Stiffness evolution of granular layers and the origin of repetitive, slow, stick-slip frictional sliding

We demonstrate the frictional behaviors of steady state sliding, stick-slip, and repetitive, slow stick-slip sliding through a carefully-designed suite of laboratory experiments focused on exploring the role of loading system stiffness in controlling the frictional response to shear. We performed tests on sheared layers of baking flour, with three configurations of loading blocks made of steel and cast acrylic to achieve different stiffnesses. Slide-hold-slide and velocity step tests were conducted and analyzed in a rate-and-state friction framework. With compliant loading blocks, the material exhibits unstable stick-slip behavior with slow-slip events of duration up to 20 s. Slow-slip has been difficult to achieve in the lab and has only been observed for a narrow variety of boundary conditions and materials. Our results suggest that this behavior is strongly controlled by the stiffness of the system, the strain history of the sample, and shear fabric evolution. We describe a new suite of automated tools that greatly improve friction analysis and provide insight to the underlying mechanisms of slow stick-slip. We demonstrate that layer stiffness evolves with shear strain and modifies the mechanical behavior of stick-slip sliding. Our work suggests that slow earthquakes in tectonic fault zones may be linked to shear fabric development and associated changes in local stiffness, likely in combination with variations in frictional constitutive properties and effective stress.