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Journal Article
Sandwell, DT, Myer D, Mellors R, Shimada M, Brooks B, Foster J.  2008.  Accuracy and Resolution of ALOS Interferometry: Vector Deformation Maps of the Father's Day Intrusion at Kilauea. Ieee Transactions on Geoscience and Remote Sensing. 46:3524-3534.   10.1109/tgrs.2008.2000634   AbstractWebsite

We assess the spatial resolution and phase noise of interferograms made from L-band Advanced Land Observing Satellite (ALOS) synthetic-aperture-radar (SAR) data and compare these results with corresponding C-band measurements from European Space Agency Remote Sensing Satellite (ERS). Based on cross-spectral analysis of phase gradients, we find that the spatial resolution of ALOS interferograms is 1.3x better than ERS interferograms. The phase noise of ALOS (i.e., line-of-sight precision in the 100-5000-m wavelength band) is 1.6x worse than ERS (3.3 mm versus 2.1 mm). In both cases, the largest source of error is tropospheric phase delay. Vector deformation maps associated with the June 17, 2007 (Father's day) intrusion along the east rift zone of the Kilauea Volcano were recovered using just four ALOS SAR images from two look directions. Comparisons with deformation vectors from 19 continuous GPS sites show rms line-of-site precision of 14 mm and rms azimuth precision (flight direction) of 71 mm. This azimuth precision is at least 4x better than the corresponding measurements made at C-band. Phase coherence is high even in heavily vegetated areas in agreement with previous results. This improved coherence combined with similar or better accuracy and resolution suggests that L-band ALOS will outperform C-band ERS in the recovery of slow crustal deformation.

Marks, KM, Sandwell DT.  1991.  Analysis of Geoid Height Versus Topography for Oceanic Plateaus and Swells Using Nonbiased Linear-Regression. Journal of Geophysical Research-Solid Earth and Planets. 96:8045-8055.   10.1029/91jb00240   AbstractWebsite

We have investigated the relationship between geoid height and topography for 53 oceanic plateaus and swells to determine the mode of compensation. The ratio of geoid height to topography was obtained from the slope of a best line fit by functional analysis (i.e. nonbiased linear regression), a method that minimizes both geoid height and topography residuals. This method is more appropriate than traditional least squares analysis that minimizes only geoid height residuals, because uncertainties are present in both data types. We find that approximately half of the oceanic and continental plateaus analyzed have low ratios that are consistent with Airy-compensated crustal thickening. The remaining plateaus, however, have higher geoid/topography ratios than predicted by the simple Airy model, and the seismically determined Moho depths beneath some of these features are too shallow for crustal thickening alone. A two-layer Airy compensation model, composed of thickened crust underlain by an anomalously low density "mantle root", is used to explain these observations. The Walvis Ridge, and the Agulhas, Crozet, and north Kerguelen plateaus have geoid/topography ratios and Moho depths that are consistent with the two-layer Airy model. The proximity of the Agulhas Plateau to a RRR triple junction during its early development, and the excessive volcanism at active spreading ridges that created the Crozet and north Kerguelen plateaus and the Walvis Ridge, may have produced regions of enhanced depletion and hence the low-density mantle anomalies. If this explanation is correct, then the low-density mantle anomaly persists over time and remains embedded in the lithosphere beneath the oceanic feature.

Cheney, RE, Douglas BC, Sandwell DT, Marsh JG, Martin TV.  1984.  Applications of Satellite Altimetry to Oceanography and Geophysics. Marine Geophysical Researches. 7:17-32.   10.1007/bf00305408   AbstractWebsite

