2. Characteristics of Landslide Phonomena in Japan
Developmental Process of Landslides and Landslide Topography
By examining the reason why landslides occur in such high frequency
from the standpoint of evolution of the landform, it must first be recognized
that mountain-forming resulting in increased erosion potential is one of
the most important factors in landslide evolution. That is attributed in
part to crustal movement of up to 1500 m during the Quaternary Period.
Secondly, as discussed earlier, the geographical location of the Japanese
archipelago produces an abundance of precipitation which accelerates the
down cutting of rivers and also provides plentiful ground water supplies
in the hillsides. As result, landslides are important elements in t he
geomorphological development in the mountain regions in Japan. Most of
the mountainous regions in Japan are underlain by the Neogene sedimentary
rocks and Mesozoizc crystalline schist, which are strongly affected by
landslides. Among the slopes that have landslide topography, there are
many static landslides with advanced erosional features and dissected scarps
and slump blocks. Active landslides often exist within static landslide
topography.
The ages of the formation of the landslide topography have been established
from published data determined by C-14 dating methods and tephra chronology,
and are shown in Fig.7. . It has been
speculated that climatic changes (warming and heavy rainfall) during the
end of the last glacial period and the beginning of the postglacial period
triggered numbers landslide. In particular Japan, located east of the Eurasian
Continent and separated by the Sea of Japan at that period causing heavy
snowfall in the winters in regions facing the Sea of Japan. As a result,
the region is highly susceptible to landslide occurrences.
Large scale landslide features can be formed by an aggregate of smaller
landslide features Fig.8. Following
the formation of a large scale landslide cluster, it has been recognized
that fissures and cracks could develop during landslide movement. That
movement could facilitate infiltration of ground water into the head portion
and flank of the landslide mass. Such conditions can cause localized instability
and result in the formation of smaller landslide within the large landslide.
Many of the currently existing active landslides represent reactivated
portions of a larger landslide mass.
As slide movement continues, fracturing and weathering of the bedrock
occurs, and creates changes in the ground water flow regime. Material composition
and mode of movement of the landslide can also be altered. Further, it
has been speculated that the landslide topography could also be modified
when associated with alteration. Next, we shall examine the relationship
between landslides and landslide topography, and the engineering geologic
divisions.
Landslide Classification Based on the Engineering Geological Divisions
I. Pre-Tertiary Accretionary Terrain Zone
Formations within this zone were lithified through diagenesis and metamorphism,
however, localized weak zones caused by shearing and fracturing can often
develop during tectonism and can be further intensified by weathering.
Five landslide types have been classified in this zone.
- (1). Debris Creep-Slow Moving Debris Slide:
- This is a movement of debris accumulated along the gentle slopes in
front of steep slopes (20 to 35 degrees) in mountainous areas of high relief
(over 300 m). Debris creep is most common in this zone, and is characterized
by an extremely slow rate of movement.
- (2). Rapid Debris Slide:
- This failure is a rapid movement of accumulated debris caused by heavy
rainfall at the headwater regions in mountain streams and small canyons.
Often, this is a source area for debris flows. The age of the debris is
considered to be younger than the debris of (1) above.
- (3). Rapid Rock Slide:
- This is a rock slide involving more than 108m3 in volume, and is caused
by seismic activity or torrential rains. It has been speculated that the
majority of the failures had experienced a creep-type deformation prior
to the catastrophic failure.
- (4). slow Rock Slide:
- For crystalline schist, the slide plane is roughly parallel to the
slope; for non-metamorphic rocks, the slide plane is sub-parallel to laminar-bedding
planes and small low angle faults.
- (5). Bedrock Creep:
- The characteristics of this landform include scarps facing towards
the ridge lines accompanied by multiple scarps that are roughly parallel
to the ridge lines. Bedrock creep generally occurs near the ridge lines
of mountain regions of high relief. The lower slopes have bulges, and the
slope as a whole exhibits a convex shape. This represents the early stage
of deformation of the landform and slope dynamics. However, it is suspected
that distinct slide planes or separation planes have not been recognized.
Numerous examples have been identified through aerial photographic interpretation
and in field reconnaissance.
II. Plutonic Zone
Although the intrusions occurred throughout geologic time, the type
of landslides and modes of failure are similar to the landslide found within
the Plutonic Zone.
- (1). Rapid Surficial Slide:
- This involves failure of surficial soils, residual soils and colluvium
triggered by intense rainfall. It is most common in the Granitic Zone.
Failed debris moves further downstream while stripping surficial materials
along the way. Depending on the amount of water available, this type of
movement can often develop into debris flows.
- (2). Rapid Debris Slide:
- This failure includes debris accumulated around the drainage head and
knick line during the Pleistocene Epoch (perhaps during the glacial periods)
by intense rainfall. Similar to (1) above, it can often develop into debris
flows.
- (3). Rapid Slide of Weathered Rock or Residual Soil:
- This type of slide involves deep and extremely weathered bedrock where
the failure is generally triggered by intense rainfall. Slide planes are
generally found within the weathered zone, and others are found along joint
surfaces and dikes.
- (4). Slow Moving Landslide:
- The slide planes in this group are found along faults and fractures
within the granitic rocks and along the weathered zone within acidic plutonic
rocks such as granodiorite. However, not many landslides are observed in
areas underlain by plutonic rocks in Japan. Even though they do exist,
they are generally small scale.
