Dilatancy toughening of shear cracks and implications for slow rupture propagation

TL;DR

This paper investigates how fault dilatancy increases the energy needed for rupture propagation, affecting slow slip events, and explores the interplay between dilatancy, fluid flow, and thermal effects in fault mechanics.

Contribution

It introduces a model combining fault dilation, fluid flow, and frictional weakening to explain dilatancy's role in rupture toughness and slow slip dynamics.

Findings

Dilatancy increases the stress intensity factor for rupture.

An undrained, strengthened zone develops near the rupture tip.

Dilatancy effects are consistent with observed slow slip events.

Abstract

Dilatancy associated with fault slip produces a transient pore pressure drop which increases frictional strength. This effect is analysed 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 pressurisation 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.