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Jacobs, A, Sandwell D, Fialko Y, Sichoix L.  2002.  The 1999 (M-w 7. 1) Hector Mine, California, earthquake: Near-field postseismic deformation from ERS interferometry. Bulletin of the Seismological Society of America. 92:1433-1442.   10.1785/0120000908   AbstractWebsite

Interferometric synthetic aperture radar (InSAR) data over the area of the Hector Mine earthquake (M-w 7.1, 16 October 1999) reveal postseismic deformation of several centimeters over a spatial scale of 0.5 to 50 km. We analyzed seven SAR acquisitions to form interferograms over four time periods after the event. The main deformations seen in the line-of-sight (LOS) displacement maps are a region of subsidence (60 mm LOS increase) on the northern end of the fault, a region of uplift (45 mm LOS decrease) located to the northeast of the primary fault bend, and a linear trough running along the main rupture having a depth of up to 15 mm and a width of about 2 km. We correlate these features with a double left-bending, right-lateral, strike-slip fault that exhibits contraction on the restraining side and extension along the releasing side of the fault bends. The temporal variations in the near-fault postseismic deformation are consistent with a characteristic time scale of 135 + 42 or - 25 days, which is similar to the relaxation times following the 1992 Landers earthquake. High gradients in the LOS displacements occur on the fault trace, consistent with afterslip on the earthquake rupture. We derive an afterslip model by inverting the LOS data from both the ascending and descending orbits. Our model indicates that much of the afterslip occurs at depths of less than 3 to 4 km.

Sandwell, DT, Sichoix L, Smith B.  2002.  The 1999 Hector Mine earthquake, southern California: Vector near-field displacements from ERS InSAR. Bulletin of the Seismological Society of America. 92:1341-1354.   10.1785/0120000901   AbstractWebsite

Two components of fault slip are uniquely determined from two line-of-sight (LOS) radar interferograms by assuming that the fault-normal component of displacement is zero. We use this approach with ascending and descending interferograms from the ERS satellites to estimate surface slip along the Hector Mine earthquake rupture. The LOS displacement is determined by visually counting fringes to within I kin of the outboard ruptures. These LOS estimates and uncertainties are then transformed into strike- and dip-slip estimates and uncertainties; the transformation is singular for a N-S oriented fault and optimal for an E-W oriented fault. In contrast to our previous strike-slip estimates, which were based only on a descending interferogram, we now find good agreement with the geological measurements, except at the ends of the rupture. The ascending interferogram reveals significant west-side-down dip-slip (similar to1.0 in) which reduces the strike-slip estimates by I to 2 in, especially along the northern half of the rupture. A spike in the strike-slip displacement of 6 m is observed in central part of the rupture. This large offset is confirmed by subpixel cross correlation of features in the before and after amplitude images. In addition to strike slip and dip slip, we identify uplift and subsidence along the fault, related to the restraining and releasing bends in the fault trace, respectively. Our main conclusion is that at least two look directions are required for accurate estimates of surface slip even along a pure strike-slip fault. Models and results based only on a single look direction could have major errors. Our new estimates of strike slip and dip slip along the rupture provide a boundary condition for dislocation modeling. A simple model, which has uniform slip to a depth of 12 km, shows good agreement with the observed ascending and descending interferograms.

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Tong, XP, Sandwell D, Luttrell K, Brooks B, Bevis M, Shimada M, Foster J, Smalley R, Parra H, Soto JCB, Blanco M, Kendrick E, Genrich J, Caccamise DJ.  2010.  The 2010 Maule, Chile earthquake: Downdip rupture limit revealed by space geodesy. Geophysical Research Letters. 37   10.1029/2010gl045805   AbstractWebsite

