THE INFLUENCE OF SUBDUCTING SEAMOUNTS: VARIATIONS IN ACCRETIONARY PRISM DEFORMATION STYLE ALONG THE NANKAI TROUGH FROM COMBINED OCEAN-BOTTOM SEISMOMETER AND MULTICHANNEL SEISMIC DATA

In this study we characterize the sediment dominated accretionary prism of the Tokai segment (TS) of the Nankai Trough (NT), offshore eastern Japan. The TS is a structurally complex region affected by the Izu-Bonin arc, which bounds it to the east. It last ruptured seismically in 1854 and is thus thought to represent an area of high seismic hazard. The Izu-Bonin arc has a series of W-NW striking sub-parallel volcanic ridges on its western flank that are oblique to the main arc. The largest and northernmost of these is called the Zenisu Ridge, which has been over-printed by southward verging intraplate thrusting that accommodates a portion of the plate convergence. Several authors have suggested that the TS is differentiated from the rest of the NT by the cyclic subduction of these volcanic ridges, and that this difference may affect the seismogenic behavior of the megathrust at this subduction zone.

We carry out a full waveform inversion of ocean-bottom seismometer data from the TS and use the resulting P-wave velocity model to carry out a pre-stack depth migration of co-located multi-channel seismic data to produce complementary images at unprecedented resolution. This combination of cutting-edge geophysical imaging and modeling methods greatly enhances our capacity to make a detailed geologic interpretation in complex structural settings.

By comparisons with previous studies done in other segments of the Nankai Trough, we show that though the overall structure of the wedge in the TS is similar to that of the rest of the NT there are some important differences. The TS is characterized by a narrower and steeper accretionary prism than the rest of the NT and by a complex bathymetry that contrasts with the smooth bathymetry of the Kumano basin to the west. The outer wedge is characterized by imbricate fans with wedge-top basins that show several internal unconformities, suggesting protracted deformation, while the northern end of the deformed Kumano basin is characterized by a large, apparently active, normal fault. In contrast with the TS, the structural style in the outer part of the accretionary prism in the NT is dominated by a forward breaking thrust system. These results will allow us to place further constraints on the role of subducted bathymetry on the properties of the seismogenic megathrust.