Scale-dependent seismic velocity and permeability in subduction zones caused by mesoscale fractures

Numerous seismic surveys have been conducted to uncover the geophysical structure and fluid distribution in subduction zones, since seismic velocities are primarily influenced by pore fluids within rocks. To link seismic velocity and the pore fluid state, laboratory measurements of elastic wave velocity using rocks samples from subduction zone environments have been conducted, revealing the role of microscopic pores and cracks in controlling seismic velocity. However, natural geological systems are heterogeneous and contain defects of different scales at each scale of observation. Therefore, the velocities observed at in situ scales should be affected not only by microscopic pore structures but also by larger-scale defects, such as fractures and faults. Such large-scale defects should also play a role in fluid drainage system, since permeability of rocks depends strongly on the dimensions of conduits. In this study, we compare laboratory-measured ultrasonic velocity measured on core samples from the Susaki area in the Shimanto accretionary complex, SW Japan, with sonic velocity measured by borehole logging experiments. P-wave velocities were measured at a frequency of 1 MHz under dry conditions at 5 cm intervals along core sections spanning a total length of 128 m. The measured values were then converted to velocities under wet conditions using an effective medium model, enabling comparison with sonic velocities acquired under groundwater-saturated conditions. 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 an 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.