Abstract
Abstract. Substantial insight into earthquake source processes has resulted from considering frictional ruptures analogous to cohesive-zone shear cracks from fracture mechanics. This analogy holds for slip-weakening representations of fault friction that encapsulate the resistance to rupture propagation in the form of breakdown energy, analogous to fracture energy, prescribed in advance as if it were a material property of the fault interface. Here, we use numerical models of earthquake sequences with enhanced weakening due to thermal pressurization of pore fluids to show how accounting for thermo-hydro-mechanical processes during dynamic shear ruptures makes breakdown energy rupture-dependent. We find that local breakdown energy is neither a constant material property nor uniquely defined by the amount of slip attained during rupture, but depends on how that slip is achieved through the history of slip rate and dynamic stress changes during the rupture process. As a consequence, the frictional breakdown energy of the same location along the fault can vary significantly in different earthquake ruptures that pass through. These results suggest the need to reexamine the assumption of predetermined frictional breakdown energy common in dynamic rupture modeling and to better understand the factors that control rupture dynamics in the presence of thermo-hydro-mechanical processes.
Highlights
Fault constitutive relations that describe the evolution of shear resistance with fault motion are critical ingredients of earthquake source modeling
We conduct numerical simulations of spontaneous sequences of earthquakes and aseismic slip (SEAS) utilizing the spectral boundary integral method (BIE) to solve the elastodynamic equations of motion coupled with friction boundary conditions, including the evolution of pore fluid pressure and temperature on the fault coupled with off-fault diffusion (Lapusta et al, 2000; Noda and Lapusta, 2010)
The values of local breakdown energy for a given amount of slip have a wide spread in our simulations, even though the constitutive properties are uniform and time-independent along the fault, highlighting the reality that breakdown energy in models with thermo-hydro-mechanical mechanisms is not fundamentally a function of slip
Summary
Fault constitutive relations that describe the evolution of shear resistance with fault motion are critical ingredients of earthquake source modeling. Saturates after a given amount of weakening (Nielsen et al, 2016) Such findings are inconsistent with the breakdown energy being a fixed fault property as often assumed in linear slip-weakening laws and as approximately follows from standard rate-and-state friction with uniform characteristic slipweakening distance (Perry et al, 2020), unless strong and very special heterogeneity is assumed in fault properties. We use numerical models of earthquake sequences with enhanced weakening due to thermal pressurization to illustrate how the inclusion of thermo-hydromechanical processes during dynamic shear ruptures makes breakdown energy rupture-dependent, in that the values of both local and average breakdown energy vary among ruptures on the same fault, even with spatially uniform and timeindependent constitutive properties. Additional fault characteristics that we do not consider here, such as heterogeneity in fault properties and dynamically induced, evolving, inelastic off-fault damage (Dunham et al, 2011a, b; Roten et al, 2017; Withers et al, 2018), should result in qualitatively similar effects and add even more variability to the breakdown energy
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