Dynamic Fault Weakening and Strengthening by Gouge Compaction and Dilatancy in a Fluid-Saturated Fault Zone

Triaxial experiments show that samples of fault gouge deform distinctly differently than those from the adjacent fault damage zone (e.g., Chester and Logan, 1986). Rock samples in the damage zone follow a characteristic elastic-brittle behavior, whereas fault gouge readily compacts and deforms in a more ductile manner. In order to explain the apparent weakness of large plate bounding faults such as the San Andreas Fault, Sleep and Blanpied (1992) proposed a mechanism in which compaction during interseismic creep reduces available pore space and hence increases fluid pressure. However, Segall and Rice (1995) questioned elevated pore pressure in the interseismic period in that it stabilizes the fault. Here we invoke a similar concept, however in this case the compaction process occurs dynamically via the stresses associated with earthquake rupture. We incorporate undrained compaction into a dynamic rupture model of a strike-slip fault with a strongly velocity-weakening friction (in a rate-and-state framework). A 20-cm thick fault gouge layer is modeled by an end-cap failure criterion (e.g. Wong et al., 1997) and experiences compaction and dilatancy, while the remainder of the model domain obeys the standard Mohr-Coulomb criterion. We show that large dynamic stresses associated with rupture propagation cause the gouge layer to compact ahead of the rupture front, leading to rapidly elevated pore pressure in the undrained fault zone and significant dynamic weakening of the principal fault surface. Compared to other dynamic weakening mechanisms such as flash heating and thermal pressurization, this mechanism does not require slip to initiate. Weakening ahead of the rupture front lowers the static friction of the fault, leading to a lower strength drop on the fault. After the passing of the rupture front, strong dilatancy of undrained fault gouge reduces the pore pressure and restrengthens the fault, promoting a more pulse-like rupture. Thus dynamic gouge compaction and dilatancy provides a simple mechanical explanation for weak mature faults and pulse-like earthquake ruptures on these faults.