Extensive fluid overpressure in the shallow slow earthquake zone of Nankai Trough off Muroto mapped with high-resolution P-wave velocity

High pore pressure ratio (λ*) has been associated with the occurrence of slow earthquakes. Many studies have estimated the λ* using taper angle, P- and S-wave velocities, and drilling but the extent is not yet clearly defined. This study utilized a recently published high-resolution P-wave velocity model off Muroto derived from a two-step tomographic inversion of ocean-bottom seismograph data to determine the λ* using empirical relationships between velocity, porosity, and effective mean stress. We determined an extensive zone of high λ* (> 0.4) from the frontal thrust up to ~ 60 km landward and to a depth of 8 km with three characteristic observations. First, the underthrust sediments in the outer wedge show patches of overpressured aquifers where λ*>0.6, consistent with previous drilling results. Second, the high λ* (> 0.6) region in the inner wedge coincides with previously reported underplated sediments composed of fluid-rich trench-fill sediments dragged down by seamounts. The high λ* may be caused by tectonic compression from the newly subducted seamounts. Lastly, vertical columns of high λ* were observed in many of the thrust faults with some faults showing a negative polarity, which are interpreted to be evidence of fluid flow.