SEA OF JAPAN: GEOGRAPHY, GEOLOGY, NAME DISPUTES

SEA OF JAPAN

Sea of Japan (日本海) on a Japanese map

The Sea of Japan is a semi-enclosed marginal sea of the northwestern Pacific Ocean, bordered by Japan to the east, the Korean Peninsula to the west, and the Russia mainland and Sakhalin Island of Russia to the north. 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). Extending from north to south about 2,255 kilometers (1,401 miles, with a maximum width of about 1,070 kilometers (660 miles), the the Sea of Japan is bounded by the Russian mainland and Sakhalin island to the north, the Korean Peninsula to the west, and the Japanese islands of Hokkaido, Honshu and Kyushu to the east and south. It is connected to other seas by five straits: 1) the Strait of Tartary between the Asian mainland and Sakhalin; 2) La Pérouse Strait between the Sakhalin and Hokkaido; 3) the Tsugaru Strait between Hokkaido and Honshu; 4) the Kanmon Straits between Honshu and Kyushu; and 5) the Korea Strait between the Korean Peninsula and Kyushu.

There are no large islands in the sea. Most of the smaller ones are near the eastern coast. The shorelines are relatively straight and are lacking large bays or capes; the coastal shapes are simplest for Sakhalin and are more winding in the Japanese islands. The largest bays in Japan Ishikari (Hokkaido), Toyama (Honshu), and Wakasa (Honshu). Prominent capes include Soya, Nosappu, Tappi, Nyuda, Rebun, Rishiri, Okushiri, Daso and Oki in Japan. As world sea level dropped during the advance of the last Ice Age, the exit straits of the Sea of Japan one by one dried and closed. The deepest, and thus the last to close, is the western channel of the Korea Strait.

Geographical Features of the Japan Sea

map that recognizes both the Japanese name (Sea of Japan) and Korean name (East Sea) names

The Japan Sea is connected to surrounding seas through several narrow and relatively shallow straits, including the Korea Strait, Tsugaru Strait, La Perouse (Soya) Strait, and Tatar Strait. Tsushima Island divides the Korea Strait into the Western Channel and the Tsushima Strait. Because most of these passages are less than 100 meters (330 feet) deep, water exchange with neighboring seas is limited, giving the Japan Sea distinctive oceanographic characteristics. [Source: Wikipedia]

The sea contains three major basins: the Japan Basin in the north, the Yamato Basin in the southeast, and the Tsushima (Ulleung) Basin in the southwest. The Japan Basin is the deepest and is largely oceanic in origin, while the Tsushima Basin is the shallowest, with depths generally less than 2,300 meters (7,500 feet). Broad continental shelves occur along much of the Japanese coast, whereas the shelves along the Korean coast are generally narrow, averaging about 30 kilometers (19 miles) in width. In the northern part of the sea, a series of submerged shelf levels descends in steps toward the deeper central basin.

The seafloor is characterized by a combination of deep basins, ridges, plateaus, and sediment-covered plains. A prominent underwater ridge system extends north to south through the central part of the sea. Along the Japanese margin are several major submarine features, including the Okujiri Ridge, Sado Ridge, Hakusan Banks, Wakasa Ridge, Oki Ridge, and Yamato Ridge. The Yamato Ridge is largely continental in origin and consists of rocks such as granite, rhyolite, andesite, and basalt, while much of the deeper seafloor is oceanic crust covered by layers of marine sediments.

The coastline of the Japan Sea is generally regular in shape and lacks many large bays or deeply indented capes. Most islands are located near the Japanese coast, although Ulleungdo lies off the Korean Peninsula. Notable islands include Rishiri, Rebun, Okushiri, Sado, Oki, Moneron, and Ulleungdo. Important coastal features include Peter the Great Gulf and Posyet Bay in Russia, East Korea Bay in North Korea, and Ishikari, Toyama, and Wakasa bays in Japan, along with numerous prominent capes along the coasts of Russia, Japan, and Korea.

Geological History of the Japan Sea

The Japan Sea is a relatively young geological feature that formed as a result of tectonic activity along the eastern margin of the Eurasian continent. During the Neogene Period, movements of the Earth's crust caused the Japanese island arc to separate from mainland Asia, leading to the opening of a back-arc basin that eventually became the Japan Sea. As the basin expanded, oceanic crust developed in its deeper portions, particularly within the Japan Basin. This tectonic evolution created the complex arrangement of basins, ridges, and continental fragments that characterize the region today. [Source: Wikipedia]

The sea's present connections with surrounding waters developed through the formation of a series of straits during the late Neogene and Quaternary periods. Among the oldest are the Tsugaru and Tsushima straits, whose opening interrupted land connections that had previously allowed large mammals, including elephants, to migrate into the Japanese islands. The formation of these waterways contributed to the increasing geographic isolation of Japan from the Asian mainland. Over time, marine conditions became established as water flowed between the newly formed sea and adjacent oceans.

