Investigating the Role of Fluid Flow Within Gouge-filled Fault Zones Using Numerical Simulations of Earthquakes
Monday, 15 December 2014
Mature fault cores contain a finely ground powder termed fault gouge where most displacement occurs. If saturated, the gouge’s resistance to shear changes due to pressurization of pore fluid that occurs when the gouge compacts or dilates. If gouge compacts at the onset of shear, fluid pressure increases, causing frictional contacts to weaken. More commonly, granular material dilates upon shear, causing frictional resistance to increase due to decreased pressure (dilation hardening). Depending on the relative rates of fluid flow and deformation, these local pressure deviations can be large enough to effect gouge strength even in drained (permeable) fault zones. To understand the conditions under which this occurs, we use a grain-scale numerical model of saturated fault zones.
We use the Goren et al. (2011) combined two-dimensional fluid flow/discrete element model to simulate earthquakes in a layer of unconsolidated grains. The periodic layer is 50 grains thick and 200 grains wide, and bounded by a fixed wall and a moveable wall. Shear stress is applied to the system by slowly pulling a spring attached to the moveable wall. When this stress exceeds the layer strength, rapid slip occurs relieving a portion of the stress. These slip events have displacements ranging from 102 to 103 grain diameters. To investigate how and when pore pressure affects the timing and characteristics of an earthquake, we conducted multiple experimental slip events on the same stressed configuration of grains, with different gouge permeability.
We find that, even when gouge permeability is as high as 10-15 m2, pressure effects are large enough to change the evolution of a slip event. As the gouge dilates at the onset of slip, lower pressure raises the effective confining stress, which hardens the system and delays the slip event. In some cases, the slip continues at reduced velocities; in others, the slip is terminated altogether. In the latter case, very small events (“foreshocks”) can be present before the main slip occurs at a higher peak stress. We will present examples of individual events and a statistical comparison of different series of events run under a range of gouge permeability and confining stress.