Volcanoes in Japan
Fig. 12 Width between volcanic zones
Volcanoes are distributed in zones parallel to a trench on island arcs. The border between the volcanic zone and the non-volcanic area (forearc) on the trench side is usually clear. This border is called a volcanic front (Figure 4). The depth of the slab (part of the plate in the mantle) underneath the volcanic front is about 110 km (see p. 1). Some island arcs have two volcanic zones parallel to each other. The zone on the backarc side is over the slab at 170 km depth. The Northeast Japan Arc has two volcanic zones but the Izu-Bonin Arc has a single zone. This difference is attributed to the angle of the slab. As shown in Figure 12, the steeper the angle of the slab is, the smaller the width between the two zones is. The angle of the slab under the Northeast Japan Arc is about 30 degrees and that under the Izu-Bonin Arc is over 45 degrees (see Figure 4 [the width between contours indicates steepness.] or Figures 5-1 and 5-2 in “Japan in a subduction zone”).
Fig. 13 Distribution of volcanoes and landform types 
As seen in Figure 13, many Quaternary volcanoes are in zones in the Kuril, Northeast Japan, and Izu-Bonin Arcs, which are parallel to the Kuril, Japan, and Izu-Bonin Trenches, respectively. The Pacific Plate is subducting at these trenches. On the other hand, there are few volcanoes in the Kinki and Chugoku regions (the Southwest Japan Arc). The volcanic activity in these regions is related to the Philippine Sea Plate subducting at the Nankai Trough. This part of the Philippine Sea Plate is younger and warmer than another part of the plate. Therefore, the slab is at shallow depths under the Southwest Japan Arc (see Figure 4 or Figure 5-2 in “Japan in a subduction zone”) and the volcanic activity is dormant. In the Ryukyu Arc, the Philippine Sea Plate subducts at the Ryukyu Trench off the Kyushu region and the Nansei Islands. The volcanic activity is active and forms a volcanic chain with the clear volcanic front parallel to the Ryukyu Trench. In central Kyushu, volcanoes are distributed east-west, not parallel to the trench. This area is a rift valley called the Beppu-Shimabara graben.
About 200 Quaternary volcanoes are found in Japan. Half of these volcanoes erupted during the past 10,000 years, defined as active volcanoes. The Japan Meteorological Agency is watching 108 active volcanoes, especially monitoring 34 very active volcanoes for 24 hours (as of Jul 2010).
In this region (the southern part of the Kuril Arc), the Shiretoko, the Kussharo-Akan, and the Taisetsu-Tokachi volcanic chains are arranged en echelon from east to west. Although the volcanic front is parallel to the Kuril Trench, each volcanic chain is oblique to the trench. This is because the Pacific Plate is obliquely subducting at the Kuril Trench. The Shiretoko volcanic chain consists of stratovolcanoes; of them, Shiretoko io-zan is a particular volcano in ejection of a large quantity of sulfur. The Kussharo-Akan volcanic chain is characterized by calderas such as the Kussharo Caldera. In the Taisetsu-Tokachi volcanic chain, volcanoes were formed on pyroclastic plateaus produced in the Pliocene to Early Pleistocene. There is no volcano between the Taisetsu-Tokachi volcanic chain and the volcanic zone of southwestern Hokkaido. However, Rishiri-zan, an isolated stratovolcano, is found as a volcanic island in the northernmost of Hokkaido more than 200-km away from the volcanic front.
Southwestern Hokkaido and Northeastern Honshu (Tohoku)
Fig. 14 Distribution of volcanoes in Tohoku 
This region is in the Northeast Japan Arc. The volcanic front is
parallel to the Japan Trench. In Tohoku, three or four volcanic zones
are found from the Pacific side to the Sea of Japan side. The zone along
the volcanic front has the largest number of volcanoes, with the most
volume of volcanic products. Volcanoes of this zone erupted on the Ou
Range, narrow and long mountain range running north and south in the
center of Tohoku. These volcanoes are divided into seven clusters; each
cluster contains several or several tens of stratovolcanoes and a few
calderas. The clusters are 30 to 50 km long and the interval between the
clusters is 70 to 100 km. Volcanoes are fewer in the zones to the west
of the Ou Range, but their locations appear to be in the west expansion of
clusters. The clusters are situated in higher altitude area of the
mountain range in which the basement rocks have high elevation, formed
on the thick crust.
This characteristic is different from other island arcs such as the Kamchatka and New Zealand in which volcanic zones with their volcanic front develop in inner arc basins and lava fields and shield volcanoes are found as well as stratovolcanoes. Volcanoes in Tohoku are mainly stratovolcanoes composed of thick andesitic lava. This distinction is attributed to the tectonic settings. The Northeast Japan Arc is compressed from both sides of the Pacific and the Sea of Japan and by the collision of the Izu-Bonin Arc from the south. The strong compression results in uplifting the Northeast Japan Arc. Consequently, magma is thought to reach the surface through fine fissures produced in the compressive stress field. Moreover, the locations of clusters correspond with the distribution of low velocity areas of seismic wave in the mantle wedge. This suggests that the formation of clusters is related to the uneven distribution of low velocity (high temperature) areas. (Figures are available on the Tohoku University website)
Central Honshu (Kanto and Chubu)
Central Honshu has extremely complex tectonic settings because the
Northeast Japan Arc meets the Southwest Japan Arc and the Izu-Bonin
Arc. The Japan Trench connects with the Izu-Bonin Trench off the Boso
Peninsula. The Pacific Plate subducts underneath the Philippine Sea
Plate at the Izu-Bonin Trench. The Philippine Sea Plate descends
under the North American Plate at the Sagami Trough and the Suruga Trough and
under the Eurasian Palate at the Nankai Trough. The Sagami and the Suruga
Troughs extend to the land as the plate boundaries and connect with
each other near Fuji-san. The Sagami Trough links with the Japan and
the Izu-Bonin Trenches at their joint, so this point is called the triple
junction. The Philippine Sea Plate subducts in directions splitting it
in the Sagami and the Suruga Troughs. The Philippine Sea Plate subducted
at the Sagami Trough
is in contact with the
leading edge of Pacific Plate (Figure
5-2 in "Japan in a subduction zone").
