[J06-4] The spectrum of fault-zone deformation processes (from slow slip to earthquake) IV

[J06-4-02] Evaluation of rock evolution process in seismogenic fault: Dynamic wave propagation modeling to the digitalized fault rocks

^○Chandoeun Eng¹, Tatsunori Ikeda², Takeshi Tsuji^1,2 (1.Earth Resources Engineering, Kyushu University, Fukuoka, Japan, 2.International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University, Fukuoka, Japan)

To understand the characteristics of the Nankai seismogenic fault in the plate convergent margins, we calculated the P- and S-wave velocities (Vp and Vs) of digital rock models constructed from core samples of an ancient plate boundary fault at Nobeoka in Kyushu, southwest Japan. We first constructed 3D digital rock models from micro-CT images and identified their heterogeneous features (cracks or veins). We replaced the cracks and veins with air, water, quartz, calcite, and other materials with different bulk and shear moduli. Using the Rotated Staggered Grid Finite-Difference Method, we performed dynamic wave propagation simulation and quantified the effective Vp, Vs, and the ratio of Vp to Vs (Vp/Vs) of the 3D digital rock models with different crack-filling minerals. Our results demonstrate that the water-saturated cracks considerably decreased the seismic velocity and increased Vp/Vs. The Vp/Vs of the quartz-filled rock model was lower than that in the water-saturated case and calcite-filled rock model. By comparing the elastic properties derived from the digital rock models with the seismic velocities (e.g., Vp and Vp/Vs) around the seismogenic fault estimated from field seismic data, we characterized the evolution of the deep seismogenic fault. The high Vp/Vs and low Vp observed at the transition region from the aseismic to seismic regimes in the Nankai Trough are caused by open cracks (or fractures) while the low Vp/Vs and high Vp at the coseismic region suggests quartz-filled cracks. The results indicate that a great earthquake could occur in the coseismic region partially due to slip velocity weakening of friction coefficient of quartz.

Key words: Digital rock, wave propagation simulation, fracture-filled materials, and earthquake fault