Spatiotemporal variations of surface deformation, shallow creep rate,
and slip partitioning between the San Andreas and southern Calaveras
Fault
Abstract
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 the 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 (0-20 mm/yr) at the SAF-CF junction 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 the subparallel sections of both faults, with the CF
coupling change lagging behind the SAF by 3 to 6 months. Similar
temporal variations are also observed in both b-values inferred from
declustered seismicity and aseismic slip rates inferred from
characteristic repeating earthquakes. The high correlation of b-value
and slip-rate changes may indicate that the SAF is extremely sensitive
to small stress perturbations.