On the mechanics of granular shear: The effect of normal stress and layer thickness on stick-slip properties

Seismic stress drop is one of the most important earthquake source parameters, playing a key role in elastic energy release and in scaling relations for seismic moment and fault length. While stress drop does not scale systematically with earthquake size, it varies greatly within seismic catalogs and there is much broad interest in understanding how such variations relate to fault zone properties. Here, we address connections between stress drop and fault zone properties via laboratory experiments that investigate the role of normal stress and layer thickness during frictional sliding. We sheared granular layers at normal stresses from 4 to 22 MPa and document both elastic and inelastic processes that couple with layer dilation to determine the granular fragility and stress drop during stick-slip failure. Stick-slip stress drop scales directly with fault normal stress and inversely with layer thickness. Thicker layers exhibit, greater dilation during shear loading, however shear driven dilatant volume strain is independent of layer thickness. We posit that force chains form rapidly after a dynamic slip event and that bulk inelastic creep occurs via formation and destruction of force chains, interparticle slip, and rolling. Stick-slip recurrence time and stress drop vary with fault normal stress and stiffness, which increases with shear strain, consistent with a model in which stiffness increases as porosity decreases and fault zone density increases. We propose a micromechanical model that accounts for force chains and spectator regions where granular processes are dominated by inelastic slip. We document the role of elastic and inelastic processes during three stages of the stick-slip cycle and show how these change systematically as a function of normal stress and layer thickness. Our work shows that dilation during shear loading contributes to frictional strength via volume strain and that apparent friction scales inversely with the granular thinning ratio.