The archipelago is separated from the Asian continent by generally shallow seas. The East China Sea is nothing but a vast expanse of shallow water (200m on average; at no point 1000m), located above an underwater plateau, on which, to the east, Formosa rises, which, keeping a short distance from the Ryū-kyū, it extends northwards to just beyond the Strait of Korea, that is to the edges of the vast depression that constitutes the bottom of the Sea of Japan, much deeper, in some points of which the probe drops even beyond 3500 m. To the NE., Along the coasts of Sakhalin and Yezo, the bottom rises to about 200 m. from the surface to descend again beyond the Strait of Sōya with the trench of the Sea of Okhotsk (200-2000 m.; somewhere more).
In the Pacific the depths are much greater; the maximums, indeed, are found right in the abysses flanking the eastern coasts of Japan. In the great “Japanese Trench”, as a rule, 7000 m. Are exceeded, and in the well-known one of Tuscarora, 8515 m were measured, not far from the island of Kunashiri; another point 8491 m deep. it was found off the coast of M. Kitakami, and finally recent soundings by the hydrographic service of the Japanese navy, measured 9950 m. south of the Bōsō Peninsula. It can thus be said that the islands of the Japanese archipelago rest on the same immense base on which the Asian continent stands, of which, therefore, they must be considered as the last offshoots towards the east; beyond, gigantic fractures carry deeps into the Pacific.
Its own characteristics, which justify its name, have the Inland Sea (Seto Naikai or Setouchi), a vast area of depression, very little deep (40 m. On average), perhaps formed in the recent Cenozoic, sown in its interior of islets, sometimes sculpted on terraces. Its very jagged coasts, often in rías, now low and marshy, now formed by the steep flank of mountains and volcanoes, have been the site of intense activity since ancient times. The Setouchi had, in fact, in the primitive history of Japan a first-rate importance, since it was thanks to its quiet and sheltered mirror and its position that the Japanese civilization, from Kyushu, where it had first settled coming from Korea, could easily spread northwards, especially on the plains of Hondo. Its function was therefore analogous to that of the Mediterranean in the development of Western civilization; that is, it served to relate countries that the relief isolated.
According to diseaseslearning, the temperature of the seas around Japan is relatively high. Surface measurements show that the eastern coast is warmer (22 ° -27 ° in August), at least in its lower portion (15 ° in February in Yokohama), than the western one, where, for example, beyond the Peninsula di Noto the thermometer drops to 10 ° in winter. Both the mountains of the country, which protect the east coast from the cold winds of the continent, and the hot current of the Kuroshio affect this condition of things., which has so much influence on the climate of the archipelago. This current originates from the north-equatorial one of the Pacific and first flows westward between 9 ° and 20 ° of lat. north. Not far from Formosa it turns north-east and widens its course so that it, about 185 km wide near this island, is already 3 times wider near Yokohama. After having lapped the coasts of the Ryū-kyū, it passes between Yakushima and Amami-Ōshima, turns towards Cape Shio-no-misaki and following the coasts of the Bōsō Peninsula, it returns to the Pacific between 30 ° and 50 ° lat. N. Kuroshio, notable for its strong salinity, undoubtedly plays its part in the summer showers of southern Japan; his name (from kuroi “black” and shio “sea water”) originates from the dark color that it has especially on the coasts of Kūyshū. Two of its branches are important: the first branches off south of Formosa and, skirting the island to the west, joins the main branch to the north of it; the other branches off to the south of Kyūshū and forms the so-called Tsushima Current which, following the western coasts of Hondo, ends up in the Mamiya Channel; it protects these coasts from the cold current of the Liman which descends from the same channel bordering the coasts of the continent up to Vladivostok. Another cold current is the Oyashio, which originates near the Camciatca peninsula and, lapping the western coasts of the Kuriles and Hondo, ends near Cape Inuboe, colliding with the Kuroshio.. This current lowers the temperature of neighboring lands and brings great fogs, but fishing owes much of its prosperity to it. The coastline is, in general, more developed on the Pacific than on the Sea of Japan. On the first, coasts abound in rías and tectonic activity has produced vast inlets, such as the bays of Tōkyō, Sagami, Suruga, Tosa, Ise and canals such as that of Kii and Bungo. On the Sea of Japan the bays of Toyama and Wakasa, the volcanic peninsula of Oga, the prominent pillar (horst) of Noto are the only variations that break the monotony of the coasts. The endogenous individual, especially earthquakes and bradyseisms, finally brings, where more where less, slow but continuous variations in the coastline of the entire archipelago.