1. Japan and Its Nature (landform, Geology and Climate)
Physiology of Japanese Archipelago (Composition and Topographic Division
of the Island Arc)
The Japanese archipelago is comprised of five main island arcs extending
approximately 3000 km in the north-south direction, and encompasses a total
area of about 378,000 square km. Seventy-five percent of the total land
area consists of mountainous terrain (Fig.1) .The
five main island arcs are, from north to south, Kurile Arc, Northeast Honshu
Arc, Izu-Mariana Arc, Southwest Honshu Arc and Ryukyu Arc. These arcs approximately
represent plate boundaries of the North America Plate, Pacific Plate, Eurasian
Plate, and Philippine Sea Plate. Because of this arc-plate relationship,
Japan is located in an area of severe crustal movement, and is situated
in one of the world's most seismically active regions. There are 77 active
volcanoes in Japan, and represent approximately 10% of the world's active
volcanoes. The deep-seated earthquakes around Japan are epicentered alone
the Kurile-Kamchatka Trench, Japan Trench, Izu-Ogasawara Trench, South
Sea Trench, and Ryukyu Trench, which dip steeply into the continent along
the Wadati-Benioff plane. Furthermore, a series of volcanic fronts are
aligned between the trenches and the continent. Typically the volcanic
belts are located about 100 to 200 km inward of the trenches (Fig.2).Based
on the volcanic fronts, the five island arcs mentioned above are classified
into volcanic inner arcs and non-volcanic outer arcs.
Based on the characteristic configuration and partial overlapping of
the five island arcs, the Japanese archipelago is divided into 12 district
physiographic zones (Fig.2).Landslides occur
frequently within six provinces: Zone A1, the main interior of Hokkaido
(inner Kurile Island Arc) which exhibits medium to small scale landslides;
Zone B1, the inner Northeast Honshu Arc (the representative landslide zone);
Zone D1, the Northeast Kyushu Island within the inner Southwest Honshu
Arc (the zone of high density landslide distribution); Zone D2, the outer
Southwest Honshu Arc and Zone DC1, the Central Western Honshu (Chubu Mountains),
represented by landslides of very slow movement and very large-scale, rapidly
moving failures; and Zone DC2, the Central Eastern Honshu (Kanto Mountains)
which exhibits a high density landslide distribution within low-lying mountains.
Engineering Geologic Structure of the Japanese Archipelago
Based on the major geologic divisions and characteristics of the earth
materials during slope movement, the Japanese archipelago is divided into
15 engineering geologic divisions (Fig.3).
Furthermore, on the basis of plate tectonic research and assessment of
slope movement, the 15 engineering geologic divisions are further subdivided
into five distinct tectonic zones, discussed below.
- I. Pre-Tertiary Accretionary Terrain Zone:
- This zone represents Mesozoic and Paleozoic sedimentary rocks (Division
4) and metamorphic rocks (Division 3), ultramafic and mafic
intrusive rocks (Division 2), Cretaceous marine deposits of turbidite
facies (Division 5), and Flysch type sedimentary rocks of Late Mesozoic
to Early Miocene age (Division 6). These divisions are typically
distributed along the extensional direction of the narrow island arcs.
The geologic divisions are juxtaposed against each other from the oldest
formations (on the continental side) to the youngest formations (on the
Pacific Ocean side). The contacts between the divisions are separated by
shear zones including thrust faults.
- II. Plutonic Zone:
- This zone consists of plutonic rocks of mostly Cretaceous of Early
Tertiary age (Division 1). However, a few parts of this zone include
Miocene and Quaternary age intrusive rocks. Plutonic rocks of pre-Tertiary
age exhibit significant weathering.
- III. Tertiary Covering Sediments Zone:
- This zone represents the areas of the highest landslide occurrence
within the Japanese archipelago, and consists mostly of Neogene (and some
Paleogene) semi-consolidated clastic materials (Divisions 8 and 10
and volcanic rocks (Division 9) which overlie the Pre-tertiary Accretionary
Terrain Zone and the Plutonic Zone. Non-siliceous mudstones easily weather
or decay into clays due to increased water content and weathering. Alteration
of the volcanic rocks changes the color to a greenish appearance, and thereafter
they are called 'green tuff";. Tuffaceous mudstones contain abundant
smectite clays, and contribute to one of the primary causative factors
of landslides.
- IV. Quaternary Volcanic Zone:
- Volcanoes consisting of lava, welded tuff and scoria beds (Division
14) often form very steep slopes. Furthermore, the volcanoes are often
associated with hydrothermal alteration and volcano-induced earthquakes.
