ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

moved by periglacial processes such as solifluction in the
cold phases of Last Glacial. On the steep slopes frost shattering
of underlying bedrock was probably predominant,
because bedding plane and joint are well developed in the
Akka limestone. It is probable that collapse of some doline
rims located at margin of the limestone plateau were caused
by periglacial processes. During these periods karst depressions
was infilled with soliflucted debris.
In order to obtain more detail reconstruction of environmental
changes in the study area, it is necessary to compare
the deposits of slopes, dolines and caves, and to date
their ages.
KAZUO OKUNISHI, MIEKO SONODA & KO]I YOKOYAMA
Geomorphic and environmental controls of
earthquake-induced landslides
Disaster Prevention Research Institute, Kyoto University,
Gokanosho, Uji, 611, Japan
The Kobe Earthquake (Southern Hyogo Prefecture Earthquake)
of a Richter scale magnitude of 7.2 struck Kobe
City and environs including a part of Osaka City, resulting
in 6,334 casualties and leaving about 320,0 homeless people.
Wooden and concrete buildings and other concrete
and steel structures in the Osaka-Kobe urban belt, which
is located between the sea and the Rokko Mountains, are
heavily damaged. Landslides were induced by the earthquake
at about 700 locations in the Rokko Mountains and
the neighboring mountains. These landslides, however,
caused little hazards compared with those by the ground
deformation in the urbanized areas. Potential hazardousness
of these landslides is still significant, since much
quantity of debris will be transported to the urban areas
which is close to the mountains, at the times of subsequent
rainstorms.Topographic conditions of the occurrence of
the earthquake-induced landslides were discussed by many
researchers but only in terms of statistics. This paper discusses
the geomorphic and other environmental controls
of these landslides in terms of material, process and present-day
trend of the geomorphic evolution.The Rokko
Mountains were once entirely deforested by over-harvesting
of wood, root and humus soil, but were reforested
between the 19th and 20th Centuries. A great rate of
weathering, due to the fracturing of granitic rock bodies
which is subject to rapid orogenic movement, and to chemical
alteration by the groundwater which is enriched with
carbonate ions, has responsible for swarms of rainfall-induced
landslides of an average return period of about 30
years. Large-scale earthquake-induced landslides are also
suggested to have occurred in the pre-historic age. The features
of the landslides induced by the Kobe earthquake in
1995 was much different from those of the landslides ever
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experienced in the Rokko Mountains and even in the other
areas in Japan.No large-scale landslide occurred. Slumpings
of considerable areas occurred at seven sites, but the
displacement was small except for one site which resulted
in 34 casualties. Most landslides were surficial and of small
areas. The landslide scars of this category suggested tensile
failure in majority of the cases (classified as debris fall),
and shear failure in other cases (classified as debris slide).
Numberless rockfalls took place but falls of the rock
blockis) of a volume of more than 30m 3 occurred only at
four sites. Debris falls were concentrated along the zone of
aftershock where the earthquake motion was heaviest at
the time of the main shock. Debris slide were more scattered
around this zone. The slumpings and the major rockfalls
were much scattered suggesting that geology and topography
were major controls.Topographic conditions of
the occurrence of debris falls and debris slides were analyzed
using the aerial photographs and a topographic map of
1:5,000. Since these landslides were of a small areas, local
topographic features were considered according to the scale
of local topographic texture without choosing a fixed
grid spacing. Three classes were identified for the curvature
of the contour lines and that of longitudinal profile, yielding
nine categories. Among the categories of convex contour
lines, landslides occurred preferably on the convex
profiles, which are frequently found along the summits.
Such topographic conditions are not preferable for rain-induced
landslides. This mode of the occurrence has been
reported in many case studies of the earthquake-induced
landslides in Japan. Among the slopes of straight contour
lines, those of straight profile (planer slope) were preferable
for landslides. Many landslides (of both types) occurred
on a steep cliffs, which are frequently found along the
major faults in the Rokko Mountains and with the aspect
coinciding with the strike of the pertinent fault (according
to Okimura, 1995), but have been reported in few other
case studies. Among the slopes of concave contour lines,
the landslides do not concentrate to a particular profile type,
although the statistics of the slope angle shows characteristic
tendency of debris falls and debris slides concentrating
on the concave profiles. Under this topographic condition,
debris falls occur on relatively gentle slopes with a
gradient range of 0.3 to 1.0. It suggests the control of the
thickness of the weathering mantle and/or the wetness. On
the other hand, most debris slides occurred on the slopes
of a gradient range over 1.0. This type of debris slide were
usually found within or in the margin of the scar of old
(undated) landslides which had presumably been induced
by rainstorms. Naturally, such landslides are much smaller
than the former landslides. Rapid weathering is responsible
for this type of debris slides which have been reported
in few other case studies in Japan.The Kobe Earthquake of
1995 occurred in the midst of dry season, and the preceding
rainy season had exceptionally small precipitation.
The control of the mechanical properties of dry weathered
granite is evident in the mode, number, distribution and
location of the earthquake-induced landslides. In such cases,
the effect of vegetation seems relatively significant, since
the cohesion by adsorbed water becomes negligible.