To understand characteristics of deep seismogenic faults in the plate convergent margins, we used P- and S-wave velocities (Vp and Vs) of the digital rocks extracted from core samples of ancient plate boundary fault at Nobeoka in Kyushu, southwest Japan. We constructed 3D digital rock models from micro-CT images. The heterogeneous texture, such as cracks and veins were identified based on comparison of the density and porosity from digital rock model with the average of porosity of Nobeoka samples from laboratory measurements. By using Rotated Staggered Grid Finite-Difference Method, we performed dynamic wave propagation simulation and measured the effective Vp, Vs, and ratio of Vp and Vs (Vp/Vs) of 3D digital rock models including cracks and veins. In particular, we investigated the sensitivity of Vp and Vs to crack-filling materials. We replaced the cracks and veins with various elastic materials, such as dry, water, quartz, calcite, and other materials with different bulk and shear modulus. By comparing the elastic properties derived from digital rock with seismic velocity (e.g., Vp/Vs) in the Nankai seismogenic fault, we characterized the deep seismogenic fault. Our results demonstrated that the cracks filled with water significantly decreased seismic velocity and increased Vp/Vs. The Vp/Vs of quartz-filling case was lower than water saturated case. Therefore, high Vp/Vs and low Vp at the transition region from aseismic to seismic regimes in the Nankai Trough are caused by open cracks (or fractures), and low Vp/Vs at coseismic region could be explained by quartz-filling cracks.

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