Abstract
We evaluated the influence of the trench-parallel component of plate motion on the active fault system within the Nankai accretionary wedge from reflection seismic profiles, high-resolution seafloor bathymetry, and deep-towed sub-bottom profiles. Our study demonstrated that a large portion of the trench-parallel component of oblique plate subduction is released by strike-slip motion along a fault located just landward of and merging down-dip with a mega-splay fault. The shallow portion of the splay fault system, forming a flower structure, seems to accommodate dominant strike-slip motion, while most of the dip-slip motion could propagate to the trenchward décollement. Numerous fractures developed around the strike-slip fault release overpressured pore fluid trapped beneath the mega-splay fault. The well-developed fractures could be related to the change in stress orientation within the accretionary wedge. Therefore, the strike-slip fault located at the boundary between the inner and outer wedges is a key structure controlling the stress state (including pore pressure) within the accretionary prism. In addition, the strike-slip motion contributes to enhancing the continuous mega-splay fault system (outer ridge), which extends for approximately 200 km parallel to the Nankai Trough.
Highlights
At the Nankai Trough, the Philippine Sea Plate subducts beneath the Eurasian Plate at a convergence rate of approximately 4.1 to 6.5 cm/year and at an azimuth of 300° to 315°
From the seafloor bathymetry around Shionomisaki Canyon, we identified an offset approximately 6 km long of the western sidewall of the canyon, which is likely to have been produced by rightlateral motion along the wedge boundary strike-slip fault’ (WBSF)
We characterized a strike-slip fault associated with oblique subduction from reflection seismic profiles, seafloor bathymetry, and deep-towed sub-bottom profiles in the Nankai Trough off the Kii Peninsula
Summary
At the Nankai Trough, the Philippine Sea Plate subducts beneath the Eurasian Plate at a convergence rate of approximately 4.1 to 6.5 cm/year and at an azimuth of 300° to 315° (as indicated by the black arrows in Figure 1; Seno et al 1993; Ito et al 1999; Miyazaki and Heki 2001). Because the trench-normal direction at the Nankai Trough off Kii Peninsula is approximately 330° to 345°, the convergence vector between these two plates deviates approximately 15° to 45° counterclockwise from the direction normal to the trough axis. This subduction zone has repeatedly generated earthquakes in excess of M8 (Ando 1975). From the coseismic region estimated from seismic and tsunami inversions (e.g., Tanioka and Satake 2001; Kikuchi et al 2003), the mega-splay fault off of Kumano, which branches upward from the plateboundary interface, may have experienced rupture during the 1944 Tonankai earthquake (Figure 2; Park et al 2002). The function of the transition zone between the inner and outer wedges in the subduction system should be reconsidered
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