To predict the stress state as well as pore pressure in the Nankai accretionary prism southwest Japan, we estimated the ratio of P-wave velocity over S-wave velocity (Vp/Vs) and shear-wave splitting using multi-component ocean bottom seismometers (OBS) data. Because it is difficult to identify PS-converted reflection waveforms for each of the geological boundaries in the deep offshore region, we focused on the more easily identified PPS-refracted waveforms. We estimated the average Vp/Vs within the sedimentary section by using the time lag between P-refracted waves and PPSconverted waves. The estimated Vp/Vs changed abruptly at the trough axis, mainly because of compaction associated with the accretion process. Vp/Vs gradually increased landward from the trough axis to the mega-splay fault. The increase in Vp/Vs might indicate abnormal pore pressure below the mega-splay fault. To estimate the stress-induced fracture orientation, we computed the fast polarization direction and principal amplitude direction from PPSconverted waves. The anisotropic characteristics change at the mega-splay fault: the fast polarization direction and principal amplitude direction are parallel to the trough (transverse) landward of the mega-splay fault. Furthermore, we observe predominant velocity anisotropy around the mega-splay fault. These observations suggest that both the preferred fracture orientation and the principal stress orientation are oblique to the direction of subduction near the mega-splay fault.
The Nankai Trough is a convergent plate margin where the Philippine Sea plate is subducting beneath southwestern Japan (Figure 1). This subduction zone has repeatedly generated great earthquakes in excess of Mw 8 (Ando 1975). Great earthquakes at convergent plate margins have been interpreted to occur along the subduction interface as well as mega-splay faults, and there have been many seismic studies of these seismogenic faults (e.g. Moore et al. 2009; Nakanishi et al. 2008). Because pore pressure near a fault influences the fault activities, several seismic studies estimated pore pressure as well as effective stress around the seismogenic faults (e.g., Tobin and Saffer 2009; Tsuji et al. 2008). Shear-wave velocity (Vs) is an important parameter in the determination of subsurface properties, including pore pressure and stress orientation (Takahashi et al., 2002). Dvorkin et al. (1999) demonstrated that Vp/Vs (Poisson’s ratio) is strongly dependent on pore pressure. However, Vs has not been accurately estimated around seismogenic faults in the Nankai Trough off Kii Peninsula because the characteristics of the deep offshore geology in this region make it difficult. Here we estimated the Vp/Vs in sediment in the Nankai accretionary prism by using multi-component OBS data acquired from the trench to the seismogenic zone. A recent drilling campaign of the Integrated Ocean Drilling Program (IODP) used borehole breakouts and core sample observations to show that the stress state (principal horizontal stress orientation) changes across the seismogenic megasplay fault (e.g., Kinoshita et al. 2008). Maximum horizontal stress is parallel to the direction of plate subduction seaward of the mega-splay fault, but perpendicular to the direction of subduction landward of the mega-splay fault. Here, we reveal further variations of stress state across the Nankai accretionary prism by using seismic anisotropy.
