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Howell, S, Smith-Konter B, Frazer N, Tong XP, Sandwell D.  2016.  The vertical fingerprint of earthquake cycle loading in southern California. Nature Geoscience. 9:611-+.   10.1038/ngeo2741   AbstractWebsite

The San Andreas Fault System, one of the best-studied transform plate boundaries on Earth, is well known for its complex network of locked faults that slowly deform the crust in response to large-scale plate motions(1-8). Horizontal interseismic motions of the fault system are largely predictable, but vertical motions arising from tectonic sources remain enigmatic. Here we show that when carefully treated for spatial consistency, global positioning system-derived vertical velocities expose a small-amplitude (+/- 2mmyr(-1)), but spatially considerable (200 km), coherent pattern of uplift and subsidence straddling the fault system in southern California. We employ the statistical method of model selection to isolate this vertical velocity field fromnon-tectonic signals that induce velocity variations in both magnitude and direction across small distances (less than tens of kilometres; ref. 9), and find remarkable agreement with the sense of vertical motions predicted by physical earthquake cycle models spanning the past few centuries(6,10). We suggest that these motions reveal the subtle, but identifiable, tectonic fingerprint of far-field flexure due to more than 300 years of fault locking and creeping depth variability. Understanding this critical component of interseismic deformation at a complex strike-slip plate boundary will better constrain regional mechanics and crustal rheology, improving the quantification of seismic hazards in southern California and beyond.

Barbot, S, Fialko Y, Sandwell D.  2009.  Three-dimensional models of elastostatic deformation in heterogeneous media, with applications to the Eastern California Shear Zone. Geophysical Journal International. 179:500-520.   10.1111/j.1365-246X.2009.04194.x   AbstractWebsite

P>We present a semi-analytic iterative procedure for evaluating the 3-D deformation due to faults in an arbitrarily heterogeneous elastic half-space. Spatially variable elastic properties are modelled with equivalent body forces and equivalent surface traction in a 'homogenized' elastic medium. The displacement field is obtained in the Fourier domain using a semi-analytic Green function. We apply this model to investigate the response of 3-D compliant zones (CZ) around major crustal faults to coseismic stressing by nearby earthquakes. We constrain the two elastic moduli, as well as the geometry of the fault zones by comparing the model predictions to Synthetic Aperture Radar inferferometric (InSAR) data. Our results confirm that the CZ models for the Rodman, Calico and Pinto Mountain faults in the Eastern California Shear Zone (ECSZ) can explain the coseismic InSAR data from both the Landers and the Hector Mine earthquakes. For the Pinto Mountain fault zone, InSAR data suggest a 50 per cent reduction in effective shear modulus and no significant change in Poisson's ratio compared to the ambient crust. The large wavelength of coseismic line-of-sight displacements around the Pinto Mountain fault requires a fairly wide (similar to 1.9 km) CZ extending to a depth of at least 9 km. Best fit for the Calico CZ, north of Galway Dry Lake, is obtained for a 4 km deep structure, with a 60 per cent reduction in shear modulus, with no change in Poisson's ratio. We find that the required effective rigidity of the Calico fault zone south of Galway Dry Lake is not as low as that of the northern segment, suggesting along-strike variations of effective elastic moduli within the same fault zone. The ECSZ InSAR data is best explained by CZ models with reduction in both shear and bulk moduli. These observations suggest pervasive and widespread damage around active crustal faults.

Price, EJ, Sandwell DT.  1998.  Small-scale deformations associated with the 1992 Landers, California, earthquake mapped by synthetic aperture radar interferometry phase gradients. Journal of Geophysical Research-Solid Earth. 103:27001-27016.   10.1029/98jb01821   AbstractWebsite

The Landers earthquake (M-w 7.3) occurred on June 28, 1992, and ruptured nearly 100 km of previously mapped and unmapped faults in the Mojave Desert. We use synthetic aperture radar interferometry (InSAR) to examine the cumulative surface deformation between April 24 and August 7, 1992, in a 100 x 100 km region surrounding the northern portion of the earthquake rupture. Also, we introduce a technique for manipulating SAR interferograms to extract short-wavelength displacement information. This technique involves computation and subsequent combination of interferometric phase gradient maps. The InSAR results show significant deformation signatures associated with faults, fractures, dry lake beds, and mountainous regions within 75-100 km of the main rupture. Using the phase gradient method, we are able to extract small-scale deformation patterns near the main rupture. Many of the preexisting, mapped faults within 50 km of the main rupture experienced triggered slip; these include the Old Woman, Lenwood, Johnson Valley, West Calico, and Calico Faults. The InSAR results also indicate right-lateral offsets along secondary fractures trending N-NE within the left-lateral zone of shear between the main rupture and the Johnson Valley Fault. Additionally, there are interesting interferogram fringe signatures surrounding Troy Dry Lake and Coyote Dry Lake that are related to deformation of dry lake beds.