Satellite-borne altimeters have had a profound impact on geodesy, geophysics, and physical oceanography. To first order approximation, profiles of sea surface height are equivalent to the geoid and are highly correlated with seafloor topography for wavelengths less than 1000 km. Using all available Geos-3 and Seasat altimeter data, mean sea surfaces and geoid gradient maps have been computed for the Bering Sea and the South Pacific. When enhanced using hill-shading techniques, these images reveal in graphic detail the surface expression of seamounts, ridges, trenches, and fracture zones. Such maps are invaluable in oceanic regions where bathymetric data are sparse. Superimposed on the static geoid topography is dynamic topography due to ocean circulation. Temporal variability of dynamic height due to oceanic eddies can be determined from time series of repeated altimeter profiles. Maps of sea height variability and eddy kinetic energy derived from Geos-3 and Seasat altimetry in some cases represent improvements over those derived from standard oceanographic observations. Measurement of absolute dynamic height imposes stringent requirements on geoid and orbit accuracies, although existing models and data have been used to derive surprisingly realistic global circulation solutions. Further improvement will only be made when advances are made in geoid modeling and precision orbit determination. In contrast, it appears that use of altimeter data to correct satellite orbits will enable observation of basin-scale sea level variations of the type associated with climatic phenomena.

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, Winterer EL, Mammerickx J, Duncan RA, Lynch MA, Levitt DA, Johnson CL.  1995.  Evidence for Diffuse Extension of the Pacific Plate from Pukapuka Ridges and Cross-Grain Gravity Lineations. Journal of Geophysical Research-Solid Earth. 100:15087-15099.   10.1029/95jb00156   AbstractWebsite

Satellite altimeter measurements of marine gravity reveal 100 to 200-km wavelength lineations over a wide area of the Pacific plate oriented roughly in the direction of absolute plate motion. At least three mechanisms have been proposed for their origin: small-scale convective rolls aligned in the direction of absolute plate motion by shear in the asthenosphere; diffuse N-S extension of the lithosphere resulting in lineated zones of extension (boudins); and minihotspots that move slowly with respect to major hotspots and produce intermittent volcanism. Recently, several chains of linear volcanic ridges have been found to be associated with the gravity lineations. Following ridgelike gravity signatures apparent in high-resolution Geosat gravity measurements, we surveyed a series of volcanic ridges that extend northwest from the East Pacific Rise flank for 2600 km onto 40 Ma seafloor. Our survey data, as well as radiometric dates on samples we collected from the ridges, provide tight constraints on their origin: (1) Individual ridge segments and sets of ridges are highly elongate in the direction of present absolute plate motion. (2) The ridges formed along a band 50 to 70-km-wide in the trough of one of the more prominent gravity lineations. (3) Radiometric dates of the largest ridges show no hotspot age progression. Moreover, the directions predicted for minihotspot traces older than 24 Ma do not match observed directions of either the gravity lineations or the ridges. Based on this last observation, we reject the minihotspot model. The occurrence of the ridges in the trough of the gravity lineation is incompatible with the small-scale convection model which would predict increased volcanism above the convective upwelling. We favor the diffuse extension model because it is consistent with the occurrence of ridges in the trough above the more highly extended lithosphere. However, the multibeam data show no evidence for widespread normal faulting of the crust as predicted by the model. Perhaps the fault scarps are buried under more than 30 m of sediments and/or covered by the elongated ridges. Finally, we note that if ridge-push force is much smaller than trench-pull force, then near the ridge axis the direction of maximum tensile stress must be perpendicular to the direction of absolute plate motion.

Marks, KM, Smith WHF, Sandwell DT.  2010.  Evolution of errors in the altimetric bathymetry model used by Google Earth and GEBCO. Marine Geophysical Research. 31:223-238.   10.1007/s11001-010-9102-0   AbstractWebsite

We analyze errors in the global bathymetry models of Smith and Sandwell that combine satellite altimetry with acoustic soundings and shorelines to estimate depths. Versions of these models have been incorporated into Google Earth and the General Bathymetric Chart of the Oceans (GEBCO). We use Japan Agency for Marine-Earth Science and Technology (JAMSTEC) multibeam surveys not previously incorporated into the models as "ground truth" to compare against model versions 7.2 through 12.1, defining vertical differences as "errors." Overall error statistics improve over time: 50th percentile errors declined from 57 to 55 to 49 m, and 90th percentile errors declined from 257 to 235 to 219 m, in versions 8.2, 11.1 and 12.1. This improvement is partly due to an increasing number of soundings incorporated into successive models, and partly to improvements in the satellite gravity model. Inspection of specific sites reveals that changes in the algorithms used to interpolate across survey gaps with altimetry have affected some errors. Versions 9.1 through 11.1 show a bias in the scaling from gravity in milliGals to topography in meters that affected the 15-160 km wavelength band. Regionally averaged (> 160 km wavelength) depths have accumulated error over successive versions 9 through 11. These problems have been mitigated in version 12.1, which shows no systematic variation of errors with depth. Even so, version 12.1 is in some respects not as good as version 8.2, which employed a different algorithm.

