InSAR Observations

We use SAR data from the European ERS-1 and ERS-2 satellites acquired between May 1992 and January 2001 to construct interferograms across the CSAF. Processing was carried out using the Caltech/JPL software ROI_PAC (Rosen et al., 2004), with a bridge unwrapping algorithm. Topographic effects were removed using a 90 m digital elevation model derived from Shuttle Radar Topography Mission (SRTM) data (Farr and Kobrick, 2000; Rodriguez et al., 2006). Agriculture in the Salinas valley and the San Joaquin basin results in temporal decorrelation in many of the interferograms, and steep topography, particularly on the northeast side of the fault, leads to geometrical decorrelation. Collectively, these zones of incoherence lead to isolated patches in the unwrapped interferograms. Figure 2.42 shows a stack of twelve interferograms which can be unwrapped consistently across the fault. The stacking process increases the signal-to-noise ratio above that of individual interferograms, so that a more robust identification of spatial variations in creep rate can be made. The stack assumes a linear velocity for each pixel. The fault is clearly delineated by the abrupt offset running northwest to southeast across the stack image. The displacement gradient near the fault is much greater than would be expected for a fault locked to the bottom of the seismogenic layer (about 12-15 km in this region), implying that significant shallow slip occurred during the decade of observation. The positive range change on the southwest part of the image is enhanced in the Salinas Valley. We surmise that this enhancement is due to subsidence caused by aquifer discharge in this highly agricultual area. The area of $>$ 10 mm/year positive range change in the southeast quadrant of the stack exactly coincides with the town of Coalinga and the nearby oil fields. It is possible that this range change anomaly is due to pumping of oil. The agreement of our 9-year creep rate with estimates obtained by other workers over earlier and/or longer periods of time show that creep rate on the years to decadal time scale has been approximately constant over the last 30 years. If there was any increase in creep rate as a result of either the 1857 Fort Tejon or the 1906 San Francisco earthquake, then presumably the rate has now levelled off.

Figure 2.42: Stack of 12 descending ERS interferograms made from SAR scenes acquired between May 1992 and January 2001. Colours show range change, which is the surface displacement in the line of sight between the satellite and the ground. Red is positive range change, i.e. motion away from the satelite. Arrows show satellite ground track (towards south-southwest) and look direction (towards west-northwest). Fault traces for the CSAF and Calaveras Fault are marked. The line-of-sight offset rate across the fault is 10 mm/year, which is equivalent to a right lateral displacement rate of 32 mm/year. Grey ellipse in bottom right outlines area of suspected subsidence, possibly related to oil pumping near Coalinga. Subsidence may also be occurring in the agricultural Salinas Valley.