GRANULAR DEFORMATION AND SEISMICITY IN LABORATORY PHOTOELASTIC FAULT ZONES

A suite of laboratory experiments provides an important addition to existing numerical simulations, rock mechanics experiments, geophysical investigations, and geologic studies. The experiments: (1) isolate the effects of intergranular friction (2) allow monitoring of the spatiotemporal evolution of both stress and strain, and (3) produce populations of both stick-slip and creep events that can be related to natural fault behaviors. The experimental shear zone is roughly two-dimensional, has a deformable area of roughly ~125 X 25 cm, and contains several thousand, ~0.5 cm diameter, circular and elliptical photoelastic particles. With motorized slider blocks and springs the granular media is subjected to a bulk non-coaxial shear with either constant volume (V), or constant dP/dV boundary conditions. The resulting strengthening, weakening, and stick-slip events are recorded by a force gauge. By using polarizing filters, the photoelasticity of the particles allows monitoring of force-chain development along with particle displacements. Force chains are roughly co-linear chains of particles through which larger-than-average stresses are transmitted. While the shear is localized to a central fault plane during stick-slip and creep events, the force-chain rearrangements nonetheless are distributed throughout the system. Observed force-chain reorganizations correspond to measurable force drops at the slider block. As in rock friction experiments, we observe an immediate rise in force (strengthening) followed by failure and steady-state friction at mu=0.4 to 0.6. Populations of experimental force drops and equivalent moment-magnitudes have power-law distributions, similar to populations of natural earthquakes. The power-law distributions for populations of smaller force-drops are more sensitive to pressure-volume boundary conditions than for larger force-drops. As the particle kinematics and force-chain geometries are explored further, the experiments will provide important information about the relationship between geologic fault structures, and earthquake processes.