A proxy implementation of thermal pressurization for earthquake cycle modelling on rate-and-state faults

International audienceThe reduction of effective normal stress during earthquake slip due to thermal pressurization of fault zone pore fluids is a significant fault weakening mechanism. Explicit incorporation of this process into frictional fault models involves solving the diffusion equations for fluid pressure and temperature outside the fault at each time step, which significantly increases the computational complexity. Here, we propose a proxy for thermal pressurization implemented through a modification of the rate-and-state friction law. This approach is designed to emulate the fault weakening and the relationship between breakdown energy and slip resulting from thermal pressurization and is appropriate for fully dynamic simulations of multiple earthquake cycles. It preserves the computational efficiency of conventional rate-and-state friction models, which in turn can enable systematic studies to advance our understanding of the effects of fault weakening on earthquake mechanics. In 2.5-D simulations of pulse-like ruptures on faults with finite seismogenic width, based on our thermal pressurization proxy, we find that the spatial distribution of slip velocity near the rupture front is consistent with the conventional square-root singularity, despite continued slip-weakening within the pulse, once the rupture has propagated a distance larger than the rupture width. An unconventional singularity appears only at shorter rupture distances. We further derive and verify numerically a theoretical estimate of the breakdown energy dissipated by our implementation of thermal pressurization. These results support the use of fracture mechanics theory to understand the propagation and arrest of very large earthquakes

The effect of axial stress in maximum sustainable fluid pressure in Andersonian and non-Andersonian crust: A field-based numerical study from the Southern Andes (39 degrees S)

Fracture opening at low differential stress controls maximum sustainable fluid pressure (lambda) within cohesive brittle crust. Standard Andersonian stress states occur when two conditions are met: (1) one of the principal stresses sigma(1)>=sigma(2)>=sigma(3) is vertical, and (2) failure occurs at optimal orientations so that the stress tensor shape ratio phi=(sigma(2)-sigma(3))/(sigma(1)-sigma(3)) is irrelevant. Here we explore the role of phi-values (axial compression, triaxial stress and axial tension) on sustainable fluid pressure driving rock failure under general stress states. We analyzed two exposures representing tectonics of the Southern Andes. Calculated failure curves in lambda-depth space indicate that the hydrostructural behavior of general stress states is governed by the steepest of the principal stresses and the phi-value. Generally, hydrostructural behavior falls within standard Andersonian lambda-depth conditions. However, field examples suggest that non-Andersonian axial stresses may sustain fluid pressures that depart from the standard Andersonian condition: the lowest fluid pressures occur under subvertical axial compression and subhorizontal axial tension; and the highest fluid pressures occur under subvertical axial tension and sub -horizontal axial compression. Since around 15% of global stress compilations correspond to one of these categories, it follows that a significant portion of tectonic regimes potentially define a hydrostructural infrastructure different from standard Andersonian crust.National Agency for Research and Development (ANID), through program ANID-FONDECYT 1180167 ANID Scholarship Program, Beca de Doctorado Nacional 21171178 National Agency for Research and Development (ANID), through the program ANID-FONDAP 1509001