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Wei, M, Sandwell DT.  2010.  Decorrelation of L-Band and C-Band Interferometry Over Vegetated Areas in California. IEEE Transactions on Geoscience and Remote Sensing. 48:2942-2952.   10.1109/tgrs.2010.2043442   AbstractWebsite

Temporal decorrelation is one of the main limitations for recovering interseismic deformation along the San Andreas Fault system using interferometric synthetic aperture radar. To assess the improved correlation properties of L-band with respect to C-band, we analyzed L-band Advanced Land Observation Satellite (ALOS) interferograms with a range of temporal and spatial baselines over three vegetated areas in California and compared them with corresponding C-band European Remote Sensing Satellite (ERS) interferograms. Over the highly vegetated Northern California forests in the Coast Range area, ALOS remains remarkably well correlated over a 2-year period, whereas an ERS interferogram with a similar temporal and spatial baseline lost correlation. In Central California near Parkfield, we found a similar pattern in decorrelation behavior, which enabled the recovery of a fault creep and a local uplifting signal at L-band that was not apparent at C-band. In the Imperial Valley in Southern California, both ALOS and ERS have low correlation over farmlands. ALOS has lower correlation over some sandy surfaces than ERS, probably due to low signal-to-noise ratio. In general, L-band interferograms with similar seasonal acquisitions have higher correlation than those with dissimilar season. For both L-and C-band, correlation over vegetated areas decreases with time for intervals less than 1 year and then remains relatively constant at longer time intervals. The decorrelation time for L-band is more than 2 years in the forest in California whereas that for C-band is less than 6 months. Overall, these results suggest that L-band interferograms will reveal near-fault interseismic deformation once sufficient data become available.

Fialko, Y, Sandwell D, Agnew D, Simons M, Shearer P, Minster B.  2002.  Deformation on nearby faults induced by the 1999 Hector Mine earthquake. Science. 297:1858-1862.   10.1126/science.1074671   AbstractWebsite

Interferometric Synthetic Aperture Radar observations of surface deformation due to the 1999 Hector Mine earthquake reveal motion on several nearby faults of the eastern California shear zone. We document both vertical and horizontal displacements of several millimeters to several centimeters across kilometer-wide zones centered on pre-existing faults. Portions of some faults experienced retrograde (that is, opposite to their long-term geologic slip) motion during or shortly after the earthquake. The observed deformation likely represents elastic response of compliant fault zones to the permanent co-seismic stress changes. The induced fault displacements imply decreases in the effective shear modulus within the kilometer-wide fault zones, indicating that the latter are mechanically distinct from the ambient crustal rocks.

O'Connor, JM, Hoernle K, Muller RD, Morgan JP, Butterworth NP, Hau F, Sandwell DT, Jokat W, Wijbrans JR, Stoffers P.  2015.  Deformation-related volcanism in the Pacific Ocean linked to the Hawaiian-Emperor bend. Nature Geoscience. 8:393-397.   10.1038/ngeo2416   AbstractWebsite

Ocean islands, seamounts and volcanic ridges are thought to form above mantle plumes. Yet, this mechanism cannot explain many volcanic features on the Pacific Ocean floor(1) and some might instead be caused by cracks in the oceanic crust linked to the reorganization of plate motions(1-3). A distinctive bend in the Hawaiian-Emperor volcanic chain has been linked to changes in the direction of motion of the Pacific Plate(4,5), movement of the Hawaiian plume(6-8), or a combination of both(9). However, these links are uncertain because there is no independent record that precisely dates tectonic events that affected the Pacific Plate. Here we analyse the geochemical characteristics of lava samples collected from the Musicians Ridges, lines of volcanic seamounts formed close to the Hawaiian-Emperor bend. We find that the geochemical signature of these lavas is unlike typical ocean island basalts and instead resembles mid-ocean ridge basalts. We infer that the seamounts are unrelated to mantle plume activity and instead formed in an extensional setting, due to deformation of the Pacific Plate. Ar-40/Ar-39 dating reveals that the Musicians Ridges formed during two time windows that bracket the time of formation of the Hawaiian-Emperor bend, 53-52 and 48-47 million years ago. We conclude that the Hawaiian-Emperor bend was formed by plate-mantle reorganization, potentially triggered by a series of subduction events at the Pacific Plate margins.