McKenzie, D, Ford PG, Johnson C, Parsons B, Sandwell D, Saunders S, Solomon SC.  1992.  Features on Venus Generated by Plate Boundary Processes. Journal of Geophysical Research-Planets. 97:13533-13544.   10.1029/92JE01350   AbstractWebsite

Various observations suggest that there are processes on Venus that produce features similar to those associated with plate boundaries on Earth. Synthetic aperture radar images of Venus, taken with a radar whose wavelength is 12.6 cm, are compared with GLORIA images of active plate boundaries, obtained with a sound source whose wavelength is 23 cm. Features similar to transform faults and to abyssal hills on slow and fast spreading ridges can be recognized within the Artemis region of Venus but are not clearly visible elsewhere. The composition of the basalts measured by the Venera 13 and 14 and the Vega 2 spacecraft corresponds to that expected from adiabatic decompression, like that which occurs beneath spreading ridges on Earth. Structures that resemble trenches are widespread on Venus and show the same curvature and asymmetry as they do on Earth. These observations suggest that the same simple geophysical models that have been so successfully used to understand the tectonics of Earth can also be applied to Venus.

McAdoo, DC, Sandwell DT.  1985.  Folding of Oceanic Lithosphere. Journal of Geophysical Research-Solid Earth and Planets. 90:8563-8569.   10.1029/JB090iB10p08563   AbstractWebsite

Folding of the lithosphere just south of the Bay of Bengal appears as (1) undulations in acoustic basement topography and (2) as linear geoid undulations in the Seasat altimeter data. From the Seasat data we find that the east-west trending folds have wavelengths ranging from 130 to 250 km and clustering about 190 km. The horizontal gravity disturbances due to the folds range in amplitude from 15 to 50 mGal. Elastic models of oceanic lithosphere have, in the past, been used to demonstrate the implausibility of lithosphere buckling, or folding, in response to compression. These elastic models typically predict that compressive stresses of about 5 GPa are required to buckle oceanic lithosphere with an age comparable to that of the northeastern Indian Ocean (40–70 Ma). These stresses exceed the strength of lithospheric rock. We use an elastic-plastic model to show that oceanic lithosphere of this age should have a net compressive strength equal to about 12% of the elastic buckling stress. We further demonstrate that loads approaching the net compressive strength cause the lithosphere to fold with a wavelength about 200 km, i.e., the wavelength observed from Seasat. Our results reinforce earlier speculation that this folding may be related to the Himalayan orogeny.

Sandwell, DT, Mackenzie KR.  1989.  Geoid Height Versus Topography for Oceanic Plateaus and Swells. Journal of Geophysical Research-Solid Earth and Planets. 94:7403-7418.   10.1029/JB094iB06p07403   AbstractWebsite

Oceanic plateaus and swells are a major component of the seafloor topography, yet they remain among the most poorly understood features. This is especially true of the oceanic plateaus which show large variations in crustal thickness. To determine the depth and mode of compensation for 53 of the largest plateaus and swells, we analyzed the relationship between geoid height and topography in polygonal areas containing each feature. Both geoid height and topography were first band-pass filtered (400 km < l < 4000 km) to isolate the signal associated with local compensation from flexural and deep mantle signals. The ratio of geoid height to topography was then determined by fitting a straight line to the data. Except for nine of the smaller features there is a high correlation between geoid height and topography that is positive in accordance with Airy and thermal compensation models. Eighteen features have high geoid/topography ratios that cannot be explained by the Airy compensation model of crustal thickening. These features (thermal swells) are partially supported by thermal buoyancy forces in the lower half of the lithosphere. The ratios are highest for active hot spot swells and decay, with the thermal age of the swell, to values consistent with Airy compensation of the enduring volcanic edifice. The remaining features (plateaus) have lower geoid/topography ratios in agreement with the Airy compensation model. Those plateaus with average height greater than 4 km are thought to be continental fragments; the shorter plateaus tend to be volcanic features. Modified continental plateaus, presumably small fragments of extended and intruded continental margin crust, cluster around heights of ∼3 km, overlapping the range associated with oceanic plateaus. Since the origin of many plateaus is poorly understood, this global geoid/topography analysis provides a new technique for comparing the deep structure of oceanic plateaus and swells.

