Shear-wave velocity and splitting within the Nankai accretionary prism off the Kii Peninsula: Insight into effective-stress and pore-pressure distribution

Abstract

To estimate stress state as well as pore pressure within the Nankai accretionary prism off the Kii peninsula, we estimated Vp/Vs and shear-wave splitting from wide-angle three-component ocean bottom seismometers (OBS) data. In this study, we focused on the PS-converted wave, which is a S-wave converted from P-wave at sediment-crust interface, recognized on the horizontal components. We estimated Vp/Vs in the sedimentary section below each OBS position from the travel-time difference between P-refracted wave and the PS-converted wave. The estimated Vp/Vs changes at the trough axis; Vp/Vs landward of the trough axis (∼1.8) is smaller than seaward of the trough axis (∼2.7) mainly due to a compaction associated with an accretion process. From the Vp/Vs, we can estimate vertical/horizontal effective-stress ratio (σv/σh) as 0.72 in the stratified sedimentary sequence seaward of the trough axis. The Vp/Vs gradually increase from the trough axis to the seismogenic mega-splay fault. The increase in Vp/Vs may represent thin-crack increasing due to abnormal pore pressure. To estimate principal stress orientation, furthermore, we calculate dominant amplitude direction of PS-converted waves from the particle motions. The results demonstrated that amplitude for line-perpendicular direction is strong landward of the megasplay fault maybe due to shear-wave splitting. The shear-wave splitting landward of the mega-splay fault may indicate that dominant fracture orientation is survey line-oblique direction. Furthermore, when we assumed the line-parallel horizontal component as SV-wave and the line-perpendicular component as SH-wave, we calculated a ratio of SV-velocity to SH-velocity (Vsv/Vsh) mainly from the time-rag between the SV- and SH-components. The trend of the estimated Vsv/Vsh is consistent with the shear-wave splitting.