Conditions immediately following the slides
(Photo courtesy of the Bhosai Chishitu Kogyo)
Location
Okushi Island, Hokkaido, (42*10'N; 139*31'E).
Date of Slide
July 2, 1993
Size of Slide
Height:120m; Width: 200m; Depth: 20 to 30 m; Volume: 1.5x105m3.
Damages
Hotel Yhoyhoso and restaurant at Okushi Harbor were destroyed, and
included 29 death.
Topographic and Geologic Conditions
The geology o fthe Okushi Harbor area consists of Upper Tertiary, Pliocene
sandstones and pyroclastic rocks and marine terrace deposits overlying
the Pliocene sediments. The terrace deposits are about 120 m high and form
a cliff slope angle of 45 degrees. The geology along the scarp area consists
of loose sandstone dipping to the southwest, and is overlain by volcanic
breccia, tuff and tuffaceous sandstone, and volcanic conglomerate.
Mechanism of Failure and Type of Movement
Based on the aerial photographic analysis, it ispossible to interpret
two separate failures: 1) bedrock slide-avalanche, and 2) debris slide.
The first failure was a vertical fall and occurred at the north side. Following
immediately was the second failure, which had some inclination and occurred
at the south side. The direction of the second failure was verified from
the orientation of the slickensides at the main scarp. finally, rock deris
that was accumulated along the way failed as a secondary slide along the
northern slope. Above the main scarp, the slide plane was formed along
the contact between the sandstone and the overlying pyroclastic rocks.
Causes of Slide
Off shore Southwest Hokkaido Earthquake-Magnitude:7.8; Epicenter: 42*47'N;
139*12'E; Depth of epicenter: 34 km. It was estimated that the Earthquake
Intensity (Japanese) was a VI at Okushiri Island; however, the earthquake
ahd two separate main shocks. The initial main shock occurred northwest
of the island and lasted for 20 seconds, then the second main shock moved
about 100 km southward and occurred west of the island and lasted for 35
seconds. Therefore, it is possible that the first and second failures may
correspond to each of the main shocks.