Becker, JJ, Sandwell DT, Smith WHF, Braud J, Binder B, Depner J, Fabre D, Factor J, Ingalls S, Kim SH, Ladner R, Marks K, Nelson S, Pharaoh A, Trimmer R, Von Rosenberg J, Wallace G, Weatherall P.  2009.  Global Bathymetry and Elevation Data at 30 Arc Seconds Resolution: SRTM30_PLUS. Marine Geodesy. 32:355-371.   10.1080/01490410903297766   AbstractWebsite

A new 30-arc second resolution global topography/bathymetry grid (SRTM30_PLUS) has been developed from a wide variety of data sources. Land and ice topography comes from the SRTM30 and ICESat topography, respectively. Ocean bathymetry is based on a new satellite-gravity model where the gravity-to-topography ratio is calibrated using 298 million edited soundings. The main contribution of this study is the compilation and editing of the raw soundings, which come from NOAA, individual scientists, SIO, NGA, JAMSTEC, IFREMER, GEBCO, and NAVOCEANO. The gridded bathymetry is available for ftp download in the same format as the 33 tiles of SRTM30 topography. There are 33 matching tiles of source identification number to convey the provenance of every grid cell. The raw sounding data, converted to a simple common format, are also available for ftp download.

Sandwell, DT, Milbert DG, Douglas BC.  1986.  Global Nondynamic Orbit Improvement for Altimetric Satellites. Journal of Geophysical Research-Solid Earth and Planets. 91:9447-9451.   10.1029/JB091iB09p09447   AbstractWebsite

The largest source of error in satellite altimetry is in the radial position of the satellite. Radial orbit errors of more than a few decimeters prohibit basin-scale studies of sea surface height variability. We explore nondynamic techniques for reducing this error. Sea surface height differences at intersections of satellite altimeter profiles (crossover data) provide a strong constraint on radial orbit error but do not uniquely define it. The portion of orbit error that is a function of latitude and longitude only produces no crossover differences and therefore cannot be recovered with crossover data. Using mathematics (inclination functions) originally developed for satellite dynamics, we determine the entire class of orbit error functions not recoverable with crossover data. These functions are mappings of surface spherical harmonics into the orbit plane. For example, the l = 1, m = 0 surface harmonic maps into sinusoidal orbit error with a frequency of once per orbit. Nonzonal harmonics map into linear combinations of three or more frequencies that are linked by the inclination functions. Between frequencies of 0 and 2.2 cycles per orbit there are nine orbit error components that cannot be recovered using crossover data. These components are uniquely defined, however, by nine globally distributed radial tracking points. Fewer tracking points are sufficient if a smoothness criteria is applied to the orbit correction curve. Our findings suggest that radial orbit error can be significantly reduced by including a few globally distributed radar reflectors (or transponders) in the tracking network.

Muller, RD, Qin XD, Sandwell DT, Dutkiewicz A, Williams SE, Flament N, Maus S, Seton M.  2016.  The GPlates Portal: Cloud-based interactive 3D visualization of global geophysical and geological data in a web browser. Plos One. 11   10.1371/journal.pone.0150883   AbstractWebsite

