OCEAN FEATURES OF JAPAN: ISLANDS, SEAS, COASTAL AREAS

SEAS OF JAPAN

Seas, Ocean and Straits Around Japan

Japan is an island chain strategically located in northeast Asia between the North Pacific Ocean to the east, the Sea of Japan to the west, the Sea of Okhotsk to the north and the East China Sea to the south. The latter three seas are extensions of the Pacific Ocean.Another extension of the Pacific, the Philippine Sea, lies to the far southeast, along the coast of the Ryukyu archipelago. Warm and cold ocean currents merge in the waters surrounding Japan. Three of Japan's four main islands — Honshu, Kyushu and Shikoku — enclose the Inland (Seto) Sea. Junior Worldmark Encyclopedia of Physical Geography, Gale Group, Inc., 2003; Library of Congress, 1994]

No point in Japan is more than 150 kilometers (92 miles) from the sea. The distance between Japan and the Korean Peninsula, the nearest point on the Asian continent, is about 200 kilometers (125 miles) at the Korea Strait. Though Japan is known historically for its isolation it has also been linked with Asia through sea trade routes, stretching in the north toward Siberia, in the west through the Tsushima Islands to the Korean Peninsula, and in the south to the ports on the south China coast.

The Korean Strait, approximately 200 kilometers (124 miles) across, separates southwest Japan from South Korea and links the East China Sea to the Sea of Japan. The Sōya Strait (La Perouse Strait) runs between northern Japan and Russia's Sakhalin Island; this strait links the Sea of Japan to the Sea of Okhotsk. Tsugaru Strait separates Hokkaido and Honshu Islands, linking the Sea of Japan to the Pacific Ocean. [Source: Junior Worldmark Encyclopedia of Physical Geography, Gale Group, Inc., 2003]

Extensive coastal shipping, especially around the Inland Sea (Seto Naikai), which separates western Honshu from Shikoku, compensates for the lack of navigable rivers. The Pacific coastline south of Tokyo is characterized by long, narrow, gradually shallowing inlets produced by sedimentation, which has created many natural harbors. The Pacific coastline north of Tokyo, the coast of Hokkaido, and the Sea of Japan coast are generally unindented, with few natural harbors.[Source: Library of Congress, 1994]

Sea of Japan

The Sea of Japan is the marginal sea between the Japanese archipelago, Sakhalin, the Korean Peninsula, and the Russia mainland. The Japanese archipelago separates the sea from the Pacific Ocean. The Japan Sea has relatively small tides due to its nearly separation from the Pacific Ocean. Its isolation also its sea life and salinity, both of which are lower than in the open ocean. The Japan Sea has no large islands, bays or capes. Few rivers discharge into the sea and their total contribution to the water exchange is only around 1 percent. The sea’s water balance is mostly determined by the inflow and outflow through the straits connecting it to the neighboring seas and the Pacific Ocean. [Source: Wikipedia]

For centuries, the Japan Sea had protected Japan from land invasions, most notably by the China-based Mongols in the 13th century. It had long been used navigated by Asians and, from the 18th century, by European ships. Russian expeditions of 1733–1743 mapped Sakhalin and the Japanese islands. The seawater in the Japan Sea has high concentrations of dissolved oxygen that results in large fisheries.. Fishing has long been the dominant economic activity in the region. Squid fishing is particularly common today as evidence by the number of squid fishing boats you see when you fly over the sea at night. Several giant squid have been fished out of the Sea of Japan. One was captured alive off Shinonsen, Hyogo Prefecture.

The Sea of Japan was a landlocked sea when the land bridge between East Asia and Japan existed is the Early Miocene (23 million to 16 million years ago). During this time the Japanese Arc formed and Japan Sea started to open. Today, the sea has a carrot-like-shape and a surface area of about 978,000 square kilometers (378,000 square miles), a mean depth of 1,752 meters (5,748 feet) and a maximum depth of 3,742 meter (12,277 feet).

