Searching for Transient Slow Slips Along the San Andreas Fault Near Parkfield Using Independent Component Analysis.

The Parkfield segment of the San Andreas Fault (SAF) sits at the transition between the locked Cholame segment to the South and the SAF creeping segment to the North. The Parkfield segment hosts regular ∼Mw $\sim {\boldsymbol{M}}_{\boldsymbol{w}}$6 earthquakes followed by postseismic deformation. Recent studies based on geodetic data have highlighted spatial and temporal variations of aseismic slip rate in addition to postseismic slip along this section of the fault. We combine Global Navigation Satellite Systems (GNSS) and seismicity data over the 2006–2018 period to detail a comprehensive picture of transient slip events. We produce a catalog of relocated seismicity and repeating earthquakes. We use a variational Bayesian independent component analysis decomposition on GNSS data to separate geodetic deformation due to non‐tectonic sources from signals of potential tectonic origin. We then reconstruct the temporal evolution of fault slip and detect potential slip transients. Those events, determined as mostly aseismic with the exception of one related to a Mw ${\boldsymbol{M}}_{\boldsymbol{w}}$4.8 earthquake, occur more frequently during the 2004 Mw ${\boldsymbol{M}}_{\boldsymbol{w}}$6 post‐seismic period than during the subsequent inter‐seismic phase. Our study illustrates the rich dynamics of seismic and aseismic slip during both post‐ and inter‐seismic periods along active faults. Plain Language Summary: Faults can slip abruptly, generating earthquakes and seismic waves, or slowly and aseismically. Such a slow motion has now been observed along multiple active faults and is recognized as one of the important processes influencing the Earth's crust stress field. Aseismic slip interacts with earthquakes, and monitoring its time evolution on faults help us in better modeling the seismic cycle. Here we focus our attention on the Parkfield fault segment, sitting along the San Andreas Fault in California in between a section that generated a large earthquake (M7.9) in 1857 and a 150‐km‐long central creeping section that slips aseismically. We dig in noisy time series of surface displacements measured by GNSS (Global Navigation Satellite System) to detect and model small variations of aseismic slip rate (10–100 mm/year) along the fault. We find multiple candidate slow slip events during the period following the 2004 Parkfield earthquake, confirming and expanding previous independent observations. While aseismic slip following earthquakes was first thought to be a steady release of tectonic stress, we show that, in Parkfield, it is evolving with sudden jumps and slowdown. Key Points: Independent Component Analysis of GPS time series highlights slip transients along the San Andreas Fault after the 2004 M6 Parkfield eventWe document the spatio‐temporal distribution of seismic and aseismic slip historySlow Slip Events are more frequent during the 2004 M6 post‐seismic period than the subsequent interseismic period [ABSTRACT FROM AUTHOR]