Effects of Damage Zone Permeability on Fluid Flow Within Gouge Zones During Earthquakes: Observations From Grain-Scale Numerical Models

Abstract:

Slip on faults occurs within very fine–grained granular gouge created by the comminution of highly fractured country rock. Due to this process of fracturing and grinding into powder, the permeability within these gouge zones naturally varies across their width, with the gouge material having relatively low permeability compared to the surrounding damage zone of fractured rock. During shear, the movement of individual grains creates local transient dilations and compactions that generate variations in fluid pressure and gouge permeability. The resulting fluid circulation, controlled by differing permeabilities across the fault, may determine aspects of the evolution of slip, such as at what position individual shear bands will materialize.

In order to investigate these phenomena, we extend the coupled discrete element / continuum numerical model of grain motion and fluid flow (Goren et al., 2011) to include fluid flow in the surrounding permeable fault block. We conducted numerical experiments with unconsolidated gouge confined between fault blocks. This system was held at a constant confining stress, and sheared at a constant rate similar to earthquake velocities (on the order of 1 m/s). We find that shear localizes into bands about 10 grains wide, and these bands prefer to form where they will create the least deviations in pressure, i.e. where permeability is highest. Preliminary results with a rigid, fixed-permeability fault block reveal a clear relationship between localization and fault block permeability. When the fault block has low permeability relative to the gouge, shear bands tend to form towards the interior of the gouge. When the fault block is more permeable, shear band formation tends to occur near the boundary between gouge and fault block. We see no further increase in boundary localization for fault block permeabilities on the order of 100 times the gouge permeability.

We will also present results from a version of the model in which the wall rock surrounding the gouge is an elastic/plastic damage zone created from cohesively bonded grains. This allows a greater degree of wall roughness, more lateral variability in wall stresses, and a heterogeneous and evolving damage zone permeability.