Mesoscale fractures control the scale dependences of seismic velocity and fluid flow in subduction zones

Natural geological systems contain porosity structures of various scales that play different roles in geophysical properties, fluid flow, and geodynamics. To understand seismic activity associated with high pore-fluid pressure and fluid migration in subduction zones, it is necessary to explore the scale dependence of geophysical properties such as seismic velocity and permeability. Here, we compare laboratory-measured ultrasonic velocity (1 MHz) measured on core samples from the Susaki area in the Shimanto accretionary complex, SW Japan, with sonic velocity (15 kHz) measured by borehole logging experiments. Results show that P-wave velocity decreases from the laboratory (~6 km/s) to the borehole scales (~5 km/s). This scale-variant effect can be explained by a differential effective medium model whereby mesoscale porosity that is undetectable at the ultrasonic wavelength is introduced into the matrix phase with microscale porosity. Assuming typical apertures for micro- and mesoscale fractures, we estimate that the effective permeability can increase to 10–12–10–11 m2 with increasing in the mesoscale porosity and decreasing P-wave velocity down to 4–5 km/s. These results indicate that seismic velocity anomalies and related seismic activity are associated with the presence of mesoscale fractures in subduction zones.