Modeling earthquakes with off-fault damage using the combined finite-discrete element method
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Modeling earthquakes with off-fault damage using the combined finite-discrete element method Kurama Okubo1
· Esteban Rougier2
· Zhou Lei2
· Harsha S. Bhat3
Received: 7 November 2019 / Revised: 9 March 2020 / Accepted: 12 April 2020 © OWZ 2020
Abstract When a dynamic earthquake rupture propagates on a fault in the Earth’s crust, the medium around the fault is dynamically damaged due to stress concentrations around the rupture tip. Recent field observations, laboratory experiments and canonical numerical models show the coseismic off-fault damage is essential to describe the coseismic off-fault deformation, rupture dynamics, radiation and overall energy budget. However, the numerical modeling of “localized” off-fault fractures remains a challenge mainly because of computational limitations and model formulation shortcomings. We thus developed a numerical framework for modeling coseismic off-fault fracture networks using the combined finite-discrete element method (FDEM), and we applied it to simulate dynamic ruptures with coseismic off-fault damage on various fault configurations. This paper addresses the role of coseismic off-fault damage on rupture dynamics associated with a planar fault, as a base case, and with a number of first-order geometrical complexities, such as fault kink, step-over and roughness. Keywords Earthquake ruptures · Fracture damage · FDEM
1 Introduction The contribution of inelastic off-fault deformation to the rupture dynamics has been pointed out since 1970s. Sibson [39] conceptually proposed a formulation for the overall energy budget of dynamic earthquake ruptures; a part of the energy released from accumulated strain energy by interseismic deformation is converted to seismic wave radiation, while the rest is expended in inelastic deformation processes within the fault zone. Then, numerous studies via field observations have shown evidence of fractured rock surrounding the fault core, which can be coseismically damaged due to dynamic earthquake ruptures (e.g., [5,10,38]). Furthermore, Mitchell and Faulkner [23] showed that microfracture density is significantly higher in the near-fault region while it exponentially decreases with distance from the fault core,
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Kurama Okubo [email protected]
1
Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA
2
EES-17 - Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM, USA
3
Laboratoire de Géologie, École Normale Supérieure/CNRS UMR 8538, PSL Research University, Paris, France
evidencing the presence of a well-defined off-fault damage zone. Figure 1 shows the hierarchical fault system in a wide range of length scales. Generally, fault geometrical complexity associated with an earthquake event is discussed in kilometric scale (Fig. 1a, b). However, when we focus on a smaller portion of the fault system, we find off-fault fractures in subkilometric scale (Fig. 1c, d). These smaller-scale off-fault fractures are either not included in kinematic and conventional
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