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