Spatiotemporal Variations of Surface Deformation, Shallow Creep Rate, and Slip Partitioning between the San Andreas and Southern Calaveras Fault

The Calaveras Fault (CF) branches from the San Andreas Fault (SAF) near San Benito, extending sub-parallel to the SAF for about 50 km with only 2-6 km separation and diverging northeastward. Both the SAF and CF are partially coupled, exhibit spatially variable aseismic creep and have hosted moderate to large earthquakes in recent decades. Understanding how slip partitions among the main fault strands of the SAF system and establishing their degree of coupling is crucial for seismic hazard evaluation. We perform a timeseries analysis using more than 5 years of Sentinel-1 data covering the Bay Area (May 2015-October 2020), specifically targeting the spatiotemporal variations of creep rates around the SAF-CF junction. We derive surface creep rates from cross-fault InSAR timeseries differences along the SAF and CF including adjacent Sargent, and Quien Sabe Faults. We show that the variable creep rates are to first order controlled by the angle between the fault strike and the background stress orientation. We further examine the spatiotemporal variation of creep rates along the SAF and CF and find a multi-annual coupling increase during 2016-2018 along the subparallel sections of both faults, with the CF coupling change lagging behind the SAF by 3 to 6 months. Similar temporal variations are observed from aseismic slip inferred from characteristic repeating earthquakes as well as b-values inferred from declustered seismicity. The high correlation between b-value and aseismic slip rate changes may indicate that the SAF is extremely sensitive to small stress perturbations.