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The southeastern Korean margin documents the processes of continental rifting and seafloor spreading that eventually led to the opening of the southern part of the East Sea (Japan Sea). Two‐dimensional crustal structure of the southeastern Korean margin was computed from ocean bottom seismometer data by tomographic inversion and iterative forward modeling. The crustal structure shows the emplacement of high‐velocity (>7 km/s) lower crust under the continental shelf and slope area associated with a rapid transition from rifted continental to oceanic crust. The high‐velocity lower crust is interpreted as magmatic underplating formed by voluminous igneous activity during rifting. Magnetic modeling confirms its primary correlation with a prominent magnetic anomaly along the edge of the Korean Peninsula. We suggest that the rifting and subsequent seafloor spreading at the Korean margin was significantly controlled by the supply of magma in a region of hotter than normal mantle temperature.

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Understanding the crustal structure of the Ulleung basin (Tsushima basin) in the southwestern Japan sea (East sea of Korea) is important for reconstructing the opening tectonics of the Japan sea. A Korea and Russia collaborative seismic experiment was carried out in 1991 to investigate the crustal structure of this basin using ocean bottom seismometers (OBSs) and large capacity air guns. The results of the experiment reveal that the Ulleung basin is floored by distinctive upper and lower crustal layers with velocities typical of layers 2 and 3. The lower crust, however, is unusually thick (7–8 km) and the Moho lies flat at 15–16 km depths. Though similar to the Yamato basin in crustal thickness, the Ulleung basin has a much thicker sedimentary sequence and a slightly thinner lower crust in the center. Geophysical evidence, thus, shows that the crust under the Ulleung basin has velocity range and crustal layering falling closer to those of normal oceanic crust than the Yamato basin. The formation mechanism of the thicker than normal lower crust should be clarified to better understand the complex opening process in the southern Japan sea.