The pace of scientific discovery is being transformed by the availability of 'big data' and open access, open source software tools. These innovations open up new avenues for how scientists communicate and share data and ideas with each other and with the general public. Here, we describe our efforts to bring to life our studies of the Earth system, both at present day and through deep geological time. The GPlates Portal (portal.gplates.org) is a gateway to a series of virtual globes based on the Cesium Javascript library. The portal allows fast interactive visualization of global geophysical and geological data sets, draped over digital terrain models. The globes use WebGL for hardware-accelerated graphics and are cross-platform and cross-browser compatible with complete camera control. The globes include a visualization of a high-resolution global digital elevation model and the vertical gradient of the global gravity field, highlighting small-scale seafloor fabric such as abyssal hills, fracture zones and seamounts in unprecedented detail. The portal also features globes portraying seafloor geology and a global data set of marine magnetic anomaly identifications. The portal is specifically designed to visualize models of the Earth through geological time. These space-time globes include tectonic reconstructions of the Earth's gravity and magnetic fields, and several models of long-wavelength surface dynamic topography through time, including the interactive plotting of vertical motion histories at selected locations. The globes put the on-the-fly visualization of massive data sets at the fingertips of end-users to stimulate teaching and learning and novel avenues of inquiry.

Schubert, G, Moore WB, Sandwell DT.  1994.  Gravity over Coronae and Chasmata on Venus. Icarus. 112:130-146.   10.1006/icar.1994.1174   AbstractWebsite

The global spherical harmonic model of Venus' gravity field MGNP60FSAAP, with horizontal resolution of about 600 km, shows that most coronae have little or no signature in the gravity field. Nevertheless, some coronae and some segments of chasmata are associated with distinct positive gravity anomalies. No corona has been found to have a negative gravity anomaly. The spatial coincidence of the gravity highs over four closely spaced 300- to 400-km-diameter coronae in Eastern Eistla Regio with the structures themselves is remarkable and argues for a near-surface or lithospheric origin of the gravity signals over such relatively small features. Apparent depths of compensation (ADCs) of the prominent gravity anomalies at Artemis, Latona, and Heng-o Coronae are about 150 to 200 km. The geoid/topography ratios (GTRs) at Artemis, Latona, and Heng-o Coronae lie in the range 32 to 35 m km(-1). The large ADCs and GTRs of Artemis, Latona, and Heng-o Coronae are consistent with topographically related gravity and a thick Venus lithosphere or shallowly compensated topography and deep positive mass anomalies due to subduction or underthrusting at these coronae. At arcuate segments of Hecate and Parga Chasmata ADCs are about 125 to 150 km, while those at Fatua Corona, four coronae in Eastern Eistla Regio, and an arcuate segment of Western Parga Chasma are about 75 km. The GTRs at Fatua Corona, the four coronae in eastern Eistla Regio, and the arcuate segments of Hecate, Parga, and Western Parga Chasmata are about 12 to 21 m km(-1). The ADCs and GTRs of these coronae and arcuate chasmata segments are generally too large to reflect compensation by crustal thickness variations. Instead, they suggest compensation by thermally induced thickness variations in a moderately thick (approximate to 100 km) lithosphere. Alternatively, the gravity signals at these sites could originate from deep positive mass anomalies due to subduction or underthrusting. Weighted linear least squares fits to GTR vs h (long-wavelength topography) data from Heng-o and Fatua Coronae, the four coronae in eastern Eistla Regio, and the arcuate segments of Hecate, Parga, and western Parga Chasmata are consistent with compensation by thermally induced thickness variations of a dense lithosphere above a less dense mantle; the fits imply an average lithosphere thickness of about 180 km and an excess lithospheric density of about 0.5 to 0.7%. Gravity anomalies at the arcuate segments of Dali and Diana Chasmata that form Latona Corona, at Artemis Chasma, and other arcuate segments of Parga and Hecate Chasmata occur on the concave sides of the arcs. By analogy with gravity anomalies of similar horizontal scale (600 km-several thousand kilometers) on the concave sides of terrestrial subduction zone arcs, which are due in large part to subducted lithosphere, it is inferred that the gravity anomalies on Venus are consistent with retrograde subduction at Artemis Chasma, along the northern and southern margins of Latona Corona, and elsewhere along Parga and Hecate Chasmata. (C) 1994 Academic Press, Inc.

