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.

engineering geologic divisions

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.