The role of heterogeneity in fault zone weakening and stability

Heterogeneity is abundant in crustal fault zones from the micron-scale to the plate interface scale. Despite this, it is still uncertain how different scales of heterogeneity interact and influence the mechanical properties of natural faults. Here we present experimental results where heterogeneous faults are simulated in the laboratory by placing patches of different fault gouge materials next to each other in a direct shear arrangement. These laterally heterogeneous experimental faults (50 mm in total length) are then sheared and the frictional strength evolution is measured with increasing displacement. Two types of fault gouge are used: (1) a fine-grained quartz gouge which obeys Byerlee friction (coefficient of friction = 0.6-0.7) and is rate weakening, and (2) a clay gouge comprised predominantly of kaolinite which has a low friction coefficient (approx. 0.25) and is rate strengthening. We find that with the addition of only a small amount of clay gouge the bulk fault strength weakens considerably after only a few millimetres of slip. Although clay is preferentially smeared along localized Y-shear bands, the observed weakening cannot be explained by clay smear as the total displacement on the fault is far too small for the clay to be smeared through the entire length of the quartz patches. Instead we propose stress concentrations at the boundary between clay and quartz patches, driven by slip on the weaker clay patch, produce enhanced weakening and shear at an overall low stress within the quartz patches.

The scale of heterogeneity also controls the frictional stability of the experimental fault. When clay patches are small and comprise <20% of the total fault area, instabilities occur within the unstable quartz gouge leading to stick-slip behaviour. However when patches of clay comprise >20% of the total sliding area, instabilities within the quartz are supressed leading to stable sliding. In this case, the bulk fault also becomes increasingly rate-strengthening with slip, tending towards the behaviour of a fault comprised of 100% clay. These results demonstrate how natural geological heterogeneity and the interplay between different geologic materials can help explain fault weakness and also control the seismogenic potential of tectonic faults.