Radar interferometry from the ALOS satellite captured the coseismic ground deformation associated with the 2010 Mw 8.8 Maule, Chile earthquake. The ALOS interferograms reveal a sharp transition in fringe pattern at similar to 150 km from the trench axis that is diagnostic of the downdip rupture limit of the Maule earthquake. An elastic dislocation model based on ascending and descending ALOS interferograms and 13 near-field 3-component GPS measurements reveals that the coseismic slip decreases more or less linearly from a maximum of 17 m (along-strike average of 6.5 m) at 18 km depth to near zero at 43-48 km depth, quantitatively indicating the downdip limit of the seismogenic zone. The depth at which slip drops to near zero appears to be at the intersection of the subducting plate with the continental Moho. Our model also suggests that the depth where coseismic slip vanishes is nearly uniform along the strike direction for a rupture length of similar to 600 km. The average coseismic slip vector and the interseismic velocity vector are not parallel, which can be interpreted as a deficit in strike-slip moment release. Citation: Tong, X., et al. (2010), The 2010 Maule, Chile earthquake: Downdip rupture limit revealed by space geodesy, Geophys. Res. Lett., 37, L24311, doi:10.1029/2010GL045805.

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Small, C, Sandwell DT.  1989.  An Abrupt Change in Ridge Axis Gravity with Spreading Rate. Journal of Geophysical Research-Solid Earth and Planets. 94:17383-17392.   10.1029/JB094iB12p17383   AbstractWebsite

The global mid-ocean ridge system shows a marked change in morphology and isostatic compensation as a function of spreading rate. Fast spreading ridges have axial highs with little bathymetric relief and low-amplitude gravity signatures indicating that they are nearly in local isostatic equilibrium. Slow spreading ridges have large axial valleys bounded by rugged topography (Macdonald, 1982) and large axial gravity troughs indicating that they are dynamically maintained. While this variation in ridge axis morphology with spreading rate has been observed, it has not been analyzed in a comprehensive manner. Moreover, it is not known whether the transition from axial valley to axial high is a continuous function of spreading rate or whether it occurs abruptly at a particular rate. Such observations would provide important constraints on models of ridge axis dynamics. Vertical deflection profiles collected by the Geosat radar altimeter have sufficient accuracy and resolution to reveal the change in ridge axis gravity with spreading rate. In this study, we have analyzed 44 Geosat profiles over ridges with spreading rates ranging from 14 to 155 mm/yr. In agreement with previous studies, we find that slow spreading ridges (<60 mm/yr) usually have high amplitude gravity troughs (40–100 μrad = 40–100 mGal), while fast spreading ridges (>70 mm/yr) are characterized by low-amplitude ridge axis highs (∼15 μrad). Unexpectedly, we find that the transition from axial trough to axial high occurs abruptly at a spreading rate of 60–70 mm/yr. Ridge axis gravity signatures are highly variable for rates less than 65 mm/yr and very uniform at higher rates. The transition of the gravity signature appears to be more abrupt than the transition of the topographic signature, suggesting an abrupt change in the style of isostatic compensation with spreading rate. Published models of ridge axis dynamics do not explain this sharp transition.

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.

Smith, B, Sandwell D.  2003.  Accuracy and resolution of shuttle radar topography mission data. Geophysical Research Letters. 30   10.1029/2002gl016643   AbstractWebsite

[1] We assess the accuracy and resolution of topography data provided by the Shuttle Radar Topography Mission (SRTM) through spectral comparisons with the National Elevation Dataset (NED) and a high-resolution laser data set of the 1999 Hector Mine earthquake rupture. We find that SRTM and the NED are coherent for wavelengths greater than 200 m, however the spatial resolution of the NED data is superior to the SRTM data for wavelengths shorter than 350 m, likely due to the application of a boxcar filter applied during final SRTM processing stages. From these results, a low-pass filter/decimation algorithm can be designed in order to expedite large-area SRTM applications.

Sandwell, DT.  1984.  Along-track deflection of the vertical from Seasat : GEBCO overlays. , Rockville, Md.: U.S. Dept. of Commerce, National Oceanic and Atmospheric Administration, National Ocean Service, National Geodetic Survey, Charting and Geodetic Services : For sale by the National Geodetic Information Center Abstract
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Sandwell, DT, Ruiz MB.  1992.  Along-Track Gravity-Anomalies from Geostat and Seasat Altimetry - Gebco Overlays. Marine Geophysical Researches. 14:165-205.   10.1007/bf01270629   AbstractWebsite

To provide easy access to the large number of Seastat and Geosat altimeter observations collected over the last decade, we have plotted these satellite altimeter profiles as overlays to the General Bathymetric Chart of the Oceans (GEBCO). Each of the 32 overlays displays along-track gravity anomalies for either ascending (southeast to northwest) or descending (northeast to southwest) altimeter passes. Where Seasat and Geosat profiles coincide, only the more accurate Geosat profiles were plotted. In poorly charted southern ocean areas, satellite altimeter profiles reveal many previously undetected features of the seafloor.

