oa Tectonic Significance of Intraoceanic Faults in the Nankai Trough: Implications for Inter- and Intra-Plate Earthquakes

Seismic reflection studies have been intensively carried out in the Nankai Trough region. However, the role of oceanic crust was not well understood in the plate convergent margin. Recently, Tsuji et al. [2009] identified intraoceanic faults developed as imbricate structures within the subducting Philippine Sea plate off the Kii Peninsula in central Japan manifesting as strong-amplitude reflections observed in an industry-standard 3D seismic reflection data set. Here we use several 2D and 3D seismic reflection data acquired in the whole Nankai Trough region in order to discuss characteristics of intraoceanic faults distributed in the Nankai Trough region. Seismic profiles demonstrate that intraoceanic faults are densely distributed in the Nankai Trough east of the Cape Shionomisaki. Large displacements of a major intraoceanic faults elevate the crust surface, and the offset due to cumulative displacements reaches >1 km. These imbricate intraoceanic faults cut through the oceanic crust as a discontinuous thrust plane. The intraoceanic faults strike nearly parallel to the trend of the trough axis. However the fault traces are bending at the western termination; the fault planes extend upward from side edges of the underlying intraoceanic faults and work as lateral faults. The deformation along the intraoceanic faults may have continued until recently because the shallow sediment as well as the seafloor is deformed due to the fault displacement. Furthermore, the locations of the intraoceanic faults recognized in the seismic data are distributed around the estimated hypocenters of the mainshocks and aftershocks of the 2004 intraplate earthquakes (Mw >7), and their geometry extracted from the 3D seismic data could explain the kind of complex rupture pattern observed during the 2004 events. These observations demonstrate that the intraoceanic faults should be seismogenically active. Furthermore the segmentation of interpolate earthquake off the Cape Shionomisaki is consistent with the ridge originated by the displacement of intraoceanic fault. Because the displacement along the intraoceanic fault is developed with subduction and cuts the plate boundary faults due to their dynamic displacements, there is a possibility that the intraoceanic faults control the interplate earthquake segmentation.