Seto Inland Sea

The Seto (Inland) Sea — Setonaikai or Setouchi in Japanese and often simply called the Inland Sea in English — is a body of water in Japan bounded by southern Honshu, northern Shikoku and northern Kyushu. Covering an area of 21,827 square kilometers, it is linked to the Sea of Japan and the Pacific Ocean by narrow channels. It is a shallow sea with an average depth of 38 meters (120 feet), seldom deeper than 380 meters (1,200 feet), and is dotted with more than 3000 volcanic and granite islands, the largest of which is Awaji-shima.

The Seto Inland Sea is famed for its scenic beauty and is the site of Setonaikai National Park The sea stretches for 490 kilometers (300 miles) from Osaka to the resort town of Beppu on northeast Kyushu. The calm, protected waters are ideal for shipping. Four major channels connects the Inland Sea to the ocean: 1) the Kanmon Straits between Honshu and Kyushu leads to the Sea of Japan; 2) the Hoyo Strait between Kyushu and Shikoku leads to Pacific; 3) the Naruto Strait and 4) Kitan Strait between Shikoku and Honshu lead to the Pacific. Strong tidal surges in the Hoyo and Kanmon Straits form currents flowing opposite directions that produce whirlpools in the Natuto Straits, between Awaji and Shikoku Islands.

Islands of Japan

(a) Location of the Seto Inland Sea; (b) Bathymetry of the Seto Inland Sea; The solid orange circles with lowercase 75 letters represent the locations of first-class rivers, and the solid purple circles represent the locations of second-class rivers; The red dashed line in the west represents the open boundary between Bungo Channel and the open ocean, and the red dashed line in the east represents the open boundary between Kii Channel and the open ocean; The solid black line represents the location of Kanmon Strait; (c) Division of sub-regions and locations of insitu observation stations; Red solid lines represent dividing lines of different sub-regions; Blue crosses represent in-situ 80 Researchgate

The mountainous islands of the Japanese Archipelago form a crescent off the eastern coast of Asia. The summits of mountain ridges uplifted near the outer edge of the continental shelf., they are separated from the mainland by the Sea of Japan, which historically served as a protective barrier. Japan's insular nature, together with the compactness of its main territory and the cultural homogeneity of its people, enabled the nation to remain free of outside domination until its defeat in World War II. [Source: Library of Congress, 1994 *]

Japan consists of four principal islands: Hokkaido, Honshu, Shikoku, and Kyushu; more than 3,000 adjacent islands and islets. Of theese four are significant: Tsushima, located in the straits between Korea and Japan; Amami Ōshima in the Nampo chain, situated in the northern Ryukyu Islands at the southern end of the Japanese archipelago; Sado Island, located in the Sea of Japan off the coast of central Honshu; and Awaji Island, positioned between Shikoku and Honshu. Other islands of note off of Kyushu and southern Honshu are Gotō, Oki, and Amakusa Islands.

The Ryukyu Archipelago includes over two hundred islands and islets, of which fewer than half are populated. They extend in a chain from southeast of Kyūshū to within 193 kilometers (120 miles) of Taiwan. Okinawa (1,256 square kilometers, 485 square miles) is the largest and most populated of the Ryukyu Islands. The Amami, Okinawa, and Sakishima chains make up the Ryukyu Islands.

The four major islands are separated by narrow straits and form a natural entity. The Ryukyu Islands curve 970 kilometers (630 miles) southward from Kyushu. Two groups of islands were returned to Japan by the United States in 1968. They are located around 1,300 kilometres (800 miles) east of the Ryukyu Islands: the Ogasawara (Bonin) Islands, which are situated around 885 kilometres (550 miles) south of Tokyo; and Iwo Jima the Kazan (Volcano) Islands, which lie directly south of the Ogasawara group and and around 1,100 kilometers (683 miles) from the main islands. [Source: Worldmark Encyclopedia of Nations, Thomson Gale, 2007]

Japan's Maritime Claims

Japan's island's make up less than 15 percent of Japan’s total claimed territory. Most of its territory is in the sea. In addition to the 200 nautical miles (370 kilometers) of ocean that Japan can also claims around the main islands according to the 1982 United Nations Convention of the Law of the Sea it can also claim 200 nautical miles around far flung Japanese islands that extend from near Taiwan and China to far out in the Pacific Ocean. Japan's total claimed area is approximately 4,850,000 square kilometers (or about 1.87 million square miles) if its territorial waters and its Exclusive Economic Zone (EEZ) are included. This about half the size of the United States including Alaska.

