Using InSAR and GPS-Constrained Block Models to Study the Garlock-San Andreas Fault Interaction
Greene, Fernando1; Wdowinski, Shimon1; Schmalzle, Gina2; Amelung, Falk1
1University of Miami - RSMAS, UNITED STATES; 2Earth and Space Sciences, University of Washington, UNITED STATES
One of the more intriguing tectonic regions in southern California is the intersection between the San Andreas Fault (SAF) and Garlock Fault (GF). The NW-SE oriented SAF accommodates most of the relative motion between the Pacific and North American plates and is seismically very active. The E-W oriented GF accommodates strain due to the relative motion of the Sierra and Mojave blocks and has low seismicity levels. Paleoseismic studies suggest that the GF ruptures in large magnitude earthquakes that may be temporally clustered. The paleoseismic evidence combined with the lack of recent activity raises concerns about likely future events on the GF.
In this study we use InSAR observations and GPS-constrained block models to study the contemporary velocity field near the intersection of the GF and SAF at the western end of the Mojave block between 1992 and 2010. We process a total of 204 ERS 1,2 and Envisat interferograms that are analyzed using the SBAS algorithm to generate time series of ground displacements and average velocities for a ~300 km long by 100 km wide swath, covering part of the Eastern California Shear Zone in the north, and the western end of Mojave block in the south. Both our ERS and Envisat time-series mean velocity maps show a broad area of Line of Sight (LOS) shortening located at the western end of the Mojave block, as well as localized areas of LOS elongation. Most regions experiencing LOS elongation, such as Antelope basin, reflect subsidence due to groundwater pumping. The broad area of LOS shortening suggests that the western Mojave block is is either uplifting or moving eastward with respect to the Sierra and the western section of the San Andreas at a rate of 3-4 mm/yr. Strain accumulation due to interseismic locking generates a long wavelength signal (>30 km) that cannot be detected by InSAR because of orbital uncertainties. We use GPS-constrained block models to evaluate the long-wavelength signal and found roughly half of the observed InSAR motion can be explained by interseismic motion. Due to the physical complexity of the study area, we observe that the GF and White Wolf fault (WWF) geometries play an important role in the stress regime. We vary the dip direction on these faults to find a low misfit model that suggests that GF is dipping 45° to the north and WWF dipping 45° to the south, or alternatively GF is dipping 45° to the north and WWF dipping 90°. However this result may be biased since vertical GPS is not used in the model inversion, and may be necessary in this physically complex region.