usu, Hokkaido, has accelerated erosion of slopes and downvalley mass movements. The upheaval of New Mountain on the summit atrio was followed by vigorous gullying that yielded the sediment at a rate of 130,000 cubic meters per square kilometersper year during 1977-1981. On the upper slope of the northern somma which was continuously thrusting northeast, the gradient increased over time due to the contractile movements. This resulted in rapid expansion of landslide scars, leading to frequent rain triggered debris flows during 1978-1981 in a watershed covered with only a few centimeters tephra. In April 1981, a debris avalanche was generated from the affected upper slope by melting of snow and rushed down at a maximum velocity of ca. 40 m/s. Based on the field work and seismic record analysis, we reconstructed the dynamic processes of this event.

Key words

Mt. Usu, crustal deformation, accelerated erosion, mud and debris flows, monitoring