Sandwell, DT, McAdoo DC.  1990.  High-Accuracy, High-Resolution Gravity Profiles from 2 Years of the Geosat Exact Repeat Mission. Journal of Geophysical Research-Oceans. 95:3049-3060.   10.1029/JC095iC03p03049   AbstractWebsite

Satellite altimeter data from the first 44 repeat cycles (2 years) of the Geosat Exact Repeat Mission (Geosat ERM) were averaged to improve accuracy, resolution and coverage of the marine gravity field. Individual 17-day repeat cycles (two points per second) were first edited and differentiated resulting in alongtrack vertical deflection (i.e., alongtrack gravity disturbance). To increase the signal to noise ratio, 44 of these cycles were then averaged to form a single, highly accurate vertical deflection profile. The largest contributions to the vertical deflection error is short-wavelength altimeter noise and longer-wavelength oceanographic variability; the combined noise level is typically 6 μrad. Both types of noise are reduced by averaging many repeat cycles. Over most ocean areas the uncertainly of the average profile is less than 1 μrad (0.206 arcsec) which corresponds to 1 mgal of alongtrack gravity disturbance. However, in areas of seasonal ice coverage, its uncertainty can exceed 5 μrad. To assess the resolution of individual and average Geosat gravity profiles, the cross-spectral analysis technique was applied to repeat profiles. Individual Geosat repeat cycles are coherent (>0.5) for wavelengths greater than about 30 km and become increasingly incoherent at shorter wavelengths. This Emit of resolution is governed by the signal-to-noise ratio. Thus when many Geosat repeat profiles are averaged together, the resolution limit typically improves to about 20 km. Except in shallow water areas, further improvements in resolution will be increasingly difficult to achieve because the short-wavelength components are attenuated by upward continuation from the seafloor to the sea surface. These results suggest that the marine gravity field can be completely mapped to an accuracy of 2 mgal and a half-wavelength resolution of 12 km by a 4.5-year satellite altimeter mapping mission.

Myer, D, Sandwell D, Brooks B, Foster J, Shimada M.  2008.  Inflation along Kilauea's Southwest Rift Zone in 2006. Journal of Volcanology and Geothermal Research. 177:418-424.   10.1016/j.jvolgeores.2008.06.006   AbstractWebsite

We report on InSAR and GPS results showing the first crustal inflation along the southwest rift zone at Kilauea volcano in over 20 years. Two independent interferograms (May 2-August 2, 2006 and June 22-Nov 7, 2006) from the ALOS PALSAR instrument reveal domal uplift located southwest of the main caldera. The uplift is bounded on the northeast by the caldera and follows the southwest rift zone for about 12 km. It is approximately 8 km wide. We use data derived from permanent GPS stations to calibrate the InSAR displacement data and estimate uplift of 7.7 cm during the first interferogram and 8.9 cm during the second with line-of-sight volumes of 2.8 x 10(6) m(3) and 3.0 X 10(6) m(3) respectively. The earthquake record for the periods before, during, and after inflation shows that a swarm of shallow earthquakes (z<5 km) signaled the beginning of the uplift and that elevated levels of shallow seismicity along the rift zones occurred throughout the uplift period. GPS data indicate that the inflation occurred steadily over nine months between mid-January and mid-October, 2006 making injection of a sill unlikely. We attribute the inflation to recharge of a shallow ductile area under the SWRZ. (c) 2008 Elsevier B.V. All rights reserved.

Malinverni, ES, Sandwell DT, Tassetti AN, Cappelletti L.  2014.  InSAR decorrelation to assess and prevent volcanic risk. European Journal of Remote Sensing. 47:537-556.   10.5721/EuJRS20144730   AbstractWebsite

SAR can be invaluable describing pre-eruption surface deformation and improving the understanding of volcanic processes. This work studies correlation of pairs of SAR images focusing on the influence of surface, climate conditions and acquisition band. Chosen L-band and C-band images (ENVISAT, ERS and ALOS) cover most of the Yellowstone caldera (USA) over a span of 4 years, sampling all the seasons. Interferograms and correlation maps are generated and studied in relation to snow depth and temperature. To isolate temporal decorrelation pairs of images with the shortest baseline are chosen. Results show good performance during winter, bad attitude towards wet snow and good coherence during summer with L-band performing better over vegetation.

