Variable Creep on the Concord fault from PS-InSAR and SBAS

The Concord fault (CF) is part of the San Andreas fault (SAF) system in California’s San Francisco Bay Area. Its long-term slip rate of 7 ± 2 mm/yr (geodetically determined) represents about one-fifth of the total SAF system rate. The Concord fault also creeps at a rate of 2.5-3.5 mm/yr, determined from measurements at two alinement arrays (AAs). The AAs were also used to observe time-variable slip, and found that creep events occur every 3-5 years. Measuring creep and its variability on the CF is important, not just for understanding the fault’s earthquake potential, but also because it may give us insight into how slip is transferred onto the Concord fault. The similarity in creep rate between the NCF and CF, noted by Galehouse and Lienkamper (BSSA, 2003) is one line of evidence for linking the two. Block modeling by d’Alessio et al. (JGR 2005) further supports a portion of the CF slip rate coming from the NCF with the majority being transferred from the Greenville fault via the Monte Diablo thrust. We use PS-InSAR (permanent scatterer InSAR) and SBAS (small baseline subset) analysis to construct time series of ground motion around the CF and measure the creep rate along several profiles. The analysis is possible because of the extensive set of ERS and EnviSAT data available through the WInSAR and GeoEarthScope archives. Preliminary results show a good agreement with AA determined creep rates at their locations. Additionally the PS-InSAR data show that the creep rate decreases to the north as the CF approaches the Green Valley fault. South of the AAs, creep continues at rates of 3-4 mm/yr along a fault trace that bends westward toward the NCF, perhaps further suggesting a link between the two structures.