Abstract
AbstractDilatancy associated with fault slip produces a transient pore pressure drop which increases frictional strength. This effect is analyzed in a steadily propagating rupture model that includes frictional weakening, slip‐dependent fault dilation and fluid flow. Dilatancy is shown to increase the stress intensity factor required to propagate the rupture tip. With increasing rupture speed, an undrained (strengthened) region develops near the tip and extends beyond the frictionally weakened zone. Away from the undrained region, pore fluid diffusion gradually recharges the fault and strength returns to the drained, weakened value. For sufficiently large rupture dimensions, the dilation‐induced strength increase near the tip is equivalent to an increase in toughness that is proportional to the square root of the rupture speed. In general, dilation has the effect of increasing the stress required for rupture growth by decreasing the stress drop along the crack. Thermal pressurization has the potential to compensate for the dilatant strengthening effect, at the expense of an increased heating rate, which might lead to premature frictional melting. Using reasonable laboratory parameters, the dilatancy‐toughening effect leads to rupture dynamics that is quantitatively consistent with the dynamics of observed slow slip events in subduction zones.
Highlights
Dilatancy is a well documented process in intact, healed or overconsolidated rocks: due to shear deformation, microcavities open, which induces an overall increase in porosity (Paterson & Wong, 2005, Section 5.3)
This analysis leads to a refined description of the various contributions to fracture energy and energy release rate of different weakening processes, including “near-tip” mechanisms such as intrinsic frictional weakening or dilatancy, which contribute to Gc, and “far-tip” mechanisms like pore fluid diffusion or thermal pressurization, which are expected to contribute to G
By analyzing a crack model incorporating slip weakening friction and slip-dependent dilatancy coupled to fluid flow, we have established that dilatancy tends to limit crack propagation by increasing the stress intensity factor required for crack growth
Summary
Dilatancy is a well documented process in intact, healed or overconsolidated rocks: due to shear deformation, microcavities (cracks, grain junctions) open, which induces an overall increase in porosity (Paterson & Wong, 2005, Section 5.3). Keeping with the eventual goal of finding a simple, usable form of fracture energy to use in a Griffith energy balance, a particular attention is paid to the contribution of fault zone dilation to fracture energy as a function of rupture propagation speed This analysis leads to a refined description of the various contributions to fracture energy and energy release rate of different weakening processes, including “near-tip” mechanisms such as intrinsic frictional weakening or dilatancy, which contribute to Gc, and “far-tip” mechanisms like pore fluid diffusion or thermal pressurization (or decomposition, or melting), which are expected to contribute to G
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