Influence of hydromechanical heterogeneities of fault zones on earthquake ruptures

Abstract

Fluid pressurization, a possible mechanism of fault weakening, exerts a critical control on earthquake rupture in the upper crust. One cause for this control is the presence of high fluid pressures inside the fault zone. However, the precise mechanisms relating high fluid pressures and fault rupture remain unclear. Here, we use 2-D hydromechanical models to show that effective stress changes induced by a transient pulse of fluid pressure along a fault zone with spatially variable material properties—conditions that are representative of natural faults—can be sufficient to produce large slip in the fault core, and fracturing in the surrounding damage volume. Rupture properties outside the limited source region are examined for ranges of values of the degree of material contrast across the fault. Our results indicate that the slip dimension is highly influenced by the contrasts of permeability and rigidity across the fault zone components, from the fault core through the various subzones of the fractured damage zone and to the less fractured host rock. Moreover, high fluid pressures may also develop locally off the narrow fault core, preferentially into the more permeable and less rigid parts of the damage zone, where the deformation is plastic, highlighting a possible mechanism for off-fault rupture. Finally, on faults with hydromechanical heterogeneities at the scale of the internal structure, the fluid pressure and rupture are highly asymmetric with propagation in a preferred direction, that is the more permeable and compliant material of the fault zones.