Detailed Bathymetric Features of Tokara and Kerama Gaps in the Ryukyu Arc

Abstract

Precise topographic features at the two major structural boundaries in the Ryukyu Arc, Tokara Gap and Kerama Gap, revealed that there exists a ridge feature of the Ryukyu Arc as a saddle topography. The feature supports the idea recently proposed that a landbridge connecting the Chinese continent, Ryukyu Islands and Japanese Islands, is thought to have been subsided since Late Pleistocene. The saddle is offset by large-scale left-lateral normal faults both of which are related to the opening of the Okinawa Trough. These topographic features and geological structure together with the gravity anomaly and regional tectonics in the Ryukyu area suggest that a north-south tensional stress regime, due to the rolling buck of the Philippine Sea Plate at the Ryukyu Trench, has dominated in this region since last 20, 000 years.