FEM-Peridynamic Modelling of Supershear Earthquake Ruptures in Dry and Fluid-Saturated Media

Since ground shaking increases with rupture speed during earthquakes, the velocity transition from sub-Rayleigh to supershear in mode II fracture is crucial for the propagation of seismic ruptures and associated strong ground motions. We employ a newly conceived 2-dimensional hybrid Finite Element Method and Peridynamic (FEM/PD-2D) model to investigate the transition from sub-Rayleigh to supershear in dry and fluid-saturated media. The FEM is used to simulate fluid flow, while PD is used to describe solid deformation. We first verify the results of the FEM/PD-2D model against two experimental configurations: (a) rupture propagation along a Homalite plate with a pre-notch subjected to impact shear loading and, (b) rupture propagation along a friction interface between two PMMA blocks. Once the robustness of the FEM/PD-2D model has been verified, we apply it to explore rupture propagation along both dry and fluid-saturated media under shear loading. Supershear crack speeds and the emergence of shear Mach cones are observed in both the dry and fluid-saturated cases. Supershear rupture can be achieved through either the indirect (mother-daughter cracks or Burridge-Andrews mechanism) or a direct transition. In particular, under fluid-saturated conditions, a frequent situation in natural fault zones but overlooked in previous studies, the direct transition from sub-Rayleigh to supershear is favored by poroelastic effects near the rupture front.