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.

Small, C, Sandwell DT.  1992.  An Analysis of Ridge Axis Gravity Roughness and Spreading Rate. Journal of Geophysical Research-Solid Earth. 97:3235-3245.   10.1029/91jb02465   AbstractWebsite

Fast and slow spreading ridges have radically different morphologic and gravimetric characteristics. In this study, altimeter measurements from the Geosat Exact Repeat Mission (Geosat ERM) are used to investigate spreading rate dependence of the ridge axis gravity field. Gravity roughness provides an estimate of the amplitude of the gravity anomaly and is robust to small errors in the location of the ridge axis. We compute gravity roughness as a weighted root mean square (RMS) of the vertical deflection at 438 ridge crossings on the mid-ocean ridge system. Ridge axis gravity anomalies show a decrease in amplitude with increasing spreading rate up to an intermediate rate of approximately 60-80 mm/yr and almost no change at higher rates; overall the roughness decreases by a factor of 10 between the lowest and highest rates. In addition to the amplitude decrease, the range of roughness values observed at a given spreading rate shows a similar order of magnitude decrease with transition between 60 and 80 mm/yr. The transition of ridge axis gravity is most apparent at three relatively unexplored locations on the Southeast Indian Ridge and the Pacific-Antarctic Rise; on these intermediate rate ridges the transition occurs abruptly across transform faults.

Sandwell, DT.  1992.  Antarctic Marine Gravity-Field from High-Density Satellite Altimetry. Geophysical Journal International. 109:437-448.   10.1111/j.1365-246X.1992.tb00106.x   AbstractWebsite

Closely spaced satellite altimeter profiles (< 5 km) collected during the Geosat Geodetic Mission (Geosat/GM), and those planned for the extended ERS-1 mission, are easily converted to grids of vertical gravity gradient and gravity anomaly. As profile spacing decreases, it becomes increasingly difficult to perform a crossover adjustment on the original geoid height profiles without introducing large cross-track gradients. If one is only interested in the horizontal and vertical derivatives of the gravitational potential, however, adjustment of the profile is unnecessary. The long-wavelength radial orbit error is suppressed well below the noise level of the altimeter by simply taking the along-track derivative of each profile. Ascending and descending slope profiles are then interpolated onto separate uniform grids. These two grids are summed and differenced to form comparable grids of east and north vertical deflection. Using Laplace's equation, the vertical gravity gradient is calculated directly from the vertical deflection grids. Fourier analysis is required to construct gravity anomalies from the two vertical deflection grids. These techniques are applied to high-density (approximately 2 km profile spacing) Geosat/GM profiles in Antarctic waters (60-degrees-S to 72-degrees-S). Gridding and interpolation are performed using the method of projection onto convex sets where the smoothness criteria corresponds to upward continuation through 4 km of ocean. The resultant gravity grids have resolution and accuracy comparable to shipboard gravity profiles. After adjustment of a DC shift in the shipboard gravity profiles (approximately 5 mGal) the rms difference between the ship and satellite gravity is 5.5 mGal. Many interesting and previously uncharted features are apparent in these new gravity maps including a propagating rift wake and a large 'leaky transform' along the Pacific-Antarctic Rise.

Sandwell, DT, Lawver LA, Dalziel IWD, Smith WHF, Wiederspahn M.  1992.  ANTARCTICA Gravity Anomaly and Infrared Satellite Image, USGS MAP 1-2284. : U.S. Geol. Survey Abstract
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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.

