The Frictional Strength and Stability of Spatially Heterogeneous Fault gouges

Along-fault lithological heterogeneity is observed in all fault zones that cross-cut compositional layering. Modelling studies of fault rupture nucleation, propagation and arrest often assume that the fault mechanical behaviour is governed by either the rheologically weakest phase or by homogeneous gouge mixtures of the juxtaposing lithologies. However, the effects of spatial heterogeneity on fault gouge composition and hence its frictional behaviour are not well understood. In this study, we conducted friction experiments to understand how material mixing and clay-smearing in fault gouges affect the frictional strength and stability of claystone-sandstone juxtaposing faults. Simulated mechanically contrasting fault gouges (Ten Boer claystone and Slochteren sandstone) were derived from the Groningen gas field in the northeast of the Netherlands, an area affected by induced seismicity. We conducted velocity stepping tests in a rotary shear configuration to facilitate substantial shear displacement, essential to study mixing and clay-smearing in large offset faults. Experiments were performed under normal stresses ranging between 2.5 and 10 MPa, imposed velocities ranging between 10 and 1000 m/s, and under drained conditions. We introduced spatial heterogeneity by segmentation of the simulated gouge in claystone and sandstone patches. Our mechanical data shows displacement-dependent changes in sliding friction and its rate-dependence. Clay smearing and shear localization on foliation planes cause transient weakening of the gouge and a shift from velocity-weakening to velocity-strengthening behaviour. Progressive shearing leads to juxtaposition of sandstone segments that are separated only by a thin clay smear. We propose that the associated increase in friction is caused by lithology mixing at the interfaces between the clay smear and the bulk Slochteren sandstone gouge, and by the disruption of continuous Y-shears. Progressive shearing does not lead to a decrease in the rate-sensitivity parameter (a-b), suggesting that, although affected by quartz and feldspar grains, deformation remains localized on phyllosilicate foliation planes. Our results show that fault friction and its rate-dependence are not simply governed by the weakest lithology along a fault plane, nor that they can be simply represented by a homogeneous mixture of the juxtaposing lithologies. Detailed knowledge of the stratigraphic layering in combination with the fault offset is required to predict the macroscopic frictional behaviour of heterogenous fault gouges.