We used a theoretical method for predicting effective stress and pore pressure based on rock physics models. To predict the pore pressure around the seismogenic mega-splay fault in the Nankai Trough, we used elastic velocities of discrete samples in addition to the drilling data. First, we developed a theoretical relationship between elastic velocity and effective stress from the crack aspect ratio spectrum estimated from laboratory and well-log data, by using differential effective medium (DEM) theory. Then we estimated in situ effective stress within the accretionary prism by iteratively fitting the theoretically calculated velocities (i.e., elastic moduli) to the seismic velocities derived from waveform tomography. Pore pressure is then obtained as the difference between the effective stress and the confining stress. Using this method, we can use information of velocity-pressure relationship of discrete samples in the pore pressure prediction. Because elastic properties of the deep mega-splay fault are not available from the drilling data, we used properties of outcrop discrete samples acquired at ancient mega-splay fault. Our results suggest an abnormally high pore pressure zone at the footwall side of the mega-splay fault. The abnormal pore pressure zone extends from the mega-splay fault to the seaward accretionary prism. This indicates that the rupture may have propagated from the mega-splay fault to the seaward region, and may have caused large tsunami (close to the trough) as in the case of the 2011 Tohoku earthquake. We further identify ancient mega-splay faults at the landward side of the active mega-splay fault. From the fault system revealed in this study, we interpret that the mega-splay faults are sequentially developed.

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