Mueller, D, Sandwell DT, Tucholke BE, Sclater JG, Shaw PR.  1991.  Depth to basement and geoid expression of the Kane Fracture Zone: A comparison. Marine Geophysical Researches. 13:105-129. AbstractWebsite

Geoid data from Geosat and subsatellite basement depth profiles of the Kane Fracture Zone in the central North Atlantic were used to examine the correlation between the short-wavelength geoid ( lambda = 25-100 km) and the uncompensated basement topography. The processing technique we apply allows the stacking of geoid profiles, although each repeat cycle has an unknown long-wavelength bias. We first formed the derivative of individual profiles, stacked up to 22 repeat cycles, and then integrated the average-slope profile to reconstruct the geoid height. The stacked, filtered geoid profiles have a noise level of about 7 mm in geoid height. The subsatellite basement topography was obtained from a recent compilation of structure contours on basement along the entire length of the Kane Fracture Zone.

Sandwell, DT.  1984.  A Detailed View of the South-Pacific Geoid from Satellite Altimetry. Journal of Geophysical Research. 89:1089-1104.   10.1029/JB089iB02p01089   AbstractWebsite

Images of sea surface undulations in the South Pacific have been constructed from GEOS 3 and SEASAT altimeter data. Height discrepancies at crossover points, associated with long-wavelength radial orbit error, were suppressed by taking along-track derivatives of the ascending and descending profiles. These geoid slopes were then rotated and scaled to produce the north and east components of the deflection of the vertical. Finally, the results are displayed by using the hill shading technique, where gray-tone images represent the innner product of the deflection vector with an assigned sun vector. Less apparent sea surface undulations can be enhanced by varying the sun's zenith and azimuth. Shorterwavelength sea surface undulations reflect seafloor topography. For instance, fracture zones (FZ's) appear as elongated sharp steps in the sea surface, while seamounts produce circular bumps. Since large areas of the South Pacific are unsurveyed, many previously undetected features appear on the images. Comparisons with bathymetric charts reveal 72 uncharted seamounts having geoid expressions greater than or equal to Easter Island's expression. The dominant features in the images, however, are the large age-offset FZ's such as the Eltanin and Udintsev FZ's. The images reveal that the Eltanin FZ is connected to the Louisville Ridge; combined they produce a continuous geoid signature across most of the South Pacific. This supports the hypothesis of Hayes and Ewing (1968) that the Louisville Ridge is the northwest extension of the Eltanin FZ.

Trugman, DT, Borsa AA, Sandwell DT.  2014.  Did stresses from the Cerro Prieto Geothermal Field influence the El Mayor-Cucapah rupture sequence? Geophysical Research Letters. 41:8767-8774.   10.1002/2014gl061959   AbstractWebsite

The M-w 7.2 El Mayor-Cucapah (EMC) earthquake ruptured a complex fault system in northern Baja California that was previously considered inactive. The Cerro Prieto Geothermal Field (CPGF), site of the world's second largest geothermal power plant, is located approximately 15km to the northeast of the EMC hypocenter. We investigate whether anthropogenic fluid extraction at the CPGF caused a significant perturbation to the stress field in the EMC rupture zone. We use Advanced Land Observing Satellite interferometric synthetic aperture radar data to develop a laterally heterogeneous model of fluid extraction at the CPGF and estimate that this extraction generates positive Coulomb stressing rates of order 15 kPa/yr near the EMC hypocenter, a value which exceeds the local tectonic stressing rate. Although we cannot definitively conclude that production at the CPGF triggered the EMC earthquake, its influence on the local stress field is substantial and should not be neglected in local seismic hazard assessments.

