Study of the Nankai seismogenic fault using dynamic wave propagation modelling of digital rock from the Nobeoka Fault

Abstract

To understand the characteristics of the Nankai seismogenic fault in the plate convergent margin, we calculated the P- and S-wave velocities (V_P and V_S) of digital rock models constructed from core samples of an ancient plate boundary fault at Nobeoka, Kyushu Island, Japan. We first constructed 3D digital rock models from microcomputed tomography images and identified their heterogeneous textures such as cracks and 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 simulations and quantified the effective V_P, V_S and the ratio of V_P to V_S (V_P/V_S) 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 V_P/V_S. The V_P/V_S of the quartz-filled rock model was lower than that in the water-saturated case and in the calcite-filled rock model. By comparing the elastic properties derived from the digital rock models with the seismic velocities (e.g. V_P and V_P/V_S) around the seismogenic fault estimated from field seismic data, we characterised the evolution process of the deep seismogenic fault. The high V_P/V_S and low V_P observed at the transition from aseismic to coseismic regimes in the Nankai Trough can be explained by open cracks (or fractures), while the low V_P/V_S and high V_P observed at the deeper coseismic fault zone suggests quartz-filled cracks. The quartz-rich fault zone characterised as low V_P/V_S and high V_P in this study could partially relate to the coseismic behaviour as suggested by previous studies, because quartz exhibits slip-weakening behaviour (i.e. unstable coseismic slip).

We quantified effective V_P, V_S and the ratio of V_P to V_S of the 3D digital rock models with different crack-filling minerals. By comparing the elastic properties derived from the digital rock models with the seismic velocities around the Nankai seismogenic fault, we characterised the evolution process of the seismogenic fault.

Key words::

• earthquake
• fractures
• modelling
• rock physics
• wave propagation

Acknowledgements

We thank G. Kimura, R. Fukuchi and M. Hamahashi (University of Tokyo) for providing the micro-CT images. We gratefully acknowledge the support of the Japan International Cooperation Agency (JICA) and I2CNER, sponsored by the World Premier International Research Center Initiative (WPI), MEXT, Japan. This study was also supported by JSPS through a Grant-in-Aid for Scientific Research in Innovative Areas (no. JP15H01143; JP17H05318), a Grant-in-Aid for Science Research B (no. JP15H02988) and a Grant-in-Aid for Scientific Research S (no. JP15H05717).