The effect of frictional heterogeneities on the seismic cycle: Insights from triaxial experiments

Deformation within the upper crust is mainly accommodated through slip on fault systems. Slip can occur in various forms, ranging from aseismic creep (i.e., stable motion) to dynamic earthquake (i.e., unstable motion). Interestingly, a single fault is not restricted to a specific slip mode. Recent geodetic observations have shown that a fault can exhibit both stable and unstable motions. The different slip behaviours have been attributed to fault spatial heterogeneity of the frictional properties, rheological transitions, or geometric fault complexity.

To comprehensively characterise the effect of frictional heterogeneities, we deformed heterogeneous fault samples in a triaxial apparatus, at confining pressures ranging from 30 to 90 MPa. The fault planes, sawcut at a 30° angle from the sample axis, are composed of two materials: granite and marble. Experiments were conducted with both marble asperities embedded in granite and vice versa, alongside homogeneous fault samples of single lithology. The selection of granite and marble was based on their different frictional properties, with granite exhibiting seismic behaviour, while marble demonstrated aseismic behaviour under the tested conditions.

Our results show that the stress drops of seismic events are dependent on fault composition, with faults containing higher granite content exhibiting larger stress drops. In addition, local strain measurements close to the fault allow us to investigate the spatial and temporal distribution of fault slip. In the case of homogeneous faults, the seismic event nucleation is relatively straightforward, initiating in the highest stressed region and propagating uniformly. Conversely, heterogeneous faults display a shorter nucleation phase, followed by a dynamic strain drop restricted close to the granite areas. Away from the dynamic event, the fault remains locked and is subjected to an increase in strain. This strain deficit is then released by a long-lasting decay similar to post-seismic afterslip observed on natural fault systems after large earthquakes. For the heterogeneous samples exhibiting post-seismic deformation, elevated confining pressure favours longer and higher amplitude of afterslip. Further data analysis demonstrated that large afterslip observed at higher confining pressures must originate from the combination of 1) larger co-seismic stress/strain drop and 2) higher frictional stability around the area of dynamic stress/strain drop, both enhanced at larger confining pressures.