Pore pressure prediction near the plate boundary fault in the Nankai Trough, southwest Japan: Insight from seismic interval velocity and well data

We developed a method for predicting the pore pressure based on rock physics theory. From crack aspect ratio distribution calibrated by well data, we calculate theoretical velocities parameterized by effective pressure via Kuster-Toksoz model and compare them with the seismic interval velocity derived from three-dimensional (3-D) reflection tomography. By iteratively fitting the theoretically calculated velocity to the seismic interval velocity by changing effective pressure, we estimate two-dimensional (2-D) in situ effective pressure. Then, 2-D pore pressure distribution can be obtained by subtracting the effective pressure from the confining pressure. This method is applied to the plate boundary decollement (a detachment that separates a deformed accretionary prism from the underthrust sediments) in the Nankai Trough off the Muroto peninsula, southwest Japan. This subduction zone has repeatedly generated great earthquakes with magnitudes in excess of M 8. Since the pore pressure along the decollement influences the frictional characteristics, it plays a key role in the earthquake mechanism and deformation features of the accretionary prism. Our results demonstrate that abnormal high pore pressure within the subducting sedimentary sequence. The pore pressure gradually increases from the deformation front to the landward region, and it is consistent with structural features.