Wdowinski, S, Smith-Konter B, Bock Y, Sandwell D.  2007.  Diffuse interseismic deformation across the Pacific-North America plate boundary. Geology. 35:311-314.   10.1130/g22938a.1   AbstractWebsite

Crustal movements and deformation within the diffuse Pacific-North America (Pa-NA) plate boundary are dominated by the right-lateral motion between the two plates. By using the Pa-NA pole of rotation (PoR) spherical coordinate system, we decompose observed crustal movements into parallel and normal components to the Pa-NA plate motion. We transformed the 840 velocity vectors of the Southern California Earthquake Center (SCEC) 3.0 velocity field into the Pa-NA PoR system in order to characterize the interseismic velocity across the plate boundary. Our results show that despite the very different deformation styles occurring across the San Andreas fault, the fault trace follows the half plate motion contour. Deviation occurs in the southern section, where the half motion contour correlates with the San Jacinto and Imperial fault segments. Our analysis yields interesting asymmetric patterns in both parallel and normal components. The parallel component shows asymmetrical velocity gradients across the San Andreas fault, and the normal component indicates compression southwest of the Big Bend, but not northeastward. The observations are compared with viscoelastic modeling results, which show a similar velocity field. The main disagreements between the observations and the model are in a narrow band along the San Andreas fault and in the Mojave block, suggesting that crustal heterogeneities and additional unmodeled fault segments should be considered in future models.

Sandwell, DT, Johnson CL, Bilotti F, Suppe J.  1997.  Driving forces for limited tectonics on Venus. Icarus. 129:232-244.   10.1006/icar.1997.5721   AbstractWebsite

The very high correlation of geoid height and topography on Venus, along with the high geoid topography ratio, can be interpreted as local isostatic compensation and/or dynamic compensation of topography at depths ranging from 50 to 350 km. For local compensation within the lithosphere, the swell-push force is proportional to the first moment of the anomalous density. Since the long-wavelength isostatic geoid height is also proportional to the first moment of the anomalous density, the swell push force is equal to the geoid height scaled by -g(2)/2 pi G. Because of this direct relationship, the style (i.e., thermal, Airy, or Pratt compensation) and depth of compensation do not need to be specified and can in fact vary over the surface. Phillips (1990) showed that this simple relationship between swell-push force and geoid also holds for dynamic uplift by shear traction on the base of the lithosphere caused by thermal convection of an isoviscous, infinite half-space mantle. Thus for all reasonable isostatic models and particular classes of dynamic models, the geoid height uniquely determines the magnitude of the swell-push body force that is applied to the venusian lithosphere. Given this body force and assuming Venus can be approximated by a uniform thickness thin elastic shell over an inviscid sphere, we calculate the present-day global strain field using equations given in Banerdt (1986); areas of positive geoid height are in a state of extension while areas of negative geoid height are in a state of compression. The present-day model strain field is compared to global strain patterns inferred from Magellan-derived maps of wrinkle ridges and rift zones. Wrinkle ridges, which are believed to reflect distributed compressive deformation, are generally confined to regions with geoid of less than 20 m while rift zones are found primarily along geoid highs. Moreover, much of the observed deformation matches the present-day model strain orientations suggesting that most of the rifts on Venus and many of the wrinkle ridges formed in a stress field similar to the present one. In several large regions, the present-day model strain pattern does not match the observations. This suggests that either the geoid has changed significantly since most of the strain occurred or our model assumptions are incorrect (e.g., there could be local plate boundaries where the stress pattern is discontinuous). Since the venusian lithosphere shows evidence for limited strain, the calculation also provides an estimate of the overall strength of the lithosphere in compression and extension which can be compared with rheological models of yield strength versus depth. At the crests of the major swells, where evidence for rifting is abundant, we find that the temperature gradient must be at least 7 K/km. (C) 1997 Academic Press.