Aseismic slip and fault interaction from repeating earthquakes in the Loma Prieta aftershock zone

Along creeping sections of the San Andreas and other faults, small asperities in the fault zone load and fail in characteristic repeating earthquake sequences which can be used as subsurface creepmeters. Here, we use these virtual creepmeters to examine and compare deep slip rates on both the northwestern end of the creeping section of the San Andreas Fault near San Juan Bautista and on the nearby subparallel Sargent Fault. While creep on the San Andreas increased dramatically due to static stress changes in response to the 1989 Loma Prieta earthquake, the Sargent showed very little immediate response, consistent with Loma Prieta finite slip models that put this section of the fault in a region of less than 3 bar Coulomb stress increase. After about 10 years, the San Andreas creep rate fell back close to the interseismic rate, and variations in creep became coherent in time with the Sargent, indicating a mutual driving force in the system.