Elastic properties of rocks in subduction zones are indicators of stress states and play important roles in earthquake nucleation. We measured elastic properties of core samples taken from Susaki area in the Shimanto accretionary complex, SW Japan, which experienced the depth condition of the seismogenic zone. We found that the samples have higher P- and S-wave velocities of 5.4-5.8 km/s and lower porosities of 1-2.5 % than other sedimentary rocks in the Shimanto accretionary complex. This may be caused by tight contacts among grains due to pressure solution processes according to microstructural observations. Comparison with seismological observations in the present Nankai Trough indicated that the samples correspond to the hanging wall rocks of the megasplay fault where now a high velocity body is resolved. We further considered velocity-porosity relation using both the samples in this study and the drilled samples from the Nankai Trough, and proposed a model explaining from low to high porosities that is applicable to shallow to deep depths toward the seismogenic zone. The obtained elastic properties provided a critical nucleation length for an earthquake along the plate boundary fault of ~0.5 m. Material and structural heterogeneities below this length scale have the potential to modulate earthquake nucleation processes and future studies on this length scale will be necessary to bridge between laboratories and natural conditions.