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Submarine topography
Ocean and marginal seas around Japan
The Pacific Ocean and marginal seas surround the Japanese Islands. The Pacific Ocean expands on the east of the islands. The sea fringed by the Izu-Bonin, Southwestern Japan, and Ryukyu Arcs is separated from the Pacific Ocean as the Philippine Sea. The seas on the west of the Japanese Islands are the Sea of Okhotsk, the Sea of Japan, and the East China Sea. These seas including the Philippine Sea are called marginal seas which are situated behind arcs (between arcs and the continent).
Pacific Ocean
The Northwest Pacific Basin expands on the east of the Kuril, Japan, and Izu-Bonin Trenches. The basin 5000 to 6000 m deep has a flat floor. The oceanic plate produced along the mid-ocean ridge part from there toward trenches. Therefore, the farther the oceanic plate moves from the ridge, the older it become. The seafloor near Japan, which is very far from the mid-ocean ridge, was yielded over 100 million ago (in the Jurassic to Cretaceous).
Philippine Sea
The Shikoku Basin is located on the south of the Nankai Trough, the eastern and western borders of which are the Izu-Bonin Arc and the Kyushu-Palau Ridge, respectively. This basin 3000 to 4000 m deep has the rugged surface to the south of the trough, implying seafloor spreading. The basin is shallower than the Pacific Basin because it is much younger. The Shikoku Basin was formed from 30 million to 15 million years ago accompanied by separating the Kyushu-Palau Ridge from the Izu-Bonin Arc. The northern end of the ridge is situated at the border between the Southwestern Japan Arc and the Ryukyu Arc. The Amami Plateau, the Daito Ridge, and the Oki-Daito Ridge are arranged to the west of the Kyushu-Palau Ridge in this order from the north. Basins lie between these plateau and ridges. The Philippine Basin is on the south of the Oki-Daito Ridge.
Sea of Okhotsk
The Sea of Okhotsk has the Chisima Basin 3000-3300 m deep in the south part which is extremely flat and covered with thick sediments.
Sea of Japan
The Sea of Japan is separated into the northern part and southern
part at the Yamato Rise situated in the middle of the Sea of Japan. In
the northern part, the Japan Basin 3000-3600 m deep expands with a flat
floor, which is covered with sediments 1000-1500 m thick. The basin is
of the oceanic crust, formed by seafloor spreading 28 million to 18
million years ago. The eastern margin of the Sea of Japan is thought of
the boundary of plates at which the plate has begun subducting (see 2)
continental slope: marginal sea side).
The southern part has complex topography including the Yamato Basin,
the Korean Plateau, and the Tsushima Basin. The crust in the Yamato
Basin is thicker than that in the Japan Basin, and the ridges and
plateaus consist of blocks of the continental crust. It is thought that
this part has spread mainly by stretching and thinning the curst.
East China Sea
In the East China Sea, the Okinawa Trough is along the Ryukyu Arc and the expansive continental shelf shallower than 150 m in depth is situated in the continental side. The Okinawa Trough is a depression about 1200 km long and 150 km wide. Knolls were formed by volcanic activity in the central area of the trough. This marginal sea is unique in Japan because of active crustal movement and volcanism recognized in the Okinawa Trough.
Topography from shoreline to deep sea floor
Fig. Generalized profile of seafloor
in active continental margins
The seafloor around the Japanese Islands geomorphologically divided into: a continental shelf, a continental slope, and a trench (forearc side) or a basin (backarc side). This submarine configuration is commonly seen in active continental margins where an oceanic plate descends. On the other hand, in passive continental margins, the seafloor has no trench but may have a continental rise following a continental slope toward an abyssal plain.
Continental shelf
Continental shelves lying between the shoreline and the continental
slope are the gentle slope, inclining at an angle of much less than 1
degree (on average, about 0.1 degree [2 m/km]). The slope abruptly
becomes steep at the outer edge. This “shelf-slope break” is at a depth
of less than 200 m (average 130 m). The width of the continental shelf
varies from place to place, the average of which is about 70 km.
In and around Japan, shelf-slope breaks are 140 m deep on average, while
the width is 20-30 km, narrower than its world average. Relatively broad
continental shelves are situated in the southern most region of the Sea
of Japan offshore from the middle part of southwestern Honshu to
northern Kyushu and the northern Sea of Japan and the Sea of Okhotsk
around northern Hokkaido. Narrow shelves are located offshore the
western coast of the Oshima Peninsula, around Toyama Bay, and offshore
from the Boso Peninsula to the Kii Peninsula. The selves in Toyama Bay,
Sagami Bay, and Suruga Bay are extremely narrow, less than 1 km in
width. The slope is not flat from the shoreline through its outer edge,
but terrace-like topography (submarine terrace) and submarine valleys
are found. The submarine valleys are often filled up with sediments
derived from land.