Volcanoes are crowded in central Honshu (Figure 13). The volcanic front extending from Tohoku curves westward and turns toward the Izu-Bonin Arc near Asama-yama which repeatedly erupted on a small scale recently. The volcanic front obliquely crosses the depth contours of the subducting Pacific Plate near the turning point, meaning that it is not parallel to the trench, because the movement of Philippine Sea Plate affects it. Three volcanic zones parallel to the volcanic front lie on the east of the turning point, continuing from Tohoku. Along the volcanic front southward, there are two volcanic chains: the volcanic front side chain including Yatsugatake, Hakone and Izu-oshima, and the back arc side chain including Fuji-san and Ashitaka-yama. Fuji-san, Hakone, and volcanoes to the south of them are on the Philippine Sea Plate. Fuji-san is the largest in volume and Yatsugatake is the second on land in Japan. The characteristics of Fuji-san, such as basaltic volcano rather than andesitic, the location on the border of plates, and particular large volume as an island arc volcano, well demonstrate the peculiarities of central Honshu (see "Fuji Volcano").
Monogenetic volcanoes (Izu-tobu Volcano Group) are found in the eastern Izu Peninsula. It is only a monogenetic volcanic group in the Pacific side in the Japanese Islands. As mentioned above, monogenetic volcanoes are formed in areas dominated by weak-compressive or tensile stress. The stress field in this area is different from that of other areas, related to the complex tectonic settings of the collision zone.
There is another volcanic zone to the west (back arc side) of above volcanic chains. This zone including Tate-yama, Norikura-dake, and Ontake-san is in the Southwest Japan Arc. These volcanoes erupted on the Hida Range, one of the highest mountain range in Japan. Furthermore, a volcanic area including Haku-san is to the west of the volcanic zone.
Izu Islands (Northern Izu-Bonin Arc)
The clear volcanic front runs parallel to the trench. Volcanic
islands, Izu O-shima and Miyake-jima, near the Izu Peninsula, are large
active volcanoes, which erupted in 1986 and 2000, respectively. There
are 19 Quaternary volcanoes along the volcanic front and 250 small
volcanoes in back arc basins of the Izu-Bonin Arc.
Basaltic volcanoes dominate this volcanic zone. The crust of the Izu-Bonin Arc is basaltic and thinner than that of other island arcs, which is about half thickness of the 30-km thick crust of the Northeast Japan Arc. Therefore, the density of basaltic magma generated in the upper mantle is much lower than the basaltic crust, readily ascending to the surface. However, rhyolitic volcanoes are also found, such as Nii-jima and Kozu-shima. In the Izu Islands, stratovolcanoes are common, and lava domes and submarine calderas have also been formed.
Southwestern Honshu (Kinki and Chugoku) and Shikoku
In this region, only Daisen is a stratovolcano, and monogenetic
volcanic groups, such as the Kan-nabe and the Abu volcano groups, are scattered
in the Sea of Japan side. These groups are small but alive for more
than four million years. Two reasons why monogenetic volcanoes have
been formed in this region are proposed: one is that the volcanism is
related to the plate subduction, and the other is that it is attributed
to the magma rising associated with the expansion of the Sea of Japan.
There is no Quaternary volcano in Shikoku.
Kyushu and Nansei Islands
In northern Kyushu, monogenetic volcanoes composed of alkali basalt were formed in the Goto Islanads and Iki Island. In central Kyushu, large volcanoes including Kuju-san, Aso-san, and Unzen-dake are arranged east and west in the Beppu-Shimabara Graben. This volcanic zone is situated in a tensile stress field, rare in Japan. No volcano is found in the Kyushu Mountains about 100 km wide, between the Beppu-Shimabara Graben and the Kirishima volcanoes. In southern Kyushu, volcanoes are distributed parallel to the Ryukyu Trench from the Kirishima volcanoes toward south. This area is characterized by a series of large calderas including Kirishima, Aira, Ata, and Kikai Calderas. These volcanoes catastrophically erupted in the Late Quaternary, with pyroclastic flows covering southern Kyushu thick. Sakura-jima in Kagoshima Bay, which has been formed in the Aira Caldera, frequently erupts and its volcanic ash falls on Kagoshima City on the opposite shore with a population of 600 thousands. The volcanic chain continues about 400 km to the south of Kyushu. There is no volcanic island further south, but submarine volcanoes have been discovered in the backarc basin of the Ryukyu Arc.