Maritime claims: territorial sea: 12 nautical miles (22.2 kilometers; 13.8 miles); between 3 nautical miles (5.6 kilometers; 3.5 miles) and 12 nautical miles (22.2 kilometers; 13.8 miles) in the international straits — La Perouse or Soya, Tsugaru, Osumi, and the Korea and Tsushima Straits; contiguous zone: 24 nautical miles (44.4 kilometers; 27.6 miles); exclusive economic zone: 200 nautical miles (370 kilometers; 230 miles).

A territorial dispute with the Soviet Union, dating from the end of World War II, over the two southernmost of the Kuril Islands, Etorofu and Kunashiri, and the smaller Shikotan and Habomai Islands northeast of Hokkaido remained a sensitive spot in Japanese-Russian relations. The Japanese government maintains that these islands belong to Japan and should be returned to Japanese administration. These islands and the Kuril Islands are occupied by Russia, whose claims are not formally recognised by Japan.

Japan’s Island Arcs

Japan's maritime claims, Japan Coast Guard

Japan is an archipelago that forms an arc in the Pacific Ocean to the east of the Asian continent. The land comprises four large islands named (in decreasing order of size) Honshu, Hokkaido, Kyushu, and Shikoku, together with many smaller islands. The Pacific Ocean lies to the east while the Sea of Japan and the East China Sea separate Japan from the Asian continent.

The four main islands account for 90 percent of Japan's land area. The four major islands of Japan are: 87,800-square-mile Honshu, or the "mainland," where Tokyo, Osaka and Kyoto are found; 30,150-square-mile Hokkaido, the northernmost of the large islands; 14,100-square-mile Kyushu, the volcanically-active southernmost large island; and 7,050-square-mile Shikoku, a rustic island on the Inland Sea between Honshu and Kyushu.

Japan is comprised of island arcs. An island arc is a string of volcanic islands in a curving arc-like pattern above a deep ocean trench. It is formed by magmatic activity after one oceanic tectonic plate subducts another one. Island arcs serve as nests for earthquakes and volcanoes. They are scattered throughout Southeast Asia, the Pacific Rim and the Caribbean Sea. In Japan, four island arcs converge on an area extending from Hokkaido through Honshu to Kyushu, to create a complex formation that has rarely occurred throughout the Earth's history. This formation makes the country always under threat from disasters. [Source: Yomiuri Shimbun, July 10, 2012]

Island arcs are vulnerable to tsunami because their land extends over a small area at relatively low altitudes. They often experience heavy rain and tropical cyclones as many are sandwiched between a continent and the high seas. However, Japan's island arcs come with unique benefits. Rich river resources have been created by rapid currents that are caused by a radical difference in height between mountainous and low-lying areas. Magma has produced a leading geothermal area blessed with undersea deposits of precious metals and rare earths raised from deep within the seabed. There are also deposits of an icy natural gas substance called methane hydrate, which was created by high-pressure low-temperature conditions.

The U.N. Commission on the Limits of the Continental Shelf upheld in April some of Japan's claims on an extended continental shelf in seabed areas around the country. The elevated seabed from the Shikoku basin and the maritime area to the south of Okinotorishima, the southernmost island of Japan--about 1,700 kilometers from Tokyo--is a remnant of the island arc that was active in the past. Due to the concentration of island arcs, Japan could obtain a continental shelf equivalent to 80 percent of its national land area.

Coastline and Coastal Features of Japan

Japan has 29,751 kilometers (18,486 miles) of coast. Much of Japan’s outer coastline consists of alternating stretches of rock shore and beach , with protected bays, marshes and mud flats here and there. Around cities and towns the coastal areas are highly developed. There are not many nice beaches near the cities. The nicest ones in remote places. Many places have been reclaimed from the sea.

