Geological and seafloor investigations of the cause of partial submergence of Yokoshima Island, Nagasaki, Japan

Abstract

Abstract Between 1950 and 1980, about half of Yokoshima Island disappeared beneath the East China Sea. The island lies about 1\u202fkm off the coast of Nagasaki and was mined for coal during the late 19th and early 20th centuries. We analyzed the geology of the island and surrounding seafloor to investigate how it became submerged. Two low-cost techniques were used to view underwater structures: a commercial drone fitted with a video camera in waters shallower than 5\u202fm and an echo sounder and side-scan sonar system designed for amateur fishers in waters deeper than 5\u202fm. We showed that the partial submergence of the island could be interpreted as a result of subsidence. Subsidence and northward tilting of sedimentary rocks north of the island, probably caused by collapse of abandoned subsea mine workings, created an extensional regime south of the island that led to the formation of steep cliffs and collapse of the southern side of the island. Our study also shows that low-cost commercially available technology can be used for rapid scientific investigations in shallow water areas.