Surface Creep Rate of the Southern San Andreas Fault Modulated by Stress Perturbations from Nearby Large Events

Shallow surface creep, observed along a few seismically active strike-slip faults, is broadly understood to result from velocity strengthening materials in the uppermost (1-2 km) fault zone atop the velocity weakening materials at seismogenic depths. Creep sometimes occurs in discrete events, spontaneously or triggered by shaking from nearby large earthquakes. A major challenge for understanding the physics of shallow creep has been to observe and model the long-term effect of stress changes on creep rate. Here we investigate the surface creep along the southern San Andreas fault (SSAF) using data from interferometric synthetic aperture radar (InSAR) spanning over 25 years (ERS 1992-1999, ENVISAT 2003-2010, and Sentinel-1 2014-present). To resolve fault creep unperturbed by passing seismic waves, we exclude interferograms spanning three major regional earthquakes for which dynamically triggered creep is well documented: the 1992 Mw 7.3 Landers, the 1999 Mw 7.1 Hector Mine and the 2010 Mw 7.2 El Mayor-Cucapah earthquakes. The main result of this analysis is that the average surface creep rate increased after the Landers event and then decreased by a factor of 2-7 over the past few decades. To further understand these pronounced variations in average creep rate, we consider quasi-static and dynamic Coulomb stress changes caused by these three major events and their effect on fault creep. The dynamic Coulomb stress changes are similar for all three events, contributing to triggered creep on the SSAF. In contrast, the static Coulomb stress changes on the SSAF are positive after the Landers and negative after the Hector Mine and El Major Cucapah, coinciding with the higher average creep rate after the Landers event and lower rates after the other two events. The elevated creep rates after the Landers can only be explained by static stress changes, indicating that even in the presence of dynamically triggered creep, static stress change has a long-lasting effect on SSAF creep rates. These results suggest time-scale-dependent complexity of shallow fault creep under stress perturbations from regional earthquakes.