Japan’s coastline is quite varied. In some places, such as Kujukurihama in Chiba Prefecture, there are long sandy beaches continuing fairly straight and uninterrrupted for 60 kilometers or so, while the coast of Nagasaki Prefecture is an example of an area characterized by peninsulas and inlets and offshore islands (like the Goto archipelago and the islands of Tsushima and Iki, which are part of that prefecture). There are also accidented areas of the coast with many inlets and steep cliffs caused by the submersion of part of the former coastline due to changes in the Earth’s crust. [Source: Web-Japan, Ministry of Foreign Affairs, Japan]

Japan's coastline has been extensively modified through land reclamation, port development, and the construction of seawalls and other coastal infrastructure. In several major urban bays, including those around Tokyo and Osaka, land subsidence has occurred as a result of groundwater extraction and urban development. In some areas, buildings have sunk by as much as 4.5 centimeters (1.8 inches) per year, while parts of the Osaka port area have subsided by approximately 3 meters (10 feet) since 1935. Rising sea levels associated with climate change pose an additional challenge for Japan's low-lying coastal areas and beaches. [Source: Junior Worldmark Encyclopedia of Physical Geography, Gale Group, Inc., 2003]

The coastline of Hokkaido is roughly diamond-shaped, marked by Cape Sōya in the north, the Shiretoko Peninsula in the east, Cape Erimo in the south, and Cape Kamui in the west. The Oshima Peninsula extends from southwestern Hokkaido, enclosing Uchiura Bay and ending at the capes of Shiragami and Esan. Honshu's Pacific coast is highly indented, featuring major peninsulas such as Bōsō, Izu, and Kii, along with large bays including Ishinomaki Bay, Tokyo Bay, and Ise Bay. Broad coastal plains occur at the heads of many bays where Japan's largest cities are located. North of Tokyo Bay, low-lying areas known as suigo ("land of water") lie at or near sea level and are protected by levees, locks, and pumping systems. In contrast, the Sea of Japan coast of Honshu is generally straighter, with notable exceptions including the Noto Peninsula and Wakasa Bay.

Shikoku has a compact, elongated shape, bordered by the Seto Inland Sea to the north and Tosa Bay to the south. Kyushu's southern and western coastlines are more irregular, with numerous bays, peninsulas, and inlets. Kagoshima Bay, formed by volcanic activity, is one of the island's most prominent coastal features. Overall, Japan's coastline displays considerable regional variation, ranging from broad plains and straight shorelines to deeply indented coasts shaped by tectonic and volcanic processes.

Japan Comprised of 14,125 Islands Rather Than 6,852

In 2023, Japan's Geospatial Information Authority (GSI) announced that the country contains 14,125 islands, more than double the previously accepted figure of 6,852 islands recorded in 1987. The revised count was made possible through the use of modern digital mapping technology, aerial photography, and improved geographic analysis. The earlier survey, conducted by the Japan Coast Guard, relied on paper maps and was unable to distinguish many small islands from larger island groups. [Source: Katherine McLaughlin, Architectural Digest, April 5, 2023; Jasmine Osby, Travel Noire, June 29, 2023; Nikki Main, Gizmodo, March 3, 2023]

The increase in the number of recognized islands does not mean that thousands of new islands suddenly appeared. Rather, many small landmasses that had previously been grouped together were identified as separate islands. The survey used the same basic definition as the 1987 count, including only naturally formed landmasses with a circumference of at least 100 meters (330 feet), while excluding artificially reclaimed land.

Volcanic activity has also contributed to changes in Japan's coastline and the formation of some new islands. Japan lies in a geologically active region where submarine volcanic eruptions can create new landmasses or divide existing islands. However, many volcanic islands are temporary features that may erode or disappear over time due to wave action and weathering.

The revised island count does not alter Japan's territorial boundaries or territorial waters. Nevertheless, accurate mapping is important because islands help define maritime zones under international law, including exclusive economic zones and other coastal rights. Of Japan's more than 14,000 islands, only about 400 are inhabited, with most remaining undeveloped because of their small size, mountainous terrain, exposure to severe weather, or unstable volcanic origins.

The discovery reflects advances in geographic surveying rather than a major expansion of Japan's territory. Similar changes in island numbers have been observed elsewhere in the world as improved satellite imagery and mapping technologies allow scientists to identify and classify landforms with greater precision. As volcanic activity and coastal processes continue to reshape shorelines, the number and shape of islands in Japan and other regions may continue to change over time.