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Sandwell, DT, Smith WHF.  2001.  Bathymetric Estimation. Satellite altimetry and earth sciences : a handbook of techniques and applications. ( Fu L, Cazenave A, Eds.).:441-457., San Diego, Calif. ; London: Academic Abstract
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Smith, WHF, Sandwell DT.  1994.  Bathymetric Prediction from Dense Satellite Altimetry and Sparse Shipboard Bathymetry. Journal of Geophysical Research-Solid Earth. 99:21803-21824.   10.1029/94jb00988   AbstractWebsite

The southern oceans (south of 30 degrees S) are densely covered with satellite-derived gravity data (track spacing 2-4 km) and sparsely covered with shipboard depth soundings (hundreds of kilometers between tracks in some areas). Flexural isostatic compensation theory suggests that bathymetry and downward continued gravity data may show linear correlation in a band of wavelengths 15-160 km, if sediment cover is thin and seafloor relief is moderate. At shorter wavelengths, the gravity field is insensitive to seafloor topography because of upward continuation from the seafloor to the sea surface; at longer wavelengths, isostatic compensation cancels out most of the gravity field due to the seafloor topography. We combine this theory with Wiener optimization theory and empirical evidence for gravity noise-to-signal ratios to design low-pass and band-pass filters to use in predicting bathymetry from gravity. The prediction combines long wavelengths (> 160 km) from low-pass-filtered soundings with an intermediate-wavelength solution obtained from multiplying downward continued, band-pass filtered (15-160 km) gravity data by a scaling factor S. S is empirically determined from the correlation between gravity data and existing soundings in the 15-160 km band by robust regression and varies at long wavelengths. We find that areas with less than 200 m of sediment cover show correlation between gravity and bathymetry significant at the 99% level, and S may be related to the density of seafloor materials in these areas. The prediction has a horizontal resolution limit of 5-10 km in position and is within 100 m of actual soundings at 50% of grid points and within 240 m at 80% of these. In areas of very rugged topography the prediction underestimates the peak amplitudes of seafloor features. Images of the prediction reveal many tectonic features not seen on any existing bathymetric charts. Because the prediction relies on the gravity field at wavelengths < 160 km, it is insensitive to errors in the navigation of sounding lines but also cannot completely reproduce them. Therefore it may be used to locate tectonic features but should not be used to assess hazards to navigation. The prediction is available from the National Geophysical Data Center in both digital and printed form.

Sandwell, DT, Smith WHF, Gille S, Kappel E, Jayne S, Soofi K, Coakley B, Geli L.  2006.  Bathymetry from space: Rationale and requirements for a new, high-resolution altimetric mission. Comptes Rendus Geoscience. 338:1049-1062.   10.1016/j.crte.2006.05.014   AbstractWebsite

Bathymetry is foundational data, providing basic infrastructure for scientific, economic, educational, managerial, and political work. Applications as diverse as tsunami hazard assessment, communications cable and pipeline route planning, resource exploration, habitat management, and territorial claims under the Law of the Sea all require reliable bathymetric maps to be available on demand. Fundamental Earth science questions, such as what controls seafloor shape and how seafloor shape influences global climate, also cannot be answered without bathymetric maps having globally uniform detail. Current bathymetric, charts are inadequate for many of these applications because only a small fraction of the seafloor has been surveyed. Modern multibeam echosounders provide the best resolution, but it would take more than 200 ship-years and billions of dollars to complete the job. The seafloor topography can be charted globally, in five years, and at a cost under $100M. A radar altimeter mounted on an orbiting spacecraft can measure slight variations in ocean surface height, which reflect variations in the pull of gravity caused by seafloor topography. A new satellite altimeter mission, optimized to map the deep ocean bathymetry and gravity field, will provide a global map of the world's deep oceans at a resolution of 6-9 kin. This resolution threshold is critical for a large number of basic science and practical applications, including: determining the effects of bathymetry and seafloor roughness on ocean circulation, mixing, climate, and biological communities, habitats, and mobility; understanding the geologic processes responsible for ocean floor features unexplained by simple plate tectonics, such as abyssal hills, seamounts, microplates, and propagating rifts;. improving tsunami hazard forecast accuracy by mapping the deep-ocean topography that steers tsunami wave energy; mapping the marine gravity field to improve inertial navigation and provide homogeneous coverage of continental margins; providing bathymetric maps for numerous other practical applications, including reconnaissance for submarine cable and pipeline routes, improving tide models, and assessing potential territorial claims to the seabed under the United Nations Convention on the Law of the Sea. Because ocean bathymetry is a fundamental measurement of our planet, there is a broad spectrum of interest from government, the research community, industry, and the general public. Mission requirements. The resolution of the altimetry technique is limited by physical law, not instrument capability. Everything that can be mapped from space can be achieved now, and there is no gain in waiting for technological advances. Mission requirements for Bathymetry from Space are much less stringent and less costly than typical physical oceanography missions. Long-term sea-surface height accuracy is not needed; the fundamental measurement is the slope of the ocean surface to an accuracy of similar to 1 prad (1 mm km(-1)). The main mission requirements are: improved range precision (a factor of two or more improvement in altimeter range precision with respect to current altimeters is needed to reduce the noise due to ocean waves); - fine cross-track spacing and long mission duration (a ground track spacing of 6 km or less is required. A six-year mission would reduce the error by another factor of two); moderate inclination (existing satellite altimeters have relatively high orbital inclinations, thus their resolution of east-west components of ocean slope is poor at low latitudes. The new mission should have an orbital inclination close to 60 degrees or 120 degrees so as to resolve north-south and east-west components almost equally while still covering nearly all the world's ocean area); near-shore tracking (for applications near coastlines, the ability of the instrument to track the ocean surface close to shore, and acquire the surface soon after leaving land, is desirable).

