Introduction to the Landforms and Geology of Japan

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Japan in a Subduction Zone

Volcanoes, earthquakes, and mountain building are major characteristics of the Japanese Islands in an active continental margin, which are closely related to plate subduction.

Convergent boundary

The boundaries of plates covering the surface of the Earth are classified into three types: convergent, divergent, and transform types. Convergent boundaries occur where one plate subducts underneath another plate with density lower than the subducting plate or collides with another plate in the case that both plates are composed of continental material. Continental plates cannot dive into the mantle because their densities are lower than that of the mantle. Divergent boundaries occur where new lithosphere (plate) is produced and plates move away from each other at spreading ridges. Transform boundaries occur where one plate laterally slides past another, displacing spreading ridge.

As an oceanic plate subducts underneath another plate at a convergent boundary, such area is called a subduction zone. The Japanese Islands are situated in a subduction zone in the northwestern margin of the Pacific Ocean where the Pacific Plate and Philippine Sea Plate are subducting. Convergent boundaries are characterized by volcanism, earthquakes, and mountain building attributed to active crustal movement.

The active crustal movement zone surrounding the Pacific Ocean is called the circum-Pacific orogenic belt, and also known as the Ring of Fire.

In and around Japan

In Japan, thousands of islands are arranged in several arc-shaped chains (island arcs). The Japanese Islands are emerged parts of volcanic island arcs, extending to about 3000 km. The four main islands and island chains are as follows: Hokkaido, Honshu, Shikoku, Kyushu, the Kuril Islands connecting with the Hokkaido island, the Izu-Bonin (Ogasawara) Islands aligning southward from central Honshu, and the Nansei Islands extending southward from the south of Kyushu. On the Pacific Ocean side, trenches run parallel to these islands: in the order from the north, the Kuril Trench, the Japan Trench, the Izu-Bonin Trench, the Nankai Trough, and the Ryukyu Trench. Short troughs, the Suruga Trough and the Sagami Trough, are situated on the west and east sides of the Izu Peninsula in central Honshu, respectively. The Suruga Trough connects with the Nankai Trough. Trenches and troughs are long, narrow submarine depressions. They are classified by depth; narrow depressions 6000 meters or more deep are trenches and those less than 6000 meters deep are troughs. (However, in this site, “trench” is mainly used as the term meaning a depression where a plate subducts in general descriptions about subduction zones and so on.)

A marginal sea is defined as a portion of ocean incompletely bordered by islands or peninsulas situated in the margin of continent. In island arc areas, marginal seas expand on basins on the back arc side (back arc basin), generally situated between island arcs and the continent (see "Island arc-trench system"). The Sea of Okhotsk, the Sea of Japan, and the East China Sea are marginal seas. The Philippine Sea is surrounded by the Philippine Islands, Taiwan, the Nansei Islands, southwest Japan, the Izu-Bonin Islands, and the Mariana Islands. This sea consists of the Shikoku Basin and the Philippine Basin, geomorphologically divided from the northwestern Pacific Basin. Although the Philippine Sea corresponds to the ocean side (forearc side) of the Southwest Japan Arc and the Ryukyu Arc, it is also a marginal sea (backarc side) of the Izu-Bonin Arc and the Mariana Arc.

Plate subduction and earthquakes

The Japanese Islands are in marginal areas of the Pacific Plate, the Philippine Sea Plate, the North American Plate, and the Eurasian Plate (Figure 3). A triple junction at which the Pacific, the Philippine Sea, and the North American Plates meet one another is situated off the Boso Peninsula in central Honshu (Kanto). Oceanic plates, the Pacific Plate and the Philippine Sea Plate, subduct at trenches or troughs. The Pacific Plate descends underneath the North American Plate at the Kuril Trench and the Japan Trench and underneath the Philippine Sea Plate at the Izu-Bonin Trench. The Philippine Sea Plate descends beneath the Eurasian Plate at the Suruga and Nankai Troughs and the Ryukyu Trench. The leading edge of the Pacific Plate reaches under the Eurasian Plate.

Plates
Fig. 3  Plates in Japan

Earthquakes very often occur in borders between plates. In Japan, over 1300 felt earthquakes were observed in 2010, but more than 2000 earthquakes were felt in some years. The frequency of M ≥5 aftershock of M 9 earthquake in eastern Japan in 2011 is over 400 two weeks after the main shock (M: magnitude).

In plate margins, the crust is stressed by plate movement. The rocks are broken and energy is released when the stress exceeds their strength. The rupture generally occurs along faults, which are considered seismic sources. Therefore, the rapid slip of rocks along a fault results in an earthquake. Volcanic activity also causes earthquakes. There are principally three types of fault motion: normal dip-slip, reverse dip-slip, and strike-slip (left or right). Fault movements may include a component of strike-slip and dip-slip (see also "Neodani fault"). As for the M 9.0 eastern Japan earthquake in 2011, Japan Meteorological Agency reported that the type of earthquake source fault was the reverse fault, the size of fault was about 450 km long and 200 km wide, the fault slippage was 20 to 30 meters (maximum), and the duration of main failure was about three minutes (The 2011 off the Pacific Coast Tohoku Earthquake [28th report]).


Fig. 4 Types of fault motion

Figures 5-1 to 5-4 is a 3-D distribution map of hypocenters. Shallow-focus earthquakes occur mainly on the landward side of trenches (near the boundaries of continental plates) and around active volcanoes and faults. Also, shallow-focus earthquakes and relatively deep-focus earthquakes often happen off the western coast of northeastern Honshu and southwestern coast of Hokkaido. This earthquake zone is along the boundary between the North American Plate and the Eurasian Plate.

