Spreading of the Okinawa Trough and its Relation to the Crustal Deformation in Kyusyu

The repeated triangulation surveys have revealed the conspicuous N-S extentional strain field in the central Kyusyu. This strain field has been caused by the horizontal crustal movement that the triangulation stations in both the sides of the Beppu-Shimabara Graben have moved away each others to the N-S direction. The Beppu-Shimabara Graben strikes across the central Kyusyu to the E-W direction. The gravity anomaly, the seismological data and other geotectonic data support the idea that the Beppu-Shimabara Graben is a rift of the crust and the north-eastern terminal part of the Okinawa Trough which is the active back arc basin of the Ryukyu Arc and is spreading to the N-S direction. Therefore, it is concluded that the N-S extentional crustal deformation in the central Kyusyu has been caused by the rifting and the spreading of the Okinawa Trough. In other words, the Kyusyu is rifting and spreading to the N-S direction along the Beppu-Shimabara Graben. The spreading ratio is estimated to be about 15mm/a from the horizontal crustal deformation.