Motochi Earth Flow, conditions immediately
following the sliding (Photo courtesy of the Hokkaido Press).
Location
Rebun Island, Hokkaido, (45K18'N; 141K2'E)
Date of Slide
October 11, 1994
Size of Slide
Length: 550m; Width: 60 to 100 m; Depth: 5 to 10 m; Area: 4.0 ha; volume:
2.8x105m3.
Damages
One house destroyed, one house damaged, road was closed for long period
of time.
Topographic and Geologic Conditions
The Motochi District is located on teh southwestern portion of the
island, and exhibits old landlside topography (North and South Blocks).
The site area is characterized by hills with flat ridges of about 250 in
relief and surrounded by monadrocks with steep peaks. Geologically, the
hills consist of Upper Tertiary andesitic hyaloclastite, and form as a
cap rocks overlying sandy tuff. The sandy tuff dips about 20 to 30 degrees
to the west. The monadnocks are comprised of pyroxene andesite dikes and
pyroxene porphyry.
Mechanism of Failure, Type of Movement and Rate of Movement
During the month of September, 1994, there were a few days with daily
rainfall exceeding 50 mm. By October 10, cumlative rainfall since early
September reached 300 mm. It has beed concluded that the cumulative rainfall
unduced the earthflow, and it is speculated that the rainfall created an
increase in the ground water level at the head area of South Block within
the Motochi landslide area.
The Motochi earthflow originated within the South Landslide Block and consists
of at least three units. The first and main flow originated along the road
knocking down power poles and power lines (October 11, 2:55 AM).The failure
started along the mid-portion of the South Landslide Block as a slump,
then transformaed into an earth flow that moved downward along a 10 to
20 degree slope. The earth flow then veered to the right at the steepest
part at the slope, and terminated near the coast line. The scarp of the
slump had receeded and eventually reached the head portion of the South
Landslide Block. Both flanks of the slump block (upper portion of the earth
flow) formed lateral scarps of 5 to 10 m high, while the foot (lower portions
of the earth flow) contained transverse ridges of 3 to 6 m. previously
installed corrugated metal pipes and gabbion cages were pushed parallel
to the side ridges (in the flow direction), while in the interior portion
of the flow the pipes were pushed perpendicullar to the flow direction
and bent towards the direction of the flow. Small scale compression ridges
were observed at the toe. It is also reported that some trees with their
roots intact had been transported over 200 m.
The second earth flow originated below the knick point of the main earth
flow. There are side ridges of about 1 m high along both sides of the flow
within the interior of the side ridges of the main earth flow. Lastly,
a small scale earth flow originated at the head section of the second earth
flow, and terminated at the mid-portion of the slide. For the second flow,
telephone poles were reportedly down at 8:15 AM, and the toe of the flow
reached the residents at 3:30 PM. With the horizontal distance between
the telephone poles and the residence being 75 m, the average movement
rate is estimated to be about 10 m/hour. The main constituent within theearthflow
consisted of white clayey soils with fragments of andesite breccia. The
white clay is a smectite that is derived from the sandy tuff, and the andesite
breccia is hyaloclastite and pyroxene porphyry.
Mitigation Measures
Ground water drainage control works; Surface drainage control works;
surface soil removal works.