The youngest major passage is the La Perouse (Soya) Strait between Sakhalin and Hokkaido. Geological evidence indicates that it formed between about 60,000 and 11,000 years ago. Before the opening of this strait, land connections or narrower water barriers may have allowed mammoths and other cold-climate animals to move into northern Japan. The creation of the strait further altered regional migration routes and strengthened the separation of ecosystems around the sea.

During the last Ice Age, global sea levels fell significantly as large amounts of water were stored in continental ice sheets. As sea levels dropped, the shallow straits connecting the Japan Sea to neighboring waters gradually became restricted and may have closed entirely. The western channel of the Korea Strait, being the deepest outlet, would have been the last connection to remain open. Some researchers have suggested that the closure of these passages may have transformed the Japan Sea into a large, isolated inland water body during glacial periods, although the extent and duration of this isolation remain subjects of scientific debate.

Sea of Japan Coast

Sea of Japan Coast on the western side of Honshu is a region of historic cities, sand dunes, traditional fishing villages and restaurants that serve giant crabs in the winter time. The waves are bigger than those on the Inland Sea but not as big as those on the Pacific. In the winter the sea brings stormy weather and lots of snow to some places. Some coast villages erect bamboo sea walls for protection In the summer the seas are relatively calm and weather can be quite humid. Websites: Sea of Japan description Wikipedia Wikipedia ; Sea of Japan photos japan-photo

The Sea of Japan has historically served as a protective barrier between Japan and Asia. A long chain of mountains runs down the middle of Japan, dividing it into two halves, the "face," fronting on the Pacific Ocean, and the "back," toward the Sea of Japan. On the Sea of Japan side are plateaus and low mountain districts, with altitudes of 500 to 1,500 meters. The Echigo Plain borders the Sea of Japan. Most of Japan’s rivers are very short. The longest, the Shinano, which winds through Nagano Prefecture to Niigata Prefecture and flows into the Sea of Japan, is only 367 kilometers long Hokuriku, a "snow country" coastal strip on the Sea of Japan, is situated in northern central Japan. Japan’s fifth largest lake, Nakaumi and adjacent Shinjiko Lake, are on the Sea of Japan coast in central western Honshsu. They, are threatened by land reclamation projects.

The Sea of Japan coast is generally unindented, with few natural harbors. The central Noto Peninsula and Wakasa Bay serve as exceptions to long curves of flat shoreline. The Korean peninsula — across the Sea of Japan from Japan — is the closest point on the Asian mainland to Japan. 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 Soya 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.

Japanese life has always been oriented toward the ocean. The currents that converge offshore create fertile and varied fishing grounds. The Tsushima Current, an offshoot of the Japan Current, transports warm water northward into the Sea of Japan. Two major ocean currents affect this climatic pattern: the warm Kuroshio Current (Black Current; also known as the Japan Current); and the cold Oyashio Current (Parent Current; also known as the Okhotsk Current). The Kuroshio Current flows northward on the Pacific side of Japan and warms areas as far north as Tokyo; a small branch, the Tsushima Current, flows up the Sea of Japan side. The Oyashio Current, which abounds in plankton beneficial to coldwater fish, flows southward along the northern Pacific, cooling adjacent coastal areas. The meeting point of these currents at 36 north latitude is a bountiful fishing ground.

Dispute Between South Korea and Japan over Japan Sea (East Sea) Name

Koreans believe the name of the sea between Japan and Korea should be called the East Sea rather than the Japan Sea, the name that appears on most maps. In the early 1990s both the U.N.'s conference on Geographical Names and the U.S. Board of Geographic names rejected suggestions by Koreans to the change the name of the body of water between Korea and Japan from the Sea of Japan to the East Sea.

In August 2007, the ninth conference for the standardization of geographical names ruled that the Sea of Japan will remain the Sea of Japan. South Korea and North Korea wanted the name to be changed to the “East Sea” or the “Sea of Korea," names which they say have been used for more than 2,000 years. The chairman of the conference said, “I encourage the three countries involved to find a solution acceptable to all of them, taking into account any relevant solutions, or else agree to differ."

North Korea and South Korea joined together to contest the name and argued that the “East Sea” and “Sea of Japan” names to be used concurrently until a common designation could be worked out. Japan argues that the “Sea of Japan” name has long been established and widely recognized as the name of the sea since the late 18th century. In August 2011, the United States said it would continue calling the waters between Japan and Korea “the Sea of Japan” despite efforts by South Korea to recognize it was “the East Sea."

Japan and South Korea have been locked in a dispute over fishing in the fish-rich waters off the Kuril islands, north of Hokkaido, which are occupied by Russia and claimed by both Japan and Russia. Japanese claim the islands and the seas around them. The Russians currently occupy the islands and have given the Koreans permission to fish in the seas off of them. Japan told South Korea that if they continued fishing off the Kurile Islands they would be denied the right to fish in Japanese waters.

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