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Creep on the San Andreas fault near San Juan Bautista and its relationship to large historic earthquakes
-- Ingrid Johanson (2000-2005) and Roland Bürgmann

InSAR stack from data spanning 5.75 years, scaled to yearly rate. White circles outline the Hollister and Santa Clara Valley Basins where groundwater recharge results in non-tectonic uplift. These and other areas located on Quaternary sediment were removed before the model inversion. White triangle is the location of creepmeter XSJ2.
The San Juan Bautista segment of the San Andreas fault is an area of moderate seismicity that forms the transition zone between the creeping section and the locked Santa Cruz segment. We examine current creep on the San Juan Bautista segment using space geodetic data to determine whether it allows a similar size and rate of earthquake production as seen in the historic record. InSAR data is taken from a stack of nine interferogram pairs made using ERS1 and ERS2 data collected between 1995-2001. We use a stack of small time span (less than 1.25 years) pairs because interferograms in this area become completely decorrelated when they span much more than two years. We choose the stacked interferograms such that the slave in one pair is the master in the next to remove the atmospheric errors from all but the very first and very last scenes. Our stack is then, in effect, a six year interferogram. Our model includes deep slip on the regional fault network, shallow creep on the nearby Calaveras fault and distributed creep on the San Juan Bautista segment. We find that below 3 km depth, the creep rate falls off considerably, causing a low slip zone at mid-seismogenic depths in the northern part of the San Juan Bautista segment that may represent the source region for some of the 19th century earthquakes.

Variations in creep rate along the central San Andreas Fault
-- Isabelle Ryder (2007-present) and Roland Bürgmann

Stack of 12 descending ERS interferograms made from SAR scenes (track 27, frame 2781) acquired between May 1992 and January 2001. Fault traces for the CSAF and Calaveras-Paicines Fault (CPF) are marked. Arrows show satellite ground track and look direction. Dashed black line marks outcrop of New Idria serpentinite (NIS). The enhanced positive range change in the Salinas Valley is likely due to agricultural activity, and the anomaly at Coalinga is probably due to oil pumping.
We use ERS InSAR measurements to record spatial variations in creep rate along the creeping segment of the San Andreas Fault, California, between 1992 and 2001. An interferometric stack is constructed from twelve interferograms which show good coherence. For the decade of observation, the total right-lateral offset across the fault is ~32 mm/year. Along most of the length of the creeping segment, this offset occurs within a narrow (< 2 km) zone close to the fault trace, which indicates that shallow creep occurs. In the northern part, a minor part of the offset is taken up by the Calaveras-Paicines Fault, which comes very close to the San Andreas Fault, and so the deformation appears more distributed. In general, the observed rates of surface creep are consistent with those obtained by several other studies for a longer and/or earlier period of time, using different geodetic methods. This suggests that average creep rate has been constant over a period of almost four decades. We invert our InSAR measurements, as well as GPS velocities from Plate Boundary Observatory permanent sites and various campaigns, for the sub-surface slip rate on the creeping segment. Along-strike moment release due to creep in the seismogenic layer is approximately inversely correlated with a profile of seismic moment release, for all earthquakes that occurred during the time covered by the stack. This may be a consequence of small earthquakes occurring on patches that slip more slowly, perhaps on account of minor asperities or different physical properties at the fault interface.