Koeberl, C, Sharpton VL, Harrison MT, Sandwell D, Murali AV, Burke K.  1990.  The Kara/Ust-Kara twin impact structure; a large-scale impact event in the Late Cretaceous. Special Paper - Geological Society of America. 247( Sharpton VL, Ward PD, Eds.).:233-238., Boulder, CO, United States (USA): Geological Society of America (GSA), Boulder, CO AbstractWebsite
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Mellors, RJ, Sichoix L, Sandwell DT.  2002.  Lack of precursory slip to the 1999 Hector Mine, California, earthquake as constrained by InSAR. Bulletin of the Seismological Society of America. 92:1443-1449.   10.1785/0120010244   AbstractWebsite

We looked for evidence of interseismic strain occurring between the 1992 Landers earthquake and the 1999 Hector Mine earthquake near the Lavic Lake and Bullion faults by using interferometric synthetic aperture radar (InSAR). Interferograms covering the Hector Mine epicentral region were studied for possible slip along the Bullion and Lavic Lake faults by both visual inspection and a matched filter technique intended to emphasize slip located at the nucleation point. Some indications of possible deformation associated with the 5 July 1992 M-L 5.4 Pisgah event was observed, but high decorrelation prevented a conclusive determination. We have seen no evidence for precursory slip in the epicentral region up to 30 days before the Hector Mine event. We estimated that the slip equivalent to a M-w 4.5 event would have been observable in the months before the Hector Mine event, and this places an upper bound on the long-term precursory slip, had it occurred. We have noted that InSAR is well suited for detecting precursory slip in general due to the high spatial resolution and the lack of ground instrumentation required but that the detection level depends on the depth and orientation of the slip.

Brooks, BA, Foster J, Sandwell D, Wolfe CJ, Okubo P, Poland M, Myer D.  2008.  Magmatically triggered slow slip at Kilauea volcano, Hawaii. Science. 321:1177-1177.   10.1126/science.1159007   AbstractWebsite

We demonstrate that a recent dike intrusion probably triggered a slow fault-slip event (SSE) on Kilauea volcano's mobile south flank. Our analysis combined models of Advanced Land Observing Satellite interferometric dike-intrusion displacement maps with continuous Global Positioning System (GPS) displacement vectors to show that deformation nearly identical to four previous SSEs at Kilauea occurred at far-field sites shortly after the intrusion. We model stress changes because of both secular deformation and the intrusion and find that both would increase the Coulomb failure stress on possible SSE slip surfaces by roughly the same amount. These results, in concert with the observation that none of the previous SSEs at Kilauea was directly preceded by intrusions but rather occurred during times of normal background deformation, suggest that both extrinsic (intrusion-triggering) and intrinsic (secular fault creep) fault processes can lead to SSEs.

Marks, KM, Sandwell DT, Vogt PR, Hall SA.  1991.  Mantle Downwelling beneath the Australian-Antarctic Discordance Zone - Evidence from Geoid Height Versus Topography. Earth and Planetary Science Letters. 103:325-338.   10.1016/0012-821x(91)90170-m   AbstractWebsite