The high relief of the volcanic regions also attracts heavy precipitation.
All of these factors contribute to slope instability. Cap rock conditions
exist when volcanic rocks overlie the clastic materials.
- V. Quaternary Regional Pyroclastics Zone:
- This zone consists of large scale eruptions of Late Quaternary acidic,
welded and unwelded pyroclastic deposits (Division 13); that are
distributed throughout much of Japan.
Climate of Japan
The Japanese archipelago is situated between North latitude 45 degrees
and 20 degrees, facing the Pacific Ocean along the southeastern side and
the Sea of Japan and Eurasian continent along the northern side. Due to
the geographical position of Japan, the climate varies considerably.
During the winter months, continental cold masses (high pressure zone)
are formed in the Siberian region due to radiation cooling. The cold seasonal
winds that are generated from the cold air masses move through the Sea
of Japan and absorb large quantities of moisture during Japans' winter
months, which cause the dominating northwest winds. When the moist seasonal
winds reach Japan, the cold air masses collide into the mountain regions.
As the air masses rise with increasing elevation, a large quantity of moisture
is precipitated as snow along the slopes facing the Sea of Japan, establishing
one of the worlds' famous snowy regions (Fig.5).
In the spring, due to the low pressure zones moving west to northeast,
the cold-warm cycles are repeated and gradual warming occurs. Numerous
landslides have been triggered by the large quantity of snowmelt along
the slopes facing the Sea of Japan. Cherry blossoms and budding from the
southernmost island moves progressively northward. In early June, the Northern
Pacific High Pressure Zones gradually move from south, and the northern
air masses move from the Sea of Ohhotsk since the springs and collide above
Japan, forming a stationary seasonal rain front. Usually, this early summer
stationary rain front (Baiu Front) lasts a couple of months, intermittently
dropping large quantities of rain. These rains often create landslide and
debris flow disaster.
In the summer months, Japan is a high temperature-high humidity region
due to the Northern Pacific High Pressure Zones that cover most of Japan.
In the fall, typhoons form in the low latitude regions of the Northern
Pacific Ocean and move northward circling the western rims of the Northern
Pacific air masses that often land in Japan (Fig.6).
These typhoons usually generate very strong winds and very heavy rainfall,
and cause frequent sedimentation disasters and flooding. The pressure distribution
pattern in early autumn is very similar to the stationary rain of early
summer. Late autumn (October-November) is generally clear and offers the
beauty of the colored hills. The annual precipitation in Tokyo (Pacific
Ocean side) is 1405 mm. At Owase, however, the Kii Peninsula records 4002
mm while Takada (Sea of Japan side) records 2880 mm (of which one-half
is snow).
Landslide Disasters in Japan
The natural conditions discussed above are multiple affects that create
unique physical conditions susceptible to landsliding which cannot be seen
any other place in the world (Fig.4).Since
only 25% of Japans' land area is flat and low lying with plateaus, the
Japanese people have suffered numerous landslide disasters since ancient
times. For example, evidence of landslide failure has been unearthed from
the site (Oshimo Shell Mound, Aso-Cho, Ibaragi Prefecture) of Jomon in
the Middle to Late Period (3000-1000 BC). Nihon Shoki (720 literature)
recorded numerous landslides and failures associated with he mega-earthquake
(along the South Sea Trough) of November 29,684. Recent disasters include
torrential downpours around Kumamoto and Nagasaki in 1972; disasters from
typhoon No.17 in 1976; torrential downpours in Nagasaki in 1982, and many
others. Human casualties from these disasters include 543 deaths in the
1972 event, 298 deaths in the 1976 event, and 493 deaths in the 1982 event.
Disasters from single landslide events include a large scale failure of
Ontake San, Nagano Prefecture in 1984 (volume:3.4x107m3, 15 deaths), Tamanoki
Landslide of Ohmi-Cho, Niigata Prefecture in 1985 (10 deaths), Jizuki Yama
Landslide, Nagano City in 1985 (26 deaths), and others. More than 300 landslides
and slope failures have been reported since the Southern Hyogo Earthquake
of January 17, 1995. The population density of Japan is 328/km2 (based
on the 1994 census population of 124 million). However, the population
density of the flat and low lying areas and plateaus of Japan is 1312/km2
indicating the severity of land use in Japan. Landslides generally occur
along gently to moderately sloping ground which is also important as these
areas include residential and agricultural use. Because of these conditions,
an active effort must be developed to protect the slopes from future landsliding
and failures in Japan.
