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A hybrid finite element-Spectral boundary integral method for modeling frictional faults

Ahmed Elbanna's picture

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Understanding patterns of earthquakes and aseismic slip that emerge in complex fault zones over seismologically relevant spatio-temporal scales is a problem of great societal relevance that continues to elicit interest in science and engineering. In this paper, we present a novel hybrid finite element (FE) - spectral boundary integral (SBI) equation scheme to address this problem. The methodology enables simulation of the slip evolution on rate and state faults, with near-field heterogeneities or nonlinearities, subjected to slow tectonic loading processes with episodes of spontaneously occurring events. This combined FE-SBI approach captures the benefits of finite elements in modeling problems with nonlinearities or small-scale heterogeneities, as well as the computational superiority of SBI. The domain truncation enabled by this scheme allows us to utilize high-resolution finite elements discretization to capture inhomogeneities or complexities that may exist in a narrow domain surrounding the fault. Combined with an adaptive time stepping algorithm, this framework opens new opportunities for modeling earthquake cycles with high-resolution fault zone physics. In this initial study, we consider a two dimensional (2-D) anti-plane model with a vertical strike-slip fault governed by rate and state friction and embedded in a medium with elastic heterogeneity. The proposed approach is first verified using the benchmark problem BP-1 from the SCEC SEAS repository. The computational framework is then utilized to model the earthquake sequence and aesismic slip of a fault embedded within a low-velocity fault zone (LVFZ) with different widths and compliance levels. Our results indicate that sufficiently compliant LVFZs contribute to the emergence of sub-surface events that fail to penetrate to the free surface and may experience earthquake clusters with nonuniform inter-seismic time. Furthermore, the LVFZ leads to slip rate amplification relative to the homogeneous elastic case. We discuss the implications of our results for understanding earthquake complexity as an interplay of fault friction and bulk heterogeneities.

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