12. Imaging/Interpretation Frontiers
Pore Pressure Prediction Near the Plate Boundary Fault in the Nankai Trough, Southwest Japan
Takeshi Tsuji(1), Hidekazu Tokuyama(1), Patrizia C. Pisani(2) and Gregory F. Moore(3)
(1) Ocean Research Institute, the University of Tokyo, JAPAN. (2) University of Hawaii, USA. (3) CDEX, JAMSTEC, JAPAN.
Abstract
We predict pore pressure distribution within the Nankai Trough accretionary prism via a theoretical based approach. The Nankai Trough is the plate boundary where the Philippine Sea plate is subducting beneath southwestern Japan. This subduction zone has repeatedly generated great earthquakes with magnitudes in excess of M 8. The d?collement (a detachment that separates a deformed accretionary prism from the underthrust sediments) in the Nankai Trough off the Muroto peninsula is developed within a hemipelagic mudstone sequence and it is well defined by a prominent reverse-polarity reflector on seismic profiles. From this polarity reversal, the d?collement is inferred to be a low acoustic impedance zone. The low acoustic impedance at the d?collement can be explained by high porosity sustained by high pore fluid pressure. Since the pore pressure along the d?collement influences the frictional characteristics, it plays a key role in the earthquake mechanism and deformation features of the accretionary prism. Many studies of pore pressure prediction rely on empirical relations. Here, we present a method for determining the pore pressure based on rock physics theory from seismic interval velocities and well data (logging data and discrete sample measurement data). In our method, we introduce the aspect ratio distribution of pore space calibrated by well data. From the aspect ratio distribution, we calculate theoretical velocity parameterized by effective pressure via Kuster-Toksoz model and compare it with the seismic P-wave velocity derived from 3-D reflection tomography. By iteratively fitting the theoretically calculated velocity to the seismic interval velocity, we estimate in situ effective pressure. The results demonstrate the abnormal high pore pressure within the subducting sedimentary sequence.
Last modified: Fri May 05 00:46:28 2006