Nucleation of Stick-Slip Instability Within a Large-Scale Experimental Fault: Effects of Stress Heterogeneities Due to Loading and Gouge Layer Compaction

Geodetic observations and large-scale laboratory experiments show that seismic instability is preceded by slow slip within a finite nucleation zone. In laboratory experiments rupture nucleation is studied mostly using bare (rock) interfaces, whereas upper crustal faults are typically filled with gouge. To investigate effects of gouge on rupture nucleation, we performed a biaxial shearing experiment on a 350 mm long saw-cut fault filled with gypsum gouge, at room temperature and a minimum horizontal stress σ 2 = 0.3-5 MPa. The gouge layer was sandwiched between polymethylmethacrylate (PMMA) plates For reference also a fault without gouge was deformed. Strain gauges and Digital Image Correlation were used to monitor the deformation field along the fault zone margins. Stick-slip behavior occurred on both the gouge-filled fault and the PMMA fault. Nucleation of instability on the PMMA fault persistently occurred from one location 2/3 to 3/4 along the fault adjacent to a slow slip zone at the fault end, but nucleation on the gouge-filled fault was more variable, nucleating at the ends and/or at approximately 2/3 along the fault, with precursory slip occurring over a large fraction of the fault. Nucleation correlated to regions of high average fault stress ratio τ/σ n , which was more variable for the gouge-filled fault due to small length scale variations in normal stress caused by heterogeneous gouge compaction. Rupture velocities and slip rates were lower for the gouge-filled fault than for the bare PMMA fault. Stick-slip persisted when σ 2 was lowered and the nucleation zone length increased, expanding from the center to the sample ends before transitioning into instability.

An investigation into the role of time-dependent cohesion in interseismic fault restrengthening

Earthquakes typically exhibit recurrence times that far exceed time-scales attainable in a laboratory setting. To traverse the temporal gap between the laboratory and nature, the slide-hold-slide test is commonly employed as a laboratory analogue for the seismic cycle, from which the time-dependence of fault strength may be assessed. In many studies it is implicitly assumed that all fault restrengthening emanates from an increase in the internal friction coefficient, neglecting contributions from cohesion. By doing so, important information is lost that is relevant for numerical simulations of seismicity on natural faults, as well as for induced seismicity. We conduct slide-hold-slide experiments on granular halite gouge at various normal stresses to assess the time-dependence of the internal coefficient of friction, and of the cohesion, independently of one another. These experiments reveal that both the internal friction coefficient and cohesion increase over time, but that these quantities do not share a common evolution, suggesting different underlying mechanisms.