National Research Institute for Earth Science and Disaster Prevention, Japan

We performed an amplitude variation with offset (AVO) analysis using 2D multichannel seismic reflection data across the Nankai Trough to determine the physical properties along the plate boundary decollement. With angle gather data obtained after a series of preprocessing, including prestack time migration, we calculated two major AVO attributes for the decollement the range from the deformation front to 45 km landward: zero-offset P-wave reflection amplitude (P) and gradient of amplitude versus sin (super 2) theta (G). Attribute P corresponding to an approximation of the reflection coefficient for normal incidence, depends on P-wave acoustic impedance contrast above and below the decollement. Attribute G mostly depends on the Poisson's ratio contrast above and below the decollement. Combining variations of these attributes (P and G) along the decollement and structural features of the accretionary wedge, the Nankai Trough decollement zone can be divided into three different physical property zones landward from the deformation front: large physical property change (LPC) zone, small physical property change zone (SPC), and moderate physical property change (MPC) zone. For these three zones, both absolute values of P and G decrease by stages landward along the decollement, suggesting landward decreases of both contrasts of P-wave acoustic impedance and Poisson's ratio above and below the decollement. A significant, landward decrease of these AVO attributes may be related to interplate coupling process during the interseismic period.