Vp/Vs and shear-wave splitting at the seismogenic plate subduction zone: Insight into effective-stress and pore pressure distribution

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 PPS-converted 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 PPS-converted 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.