STRESSED AND UNSTRESSED FRICTIONAL AGING: USING MICROSTRUCTURES AND GOUGE PARTICLE SIZE DISTRIBUTION TO UNDERSTAND THE ROLE OF SHEAR STRESS IN FAULT HEALING

During the seismic cycle of repeated earthquake failure, faults regain strength in a process known as fault healing. Existing works suggest that healing plays an important role in determining the mode of fault slip ranging from dynamic rupture to aseismic creep and slow earthquakes. Laboratory studies play an important role in identifying the underlying processes of fault healing, including but not limited to friction, pressure solution, cementation, and their evolution through the seismic cycle.

Previous laboratory studies suggest that frictional healing varies with shear stress during the interseismic period. In slide-hold-slide (SHS) tests, the magnitude of frictional healing scales inversely with shear stress during the hold period, such that unstressed aging is greater than stressed aging. The processes that cause shear stress dependent healing are not well understood. To investigate these processes and quantify frictional healing we conducted laboratory experiments with granular quartz and ground Westerly granite. SHS tests serve as a simple laboratory analog for the seismic cycle in which earthquakes (slide) are followed by interseismic quiescence (hold). We quantify frictional healing as the difference between the peak friction value upon reshearing and the pre-hold steady state friction value. We performed multiple SHS tests in each experiment while holding normal stress constant at 25 MPa. Samples were loaded at a constant shear displacement rate of 10 μm/s and unloaded to a prescribed shear stress at 300 μm/s. Sheared layers were recovered from experiments for scanning electron microscope analysis and particle size distribution measurements. We investigate variations in fabric development for stressed and unstressed aging and compare differences as a function of gouge composition.