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How did back-arc spreading form the Sea of Japan?

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World map highlighting major features of the ocean basins, including trenches, ridges, basins, and plateaus labeled throughout the Pacific, Atlantic, Indian, Southern, and Arctic Oceans. Continents are shown with shaded terrain, and prominent undersea features such as the Mid-Atlantic Ridge, Mariana Trench, and Kerguelen Plateau are marked across the map. — ocean basin Major features of the ocean basins.

The Sea of Japan formed as a back-arc basin through the process of back-arc spreading, which occurred when Japan moved away from the Asian mainland during the Middle Miocene epoch.1

Back-arc spreading is driven by the subduction of one tectonic plate beneath another. As the subducting slab sinks into the asthenosphere, it may "roll back" oceanward, creating extensional forces in the overlying plate. This extension causes the crust behind the island arc to thin and rift. As the crust stretches, decompression of the underlying mantle triggers melting and the formation of magma. This magma rises to the surface, initiating seafloor-spreading processes that widen the basin and push the island arc away from the continent.23

In the case of the Sea of Japan, this tectonic activity resulted in the opening of the basin behind the Japanese archipelago. While many such basins continue to expand, the Sea of Japan is unique among young marginal basins as it may have begun to close again.1