A unified model for thermally-activated fault weakening during nonlinear dynamic earthquake rupture and off-fault fracturing in 3D diffuse fault zones

Earthquake fault zones are more complex, both geometrically and rheologically, than an idealized infinitely thin plane embedded in linear elastic material. Field and laboratory measurements have revealed intense fault weakening induced by flash heating and melting on natural fault (Di Toro et al., 2006; Goldsby & Tullis, 2011) and complex fault zone structure involving both tensile and shear fractures spanning a wide spectrum of length scales (e.g., Mitchell & Faulkner, 2009). Previous 2D numerical models explicitly accounting for off-fault fractures have demonstrated important feedback with rupture dynamics and ground motions (e.g., Thomas & Bhat 2018, Okubo et al., 2019). However, numerical studies of thermal-related weakening mechanisms usually avoid frictional melting due to the lack of the solid-fluid phase transition. 

In the work of Gabriel et al. (2021), we have presented our first-order hyperbolic and thermodynamically compatible mathematical model, namely the GPR model (Godunov & Romenski, 1972; Romenski, 1988), combined with a diffuse crack representation to incorporate finite strain nonlinear material behavior, natural complexities and multi-physics coupling within and outside of fault zones into dynamic earthquake rupture modeling. We compare our novel diffuse interface fault models of kinematic cracks, spontaneous dynamic rupture, and dynamically generated off-fault shear cracks to sharp interface reference models. Pre-damaged faults, as well as dynamically induced secondary cracks are therein described via a scalar function indicating the local level of material damage (Tavelli et al., 2020); arbitrarily complex geometries are represented via a diffuse interface approach based on a solid volume fraction function (Tavelli et al., 2019). 

Here we further extend the diffuse crack representation to more complicated scenarios including severe dynamic fault zone weakening as activated by flash heating, the effect of locally melting rocks, and off-fault cracks with complex topology in 3D materials, by taking advantage of adaptive Cartesian meshes (AMR) embedded in the extreme-scale hyperbolic PDE solver ExaHyPE (Reinarz et al., 2019). We intend to compare our thermally-weakened rupture in diffused fault zone with the semi-analytical thermal pressurization weakening implemented in the linear elastodynamic rupture on an infinitely-thin fault surface, using SeisSol (https://github.com/SeisSol). We will further qualitatively verify our model using the up-to-date observations in the 2020 M8.2 Chignik, Alaska, to illustrate the importance of thermal weakening on relatively deeper faults.

Our approach is part of the TEAR ERC project (www.tear-erc.eu) and will potentially allow to fully model volumetric fault zone shearing during earthquake rupture, which includes spontaneous partition of fault slip into intensely localized shear deformation within weaker (possibly cohesionless/ultracataclastic) fault-core gouge and more distributed damage within fault rocks and foliated gouges.