The Australian-Antarctic discordance zone (AAD) is an anomalously deep and rough segment of the Southeast Indian Ridge between 120-degrees and 128-degrees-E. A large, negative (deeper than predicted) depth anomaly is centered on the discordance, and a geoid low is evident upon removal of a low-order geoid model and the geoid height-age relation. We investigate two models that may explain these anomalies: a deficiency in ridge-axis magma supply that produces thin oceanic crust (i.e. shallow Airy compensation), and a downwelling and/or cooler mantle beneath the AAD that results in deeper convective-type compensation. To distinguish between these models, we have calculated the ratio of geoid height to topography from the slope of a best line fit by functional analysis (i.e. non-biased linear regression), a method that minimizes both geoid height and topography residuals. Geoid/topography ratios of 2.1 +/- 0.9 m/km for the entire study area (38-degrees-60-degrees-S, 105-degrees-140-degrees-E), 2.3 +/- 1.8 m/km for a subset comprising crust less-than-or-equal-to 25 Ma, and 2.7 +/- 2.0 m/km for a smaller area centered on the AAD were obtained. These ratios are significantly larger than predicted for thin oceanic crust (0.4 m/km), and 2.7 m/km is consistent with downwelling convection beneath young lithosphere. Average compensation depths of 27, 29, and 34 km, respectively, estimated from these ratios suggest a mantle structure that deepens towards the AAD. The deepest compensation (34 km) of the AAD is below the average depth of the base of the young lithosphere (approximately 30 km), and a downwelling of asthenospheric material is implied. The observed geoid height-age slope over the discordance is unusually gradual at -0.133 m/m.y. We calculate that an upper mantle 170-degrees-C cooler and 0.02 g/cm3 denser than normal can explain the shallow slope. Unusually fast shear velocities in the upper 200 km of mantle beneath the discordance, and major-element geochemical trends consistent with small amounts of melting at shallow depths, provide strong evidence for cooler temperatures beneath the AAD.

Sandwell, DT, McAdoo DC.  1988.  Marine Gravity of the Southern-Ocean and Antarctic Margin from Geosat. Journal of Geophysical Research-Solid Earth and Planets. 93:10389-&.   10.1029/JB093iB09p10389   AbstractWebsite

In November of 1986 the U.S. Navy satellite Geosat began collecting unclassified (gravity) altimeter data as part of its exact repeat mission (ERM). For national security reasons the Geosat orbit was arranged so that it closely follows the Seasat satellite altimeter ground track. However, there are two advantages of the Geosat data over the Seasat data. First, because of improvements in altimeter design, Geosat profiles are about 3 times more precise than Seasat profiles. This corresponds to an accuracy of 2–3 μrad (i.e., 2–3 mGal) for wavelengths greater than 20 km. Second, the Geosat altimeter data were collected when the Antarctic ice coverage was minimal (February 1987 to March 1987), while Seasat was only active during an Antarctic winter (June 1978 to September 1978). These new data reveal many previously uncharted seamounts and fracture zones in the extreme southern ocean areas adjacent to Antarctica. Seven large age-offset fracture zones, apparent in the Geosat data, record the early breakup of Gondwana. Finally, the new data reveal the detailed gravity signatures of the passive and active continental margins of Antarctica. These data are an important reconnaissance tool for future studies of these remote ocean areas.

Sandwell, DT, Sichoix L, Agnew D, Bock Y, Minster JB.  2000.  Near real-time radar interferometry of the Mw 7.1 Hector Mine Earthquake. Geophysical Research Letters. 27:3101-3104.   10.1029/1999gl011209   AbstractWebsite

The Hector Mine Earthquake (Mw 7.1, 16 October 1999) ruptured 45 km of previously mapped and unmapped faults in the Mojave Desert. The ERS-2 satellite imaged the Mojave Desert on 15 September and again on 20 October, just 4 days after the earthquake. Using a newly-developed ground station we acquired both passes and were able to form an interferogram within 20 hours of the second overflight. Estimates of slip along the main rupture are 1-2 meters greater than slip derived from geological mapping. The gradient of the interferometric phase reveals an interesting pattern of triggered slip on adjacent faults as well as a 30 mm deep sink hole along Interstate 40.

Sandwell, DT, Müller DR, Smith WHF, Garcia E, Francis R.  2014.  New global marine gravity model from CryoSat-2 and Jason-1 reveals buried tectonic structure. Science. 346:65-67.   10.1126/science.1258213   AbstractWebsite

Gravity models are powerful tools for mapping tectonic structures, especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment. We combined new radar altimeter measurements from satellites CryoSat-2 and Jason-1 with existing data to construct a global marine gravity model that is two times more accurate than previous models. We found an extinct spreading ridge in the Gulf of Mexico, a major propagating rift in the South Atlantic Ocean, abyssal hill fabric on slow-spreading ridges, and thousands of previously uncharted seamounts. These discoveries allow us to understand regional tectonic processes and highlight the importance of satellite-derived gravity models as one of the primary tools for the investigation of remote ocean basins.