Detecting seismogenic stress evolution and constraining fault zone rheology in the San Andreas Fault following the 2004 Parkfield earthquake

We investigate temporal changes in seismic scatterer properties at seismogenic depth attributed to the 2004 M 6 Parkfield earthquake, making use of the San Andreas Fault Observatory at Depth repeating-earthquake target sequences, as well as nearby similar-earthquake aftershock clusters. We use a two-step process: (1) observing temporal variations in the decorrelation index, D(t), reflecting changes in the scattered wavefield of repeating-earthquake sequences and (2) estimating the spatial distribution of time-dependent scatterers by using a larger-aperture source array. We focus on three scatterers exhibiting clear time dependence, using pairs of earthquakes that span or follow the 2004 Parkfield earthquake. They are found to be located on the fault at the northernmost extent of coseismic rupture, beneath Middle Mountain, with a depth range of 11 to 17 km. The shallowest and most prominent scatterer is located near a region of increased Coulomb stress, as well as significant postseismic slip following the 2004 Parkfield earthquake, and a large M = 5 aftershock. The other two deeper ones are also in regions of increased Coulomb stress. We show that D(t)(1/2) is expected to be proportional to the level of stress in the fault zone, and then we constrain the form of fault zone rheology by comparing the time dependence of D(t)(1/2) with geodetic or seismic measures of strain rate, assuming a power law rheology between stress and strain rate characterized by exponent n. Such a comparison yields n ranging from 1.6 through 3.3, a value that is more consistent with ductile behavior, rather than frictional sliding, at the base of the seismogenic zone.