Interseismic coupling on the Hayward-Calaveras fault zone from InSAR

Evaluation of interseismic deformation traditionally relies on GPS and alignment arrays (AA) providing precise but spatially sparse measurements. Here, we resolve with a high spatial resolution interseismic strain accumulation and fault creep associated with the Hayward and Calaveras Faults (HF and CF) in the Eastern San Francisco Bay Area using InSAR. The large 1992-2011 ascending and descending ERS-Envisat dataset enables characterization of short- and long-wavelength horizontal and vertical deformation as small as 2 mm/yr without alignment to a GPS-based model. A comparison between independent InSAR, GPS, and AA datasets shows that the remaining noise is negligible in mean velocity maps and that the creep rates are mostly constant between 1992-2011. Creep rates vary from 0±2 mm/yr on the northern CF to 14±2 mm/yr on the central CF south of the HF surface junction. The high spatial resolution velocity map also highlights the southernmost occurrence of creep on the HF, located ~15 km farther south than prior determinations based on AA and field mapping. We remove the long-wavelength deformation using a deep-dislocation model and estimate the shallow slip due to creep by inverting the remaining InSAR fault-parallel motion. We confirm a good agreement between our model and surface slip rates measured with AA and slip at depth from characteristically repeating earthquakes. The distribution of aseismic slip is comparable to previous models focused on the HF, confirming that the distribution of creeping and locked patches is stable. We find that the northern CF is mostly locked, explaining the absence of seismicity and that most of the aseismic slip is limited to the shallowest 5 km on the HF and CF, suggesting partial or full locking at deeper levels. Considering the time since the last earthquakes and the difference between the long-term slip rates and the shallow aseismic slip, we infer that a joint rupture of the HF and central CF could currently produce a M7.1±0.1.