Investigating fault creep variability along the southern San Andreas fault through numerical modeling and space geodetic observations

Shallow fault creep as part of aseismic faulting processes has been commonly observed on several strike-slip faults in California including the southern San Andreas fault (SSAF). This shallow creep can be continuous, episodic, or a combination of the two. To better quantify and understand the spatiotemporal variation of fault creep along the SSAF, we analyze decades long multiple satellite InSAR data including ERS, Envisat and recent Sentinel-1, which jointly span more than 25 years. Our 18-year InSAR time series from the combined analysis of ERS and Envisat satellites, along with recent observations from Sentinel-1 and in-situ creepmeter measurements, shows clear spatiotemporally varying transient behavior of surface creep along the SSAF. We develop laboratory-based rate-and-state friction earthquake cycle models to investigate what controls the observed fault creep variability along the SSAF. Especially we attempt to answer: To what extent is such variation controlled by inherent frictional properties versus external stress perturbations? How does the shallow creep regime interact with the seismogenic zone at depth? What are the implications toward a future large earthquake on the SSAF that is thought overdue? Our numerical modeling results show that shallow creep events are prone to external stress perturbations. Even a relatively small perturbation, on the order of several KPa, can lead to a noticeable change of transient fault creep behavior. Longer term transient fault creep behavior occurs following each earthquake. We find the interseismic fault creep represents a small fraction of the long term slip rate. This may imply that the slip is more distributed at shallow depth, or that a large portion of the shallow slip deficit will be eventually released by a future large earthquake.