Cooperative research among Japan, Russia and Korea in crustal structure in transient areas between land and ocean around Japan Sea

日本海位于亚洲大陆和日本列岛之间，是西北太平洋的弧后盆地。它包括北部和西北部的日本盆地，东南部的大和盆地和西南部的对马盆地。日本海由三个深盆地组成，它们被朝鲜高原、冲岐浅滩和大和浅滩等断裂的大陆遗迹分开。日本海的开放是由早渐新世日本盆地东北部的地壳减薄开始的，随后是晚渐新世的海底扩张。在日本海盆和后来的南日本海海盆的扩张过程中，地壳发生了伸展，形成了大和海盆和郁陵海海盆。晚中新世，日本列岛东部和西南部的俯冲带沿着板块运动和俯冲方式的变化，导致了持续至今的皮弧闭合和地壳缩短。为了弄清海陆过渡区的地壳结构，我们进行了海底地震仪（OBS）实验、磁力和重力测量以及单气枪测量。结果表明，该地区存在大洋和大陆混合结构，并认识到利用DTCM方法获取近海底磁异常信息的重要性，从而使我们能够顺利地对结果进行确认。

Japan Sea, which is a back-arc basin in the northwestern Pacific, is located between Asian continent and Japanese islands. It comprises the Japan Basin in the north and northwest, the Yamato Basin in the southeast, and the Tsushima Basin in the southwest. Three deep basins comprise the Japan Sea ; they are separated by rifted continental remnants such as the Korea Plateau, Oki Bank, and Yamato Bank. The opening of the Japan Sea was initiated by crustal thinning in the northeastern Japan Basin in Early Oligocene, followed by seafloor spreading in the Late Oligocene. During initial spreading of the Japan Basin, and later one of the southern Japan Sea there underwent crustal extension, forming the Yamato and Ulleung Basins. In the Late Miocene, changes in plate motions and subduction mode along the subduction zones, east and southwest of Japanese Islands, caused bark-arc closing and crustal shortening that has continued till the present. We have conducted the OBS (Ocean Bottom Seismometer) experiments, magnetic and gravity surveys, and single air-gun survey to make clear the crustal structure in the transient area between land and ocean. As the result, we got the mixed structure with oceanic and continental ones, and understood importance of magnetic anomaly information obtained at near bottom using DTCM which will lead us smoothly to confirm the result.