[S-CG40] Science of slow-to-fast earthquakes

[SCG40-22] Granular simulation for earthquakes in geological scale structure evolution

*古市 幹人¹、Chen Jian¹、西浦 泰介¹、新井 隆太¹、山本 由弦²、井出 哲³ (1.海洋研究開発機構 、2.神戸大学、3.東京大学)

キーワード：地震モデル、スラスト形成、シームレスモデル、DEM、HPC

We present virtual earthquake simulation using a numerical granular rock box experiment based on the Discrete Element Method (DEM). This simulation extends the numerical sandbox test by incorporating cohesive contact forces. We simulate a thin granular rock layer sized 100km in convergence direction, 250m laterally, and about 2.0km in gravity direction, using about 5.6 million DEM elements. The horizontal shortening of this layer generates sequential thrust formation like that of an accretionary wedge [1]. Intermittent fast motions of elements generating seismic waves, are observed through convergence tests following the development of geological-scale structures [2]. These results suggest that the granular rock box simulation seamlessly reproduced seismological and geological-scale phenomena. The hypocenter, which is not predefined by the model, emerges within the active fault damage zone and shifts as the geological structures evolved. The seismic event shows the 3D elementwise rotation and the double-couple behavior, and Gutenberg–Richter law regarding kinetic energy. Quantitative analysis of a seismic event shows that the fracture propagation speed is ~2.6 km/s, and the P-wave and S-wave velocities are ~3.7 and ~2.7 km/s, respectively. The slip distance and the mean stress drop at the hypocenter are ~1.8 cm and ~0.46 MPa, respectively. These characteristics demonstrate the feasibility of granular rock box simulation for reproducing multiscale mechanisms that bridge seismological and geological phenomena. On the other hand, we acknowledge the limitations of the model in terms of fast convergence speed, large element size, and element shape.

[1] Furuichi, M., Chen. J., Nishiura, D., Arai, and R., Yamamoto, Y., Tectonophysics, 862, 5, 229963 (2023). 654 DOI:10.1016/j.tecto.2023.229963.
[2] Furuichi, M., Chen. J., Nishiura, D., Arai, and R., Yamamoto, Y., Ide, S., Tectonophysics, 874, 11, 230230 (2024). DOI:10.1016/j.tecto.2024.230230