Japan’s Southernmost Island

The southernmost point of Japan is a heliport on Okinotori Island — about 1,700 kilometers south of Tokyo. The landing pad sits on a 50-meter-wide foundation intended for an unbuilt lighthouse planned built before World War II. Japan claims the waters around the island — an area of 400,000 square kilometers, which is the total land area of Japan.

Okinotori island embraces a coral reef that has a circumference of 11 kilometers during low tide. At high tide just two peaks — including the one with the heliport’sit above the ocean surface. Located in a strategic spot between Taiwan and Guam, it requires regular “maintenance” to keep it from submerging. China claims that Okinotori is a rock not an island and Japan has no right to claim it or the waters around it. It opposes the “maintenance” efforts.

There has been some discussion of employing new technology that creates sand from substances in seawater to help protect Okinotori from erosion. The technology — called the electrodeposition method — uses electrodes with negative charges that attract calcium and magnesium ions and they in turn form sandlike chemical compounds and they in turn attract coral

Japan's Nationalization of Remote Islands

Beginning in the 2000s and continuing through the 2010s, the Japanese government undertook a series of measures to strengthen administrative control over remote islands that serve as base points for defining the country's territorial waters and exclusive economic zone (EEZ). These islands are important because, under international law, they help determine the extent of Japan's maritime jurisdiction and rights to marine resources. [Source: Yomiuri Shimbun, April 7, 2017; Ankit Panda, The Diplomat, January 9, 2014; Yomiuri Shimbun, March 8, 2012]

In August 2012, the Japanese government placed 23 remote islands under state ownership after finishing necessary legal procedures in which the islands were placef under the jurisdiction of the village of Ogasawara, Tokyo; Ishigaki, Okinawa Prefecture; and Tsushima, Nagasaki Prefecture. According to the Yomiuri Shimbun: “Under the Civil Code, unowned land is vested in state coffers. The government has confirmed that the isles had no owners after checking property registers. The legal steps were taken based on the National Property Law. In a related move, the government announced a list of 39 newly named islands. These islands had been previously unnamed despite being used as reference points to draw Japan's EEZs.

In 2014, the government announced plans to formally register about 280 uninhabited islands as state property. Many of these islands were located within Japan's exclusive economic zone and had previously lacked clear administrative ownership. The objective was to improve management of the islands and ensure accurate records of their legal status. The policy formed part of a broader effort to monitor and preserve remote territories throughout the Japanese archipelago.

The initiative followed government surveys that found many remote islands had unclear ownership, inconsistent mapping records, or no officially recognized names. To address these issues, government agencies conducted detailed investigations of approximately 400 islands. The surveys examined ownership records, geographic characteristics, and the role of the islands in establishing maritime boundaries. Unnamed islands were given official names, and islands without clear ownership were considered for nationalization.

In April 2017, Japan's Headquarters for Ocean Policy announced that 273 uninhabited remote islands had been registered as national property. The government stated that the move was intended to clarify property rights and improve the management of territorial waters and exclusive economic zones. During the same period, Japan also adopted policies to support residents living on strategically important border islands and to strengthen infrastructure and public services in these remote communities.

The preservation of remote islands has become increasingly important because Japan's territorial waters and EEZ cover an area much larger than its land territory. The islands provide the legal basis for maritime claims, fisheries management, resource exploration, and environmental protection. Government officials have also emphasized the role of remote islands in maritime surveillance, security monitoring, and disaster preparedness.

Japan's island policies have attracted international attention because of territorial disputes in surrounding seas, particularly in the East China Sea. Although most of the islands nationalized by the government are not disputed territories, the measures have been viewed as part of Japan's broader effort to reinforce administrative control over its maritime domain. The government has maintained that the policies are administrative and legal measures designed to preserve national territory and ensure effective management of its coastal and island regions.

Image Sources: Wikimedia Commons, CIA map, USGS

Text Sources: Japan National Tourist Organization (JNTO), CIA World Factbook], Library of Congress, Daily Yomiuri, National Geographic, New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Encyclopedia.com, Live Science, The Conversation, The New Yorker, Time, BBC, CNN, Reuters, Associated Press, AFP, Lonely Planet Guides, Google AI, Wikipedia, The Guardian and various websites, books and other publications.

Last updated June 2026