[S-CG52] Inter-segment Tectonics: Interdisciplinary Research on Responses to Plate Subduction

[SCG52-P03] Pore pressure estimation in Nankai Trough using full waveform inversion of ocean-bottom seismometer data

*Paul Caesar Mason Flores^1,2, Shuichi Kodaira¹, Kazuya Shiraishi¹, Gou Fujie¹, Ryuta Arai¹, Yasuyuki Nakamura¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.Yokohama National University)

Keywords:seamount subduction, sediment underplating, pore pressure, full wave inversion

Pore pressure is an important factor that controls the strength and sliding stability of faults. Elevated pore pressure has been invoked to explain the occurrence of slow earthquakes and shallow earthquake rupture during megathrust events. In the Nankai Trough offshore Muroto, elevated pore pressures have already been reported by previous studies based on taper angle, P- and S-wave velocities, and drilling. However, the extent of the high pore pressure zone is not yet clearly defined. This study utilized a high-resolution seismic velocity model derived by full waveform inversion (FWI) of wide-angle ocean-bottom seismograph (OBS) data to determine the pore pressure ratio (λ*) in the underthrusted sediments of Nankai Trough. The λ* was calculated using empirical relationships between P-wave velocity, porosity, and effective mean stress. We observed an extensive zone of high λ* (0.5 – 0.8) from the frontal thrust up to ~65 km landward and up to a depth of ~10 km. Within this high λ* zone in the outer wedge, a patchy distribution of overpressured aquifers (λ* > 0.6) can also be observed which is consistent with previous drilling studies. Comparison with seismic reflection images also show that the high λ* zone in the inner wedge coincides well with a region of strong reflections. This region is interpreted to be comprised of trench fill sediments that were underplated due to seamount subduction. The high λ* (>0.6) in the underplated sediments is interpreted to be caused by tectonic compression from younger subducted seamounts.