Variation in Sediment Thickness, Subducting Basement Topography, and Frontal Prism Structure Along the Nankai Trough

Large, tsunamigenic earthquakes repeatedly occur at the Nankai Trough, off the southeastern coast of Japan where the Philippine Plate is subducting to the northwest under the Eurasian Plate. It has been hypothesized that variations in sediment thickness and type on the Philippine Plate can affect the structure of the upper plate at the deformation front and may additionally control frictional properties along the plate boundary thrust. Recent seismic surveys by JAMSTEC have provided an expanded data set that allows us to refine previous maps of sediment cover on the northern Philippine Sea plate. Seismic lines taken parallel to the accretionary prism show that the total sediment thickness increases from the southwest to the northeast and that underlying basement topography variations have produced remarkable local variations in sediment thickness from <500 m over basement ridges to >5500 m in adjacent troughs. The total thickness of sediment being accreted at the frontal thrust and thickness of sediment subducting beyond the frontal thrust show similar high variability.