Probing the Damage Zone at Parkfield

Rocks are heterogeneous materials that exhibit nonlinear elastic (anelastic) behavior at scales ranging from the laboratory to Earth. In the laboratory, typical, complex relationships exist between stress and strain that include hysteresis, finite relaxation times, strain rate, and history dependence. These behaviors are linked to important characteristics such as stress, porosity, permeability, material integrity, and material failure. We adopted a “pump‐probe” type experiment common in laboratory studies, using solid earth tides as the low‐frequency pump and empirical Green's function as the high‐frequency probe. By probing the velocity at different points in the pump cycle, we constrained important information about the strain‐modulus relationship. Near the San Andreas Fault, we observed strongly nonlinear elastic behavior that characterizes the damage zone. We also constrained important aspects of hysteretic behavior that are related to damage properties and possibly pore pressure. Away from the fault, the nonlinear behavior is diminished. Plain Language Summary When intact materials are slightly squeezed or stretched, the amount of the material compresses or extends is typically some multiple of the applied force. Damaged materials have internal fractures, which makes this relationship more complicated. This is because fractures can be open or closed, each contributing differently to the overall material behavior. The more internal fractures exist, the more complicated the material behavior will be. By measuring complexity in how a material deforms when a force is applied, we can learn something about how damaged it is. We apply this analysis to rocks in the subsurface using seismic waves and solid earth tides. Solid earth tides are the stretching and compressing of the Earth due to the gravitational pull of the sun and moon. As the Earth is stretched and compressed, fractures in the Earth can open and close. We use seismic waves to measure how the Earth becomes softer or stiffer as these fractures open and close. The amount of this softening or stiffening tells us something about the material damage along the San Andreas Fault, and may also tell us something about the earthquake cycle. Key Points The subsurface along the San Andreas Fault near Parkfield, California exhibits inhomogeneous nonlinear elastic behavior The magnitude of nonlinear response increases with decreasing distance to the San Andreas Fault We constrain key properties of modulus‐str