Many landslides occurred adjacent to the active Nojima Fault which
ruptured during the South Hyogo Earthquake on January 17, 1995. One of
the landslides that occurred was the Nojima Ohkawa Landslide measuring,
Length: 160 m; Average Width: 370 m; Average Thickness: 15 m; Area: 6 ha;
Volume: 8.0 X 10_m_. Residential structures, chicken coops and other structures
were damaged by the slide, and the other damages included the complete
destruction of one water tank. One of the other landslides to occur was
the Todoroki Landslide measuring, Length: 80 m; Maximum Width: 150 m; Average
Width: 100 m; Area: 5 ha; Volume: 7.0 X 10_m_. There are very few structures
within the landslide area, and therefore no direct damage was inflicted.
Geology, Mechanism of Failure and Type of Movement
Granitic rocks of the Mesozoic Era-Cretaceous Period are widely distributed
on the northern portion of Awaji Island. The northwest side of the Nojima
Fault (where the landslide occurred) is underlain by Upper Tertiary sandstones
and siltstones that form a gentle dip slope that faces the ocean. The Nojima
Fault is a relatively active fault that runs through the northwest portion
of Awaji Island, and extends to the Akashi region of the main Honshu. The
rupture along a segment of this fault caused the South Hyogo Earthquake,
and exhibits a distinct right-lateral movement at the site area. The landslide
deposits consist of the highly weathered granite while the Todoroki Landslide
consists of partly weathered sandstone that was deposited over Tertiary
sandstone. Further, the basement rocks near the fault have been severely
fractured by past fault activity, forming numerous splinter faults. It
is suspected that the fracture zone was caused by continuous seismic activity,
and subsequently created avenues for liberal ground water movement. It
is speculated that the landslide was caused by relaxing of the bedrock
in the region due to activity along the active trace of the Nojima Fault,
and a resulting change in the ground water flow directions and characteristics
caused the influx of a large volumes of ground water along the bedding
planes and/or joint surfaces.
Mitigation Measures
To reduce the pore water pressure around the slide plane, installation
of horizontal gravity drains, drainage wells and drainage borings have
been installed and more are planned. Furthermore, in order to augment the
deficiency of the design safety factor, steel pile works for the Nojima
Ohkawa Landslide and soil removal at the head area for the Todoroki landslide
are also planned.
Nojima Ohkawa
Landslide
