We continue to amass a large database of Synthetic Aperture Radar (SAR) data for the San Francisco Bay Area. This extensive data set provides a unique opportunity to monitor crustal deformation caused by land subsidence from aquifer withdrawal, seasonal cycles of aquifer rehydration, surface slip on Bay Area faults, and regional strain accumulation (Figure 26.1). As the number of potential interferograms increase, organizing the available information into a manageable state that can be easily interpreted becomes a difficult task. We have developed a technique that allows us to synthesize differential Interferometric SAR (DInSAR) images of varying time windows from 1992 to the present using a linear inversion approach. The technique uses information from multiple differential interferograms to infer the deformation during a given time step for a given region. The result is a timeseries of crustal deformation covering most urban regions within the Bay Area. This technique allows seasonal deformation to be easily distinguished from linear deformation such as the build up of strain across the plate boundary.
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While producing a spatially dense data set, InSAR provides only one component of the displacement field along a baseline from the satellite to the ground. When interpreting a differential interferogram, assumptions must be made about the contributions from vertical versus horizontal deformation. These assumptions are valid in regions where one process dominates. For example, most deformation is vertical in basins where fluctuations in the water table dominate deformation while horizontal deformation dominates along creeping, strike-slip faults. In several regions, both processes are active and making assumptions regarding the ratio of vertical to horizontal motion can result in inaccurate results. This inadequacy can be overcome by utilizing SAR data collected from different satellite orientations. We attempt to resolve this ambiguity by analyzing data from both ascending and descending trajectories of the European Space Agency's ERS spacecraft. By assuming zero motion along a third component, this technique allows two components of the displacement field to be resolved. Assigning which component has nonzero displacement must be evaluated carefully since any nonzero value will be mapped into either of the other two components. In the Bay Area we can assume deformation occurs either in the vertical or fault parallel components. Using this technique we have attempted to resolve the magnitude of deformation along the southern Hayward fault that is fault parallel versus vertical deformation believed to reflect subsidence in the adjoining basin bounded by the fault.
Schmidt, D. A., and R. Bürgmann, Observations of time-dependent fault creep using InSAR, EOS TRANS. AGU, 80 (46), 267, 1999.
Schmidt, D. A., and R. Bürgmann, Deciphering the transient dip-slip behavior on the southern segment of the Hayward Fault, California, The International Symposium on GPS, Tsukuba, Japan, 1999.