Combining InSAR and GPS data to distinguish coseismic and postseismic slip in the 2003 San Simeon and 2004 Parkfield earthquakes

Johanson, Ingrid A.; Bürgmann, Roland
Berkeley Seismological Laboratory; Ingrid@seismo.berkeley.edu

InSAR provides deformation measurements with tens-of-meters spatial resolution, providing detailed information on the locations and spatial distribution of deformation sources. However, interferograms provide a snapshot of ground motions within the time spanned by the two satellite acquisitions, with a minimum span of 1 month (for most satellites). Continuous GPS and other ground-based instruments can provide data with time resolutions of at least 1 second, but at a limited number of locations. By using information from GPS data on the temporal evolution of postseismic slip, we construct a model inversion scheme that separates coseismic from postseismic slip in the InSAR data. This allows us to leverage the high resolution of InSAR data to determine separate spatial distributions for each process. For the 2004 M6.0 Parkfield earthquake, continuous GPS data from 12 sites show a consistent postseismic decay pattern that is well fit by an exponential function. The decay time constant determined from the GPS data is used to construct Green?s functions relating the total amplitude of the postseismic slip to deformation within the given time periods of eight interferograms. Separate slip distributions for coseismic and postseismic slip are determined simultaneously during the model inversion and reveal that postseismic slip tended to occur in areas that did not slip coseismically and especially occurred up-dip and to the north of the coseismic rupture. The thrust-mechanism 2003 M6.5 San Simeon earthquake was more spatially and temporally complex. In this case, the InSAR data is not well fit by a single postseismic decay function and less GPS data is available to constrain the time dependence of the postseismic slip. Nonetheless a simultaneous inversion suggests shallow afterslip occurred on multiple structures including the main rupture plane and two backthrusts, and allows the possibility of coseismic slip on a backthrust structure.