III. Tertiary Covering Sediments Zone
The clastic materials observed within this zone are semiconsolidated.
Mudstones could easily turn to clay by hydration and weathering. Tuffaceous
mudstones contain abundant smectite that contributes to sliding. The types
of landslides observed within this zone are discussed below.
- (1). Clayey Soil Creep-Slow slide Movement:
- This type of slide often occurs in areas underlain by mudstone and
colluvium along valleys of low relief. Depending on the velocity, the sliding
can change to "mud flow". This type of movement is relatively
small, and ranges in size measurements of width: 5-100 m; length: 100-500
m and depth: 5-20 m. The causes of initial movement include increased pore
water pressure from snowmelt, stationary weather fronts of early summer
(Baiu Front) and typhoons.
- (2). Rapid Slide of Semi-consolidated Sediments:
- This is a rapidly moving slide along very steep slopes triggered by
snowmelt, intense rainfall or earthquakes with materials comprised of clastic
materials such as gravels (conglomerate), sands (sandstone), and silts
(siltstone) deposited during the Pleistocene and Upper Pliocene Epochs.
In some cases the rapid slide changes to mud flows with an average velocity
of up to 8 m/sec.
- (3). Slow Slide of Consolidated Sediments:
- The basic composition of the slope-forming materials includes the overlying
hard-competent cap rock formation with underlaying softincompetent formations.
The competent formations include thick sandstone, silicified shale, and
thick volcanic rocks, which the incompetent formations include unsilicified
mudstone and altered tuff. The maximum dimensions of this type of slide
could be on the order of width: 4 km; length:5 km; and depth; 100m.
- (4). Bedrock Creep:
- This is the same type of slope deformation defined in I,(3) above and
is also observed in the Green Tuff Zone.
IV. Quaternary Volcanic Zone
There are four types of slope movement that have been recognized within
this volcanic zone.
- (1). Large-Scale Rapid Failure of Volcanic Rocks / Debris Avalanche:
- This type of failure is involved only with composite volcanoes and
lava domes of high relief. As new volcanic activities centered within the
deeper portion of the volcano that contain upward-migrating magma, earthquakes
and fluctuating ground water levels could induce a catastrophic collapse
of the upper volcanic body. This type of failure often deposits relatively
large, unbroken blocks as mudflow hills along the foothills and nearby
low lying areas. The volume of the failure is on the order of 108 to 1010
m3. Most of the cases involve andesitic volcanoes (for example, Mayuyama
at Shimabara and Bandai-San), but there is a case that involves basaltic
volcanoes (Mt. Fuji, for example). Smaller scale failures ranging from
106 to 107 m3 triggered by earthquakes (for example, South Slope
Ontake-San of 1984).
- (2). Rapid Clay Slide in Hydrothermal Altered Zone:
- This type of landslide occurs within zones that receive severe hydrothermal
and fumarolic alteration near the crater bottom which is an opening to
the lower slope. Examples of this type of failure include Hakone Sounzan
of 1953, Hakone Ohwakudani, Kirisima Volcanoes, and Myokyo San.
- (3). Slow Moving Slide of Sedimentary Rocks Underlain by Volcanic
Deposits:
- This is a type of landslide formed along the foothills of volcanoes
where terrains are underlain by sedimentary rocks in turn overlain by lava
and pyroclastic rocks that protect the slope from erosion (cap rock structure).
As localized active down cutting of channels proceeds, oversteepened and
unstable areas transporting the volcanic bodies would fail. The scale of
this failure type could be very large, measuring up to width: 2 km; and
length: 4 km.
- (4). Debris Flow:
- There are two types of debris flows. One type involves recently deposited
volcanic ash that is subjected to rainfall which could flow downstream
causing frequent debris flows (Yakedake, Sakurajima, Unzen Fugendake, and
others). The second type is associated with high temperature pyroclastic
flows that melt the existing snow and ice, and flow downslope with the
melted water. The flowing body erodes and consumes materials along the
way, becoming a large scale mudflow-debris flow (Tokachidake).
V. Quaternary Regional Pyroclastic Zone
The slope movements within this zone are represented by the failure
of unwelded pyroclastic flow deposits accompanied with intense rainfall.
Furthermore, in this zone the failures are frequently induced by seismic
activity. However, it is considered that the majority of these failures
involve not only the rapid slide/topple/fall of the pyroclastic flow deposits,
but also include the rapid slide of the tephra and surficial deposits underlain
by the pyroclastic flow deposits.
Sliding and Rapid Failure
Among the slope movements discussed in the previous sections, there
are significant differences in the mode of movement assigned "rapid"
and "creep" or "slow". In Japan, it is traditional
to classify slope movements into two broad categories; Jisuberi=sliding,
and Hokai=rapid failure. As shown in Fig.9, the areal extent of sliding
is comparatively large (over 1000 square meters), gently sloping (less
than 30 degrees, and generally between 5 to 20 degrees), and slow velocity
(less than cm/minute) with a longer duration (over 100 hours). On the other
hand, the rapid failure is quite the opposite. The size is less than 1000
m2 occurs on slopes ranging between 30 to 60 degrees, and has
a velocity faster than 1 m/sec with a duration that is less than one hour.
The definite difference in damage is that sliding sustains large economic
damages while the rapid failures are more of a threat to human casualties.
Therefor, any measure for such slope movements have to be responded according
to the characteristics of each type. In fact, the mitigation measures must
be implemented according to the characteristics of the type of movement.
