Effects of physical fault properties on frictional instabilities produced on simulated faults

Abstract

Laboratory studies of large‐scale simulated faults show that physical properties of the fault, specifically normal stress and fault roughness, significantly influence the unstable shear failure behavior of the fault. In addition, the experiments provide insights into important length‐scaling effects that are useful for assessing concepts such as critical crack length or rupture nucleation dimension. Stick‐slip shear failures have been generated along a 2‐m‐long simulated fault in a block of Sierra white granite. Experiments were conducted at normal stresses between 0.6 and 4 MPa with two different prepared roughnesses of the simulated fault: a smooth fault with a profilometer measured roughness of 0.2 μm and a rough fault with a measured roughness of ∼80 μm. High‐speed records of shear strains and slip velocities clearly show both the propagation of the stick‐slip failures and details of the local deformations and motions from which slip weakening of the fault at the onset of sliding can be resolved. Both the dynamic stress drops of the stick‐slip events and the apparent fracture energies of the events determined from the records of the fault slip weakening increase with increasing normal stress. Although critical slip‐weakening displacements are insensitive to normal stress, they increase with increased fault roughness such that the apparent fracture energies are also larger on the rough fault than on the smooth fault. Slip velocities in these experiments are influenced by roughness and are proportional to stress drop. Although the onset of slip is sharply defined, the termination of slip is more gradual without a clear stopping phase. Measured rupture velocities on the rough fault are lower than those on the smooth fault. When a length scale based on rupture nucleation dimensions, directly proportional to critical displacements, is introduced, the data suggest that at fault lengths close to the nucleation lengths, rupture velocity and slip velocity increase with increasing fault length.