Hypocenters of earthquakes occurring at depths greater than 60 km are found in a zone tilted landward, which are characteristic of subduction zones (Figures 5-2, 5-3, and 5-4). The zone is called a deep seismic zone or the Wadachi-Benioff zone and is situated in a slab (a sinking part of oceanic plate in the mantle). Therefore, the distribution of hypocenters provides information on subducting plates.

Fig. 5 3-D distribution maps of hypocenters

Distribution map of hypocenters (5-1)  Distribution map of hypocenters (5-2) 
Fig. 5-1 Hypocenter distribution [Another window] Fig. 5-2 Viewed from the south [Another window]
Distribution map of hypocenters (5-3)  Distribution map of hypocenters (5-4) 
Fig. 5-3 Viewed from the northwest [Another window] Fig. 5-4 Viewed from the north [Another window]

In the maps, the largest slope is the slab of the Pacific Plate. The deepest hypocenters are found at depths about 600 km, showing the slab is diving extremely deep into the mantle. Seismic tomography indicates that the slab reaches 1200 m deep under the ground in a region from the southern Kamchatka to the northern Kuril Islands and the Mariana region. The angle of the deep seismic zone differs among island arcs (Figure 5-3). The deep seismic zone under the Northeast Japan Arc is the gentlest (about 30 degrees) and that under the Izu-Bonin Arc is the steepest (45 degrees or more). The subduction rate of the Pacific Plate is about 10 cm/year.

On the other hand, the leading edge of the Philippine Sea Plate is at much shallower depths than that of the Pacific Plate (Figures 5-3 and 5-4). The depths of deepest hypocenters in the Philippine Sea Plate subducting at the Nankai Trough are 60 km under Chubu, 70 km under Kinki and 40 km under Shikoku, and the angle of the deep seismic zone is 20 to 30 degrees. Data of hypocenters suggest the flexure of the slab from Chubu through Shikoku. The Philippine Sea Plate subducted at the Sagami Trough is in contact with the slab of the Pacific Plate. The plate is subducting from the Nankai Trough at about 3.2 cm/year. As for the Philippine Sea Plate subducting at the Ryukyu Trench, the deep seismic zone reaches to a depth of 160 km to 200 km, and the angle is about 70 degrees in an area to the north of Tokara Straight (around lat. 29 N) and 40 to 50 degrees in an area to the south of Tokara Straight. The subduction rate is about 4.6 cm/year.

Plate subduction is a phenomenon that lithosphere with density increased by cooling sinks into the upper mantle with a high temperature and relatively low density. Generally, the older a plate is, the colder and denser it is. It is known that a subduction rate correlates with an age of plate. In fact, this relationship is true around Japan (Table 1). As mentioned above, the approximate subduction rate is 10 cm/year for the Pacific Plate, 4.6 cm/year for the Philippine Sea Plate subducting at the Ryukyu Trench, and 3.2 cm/year for the Philippine Sea Plate subducting at the Nankai Trough. The age of the seafloor in the northwest Pacific Basin is 100 million years or older. The seafloor of the Philippine Sea near Japan is divided into the Philippine Basin and the Shikoku Basin (see Submarine topography). The Philippine Basin was formed in the Paleogene (49 million to 40 million years ago) and the Shikoku Basin was formed in the Late Paleogene to Middle Miocene (30 million to 15 million years ago). However, an angle of deep seismic zone does not correlate with an age of plate.

Table 1
Table 1 Subduction rate and age of seafloor

Island arc-trench system

In a subduction zone, plate subduction forms a trench and uplift area parallel to the trench and causes igneous activity and earthquakes. Such uplift area is called a volcanic island arc or island arc because it is an arc-shaped chain of islands and volcanoes. Volcanic arcs formed on the edge of continent without marginal seas are continental margin arcs. 

The arrangement of major landforms, distribution of volcanoes, and geotectonic subdivisions of southwest Japan are parallel to trenches off the Japanese Islands. Plate subduction at trenches is responsible for these features, and island arcs and trenches, therefore, can be regarded as an island arc-trench system. It is mainly characterized by the following: (1) major landform arrangement, ocean–trench–island arc–marginal sea (backarc basin)–continent, (2) igneous activity, (3) earthquakes occurring at depths of ≥70 km in addition to shallow-focus earthquakes, and (4) accretionary prisms and metamorphic rocks produced by plate subducting.

The Japanese Islands consist of five island arcs: in the order from the north, the Kuril, the Northeast Japan, the Izu-Bonin, the Southwest Japan, and the Ryukyu Arcs (Figure 2). The Kuril Arc collides with the Northeast Japan Arc in central Hokkaido. The Northeast Japan Arc meets the Southwest Japan Arc in central Honshu, and the Izu-Bonin Arc collides with these two arcs. The Southwest Japan Arc meets the Ryukyu Arc in central Kyushu. In these areas called the arc-arc collision zone, the directions of the island chains change and geomorphological and geological structures are complicated.

Each island arc is accompanied with a trench in parallel: Kuril Arc–Kuril Trench, Northeast Japan Arc–Japan Trench, Izu-Bonin Arc–Izu-Bonin Trench, Southwest Japan Arc–Nankai Trough, and Ryukyu Arc–Ryukyu Trench. These trenches are divided into two series. The first series is the line of the Kuril, the Japan, and the Izu-Bonin Trenches, and the second is the line of the Nankai Trough and the Ryukyu Trench. The island arc-trench system in Japan, therefore, is classified into two systems: the eastern Japan arc system (the Kuril, the Northeast Japan, and the Izu-Bonin Arcs) and the western Japan arc system (the Southwest Japan and the Ryukyu Arcs). Tectonism and volcanism in the eastern Japan arc system and in the western Japan arc system are mainly regulated by the Pacific Plate movement and the Philippine Sea Plate movement, respectively.

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