Sandwell, DT.  1987.  Biharmonic Spline Interpolation of Geos-3 and Seasat Altimeter Data. Geophysical Research Letters. 14:139-142.   10.1029/GL014i002p00139   AbstractWebsite

Green functions of the biharmonic operator, in one and two dimensions, are used for minimum curvature interpolation of irregularly spaced data points. The interpolating curve (or surface) is a linear combination of Green functions centered at each data point. The amplitudes of the Green functions are found by solving a linear system of equations. In one (or two) dimensions this technique is equivalent to cubic spline (or bicubic spline) interpolation while in three dimension it corresponds to multiquadric interpolation. Although this new technique is relatively slow, it is more flexible than the spline method since both slopes and values can be used to find a surface. Moreover, noisy data can be fit in a least squares sense by reducing the number of model parameters. These properties are well suited for interpolating irregularly spaced satellite altimeter profiles. The long wavelength radial orbit error is suppressed by differentiating each profile. The shorter wavelength noise is reduced by the least squares fit to nearby profiles. Using this technique with 0.5 million GEOS-3 and SEASAT data points, it was found that the marine geoid of the Caribbean area is highly correlated with the sea floor topography. This suggests that similar applications, in more remote, areas may reveal new features of the sea floor.

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Yale, MM, Sandwell DT, Smith WHF.  1995.  Comparison of Along-Track Resolution of Stacked Geosat, Ers-1, and Topex Satellite Altimeters. Journal of Geophysical Research-Solid Earth. 100:15117-15127.   10.1029/95jb01308   AbstractWebsite

Cross-spectral analysis of repeat satellite altimeter profiles was performed to compare the along-track resolution capabilities of Geosat, ERS 1 and TOPEX data. Geophysical Data Records were edited, differentiated, low-pass-filtered, and resampled at 5 Hz. All available data were then loaded into three-dimensional files where repeat cycles were aligned along-track (62 cycles of Geosat/Exact Repeat Mission; 16 cycles of ERS 1, 35-day orbit; 73 cycles of TOPEX). The coherence versus wave number between pairs of repeat profiles was used to estimate along-track resolution for individual cycles, eight-cycle-average profiles, and 31-cycle-average profiles (Geosat and TOPEX only). Coherence, which depends on signal to noise ratio, reflects factors such as seafloor gravity amplitude, regional seafloor depth, instrument noise, oceanographic noise, and the number of cycles available for stacking (averaging). Detailed resolution analyses are presented for two areas: the equatorial Atlantic, a region with high tectonic signal and low oceanographic noise; and the South Pacific, a region with low tectonic signal and high oceanographic variability. For all three altimeters, along-track resolution is better in the equatorial Atlantic than in the South Pacific. Global maps of along-track resolution show considerable geographic variation. On average globally, the along-track resolution (0.5 coherence) of eight-cycle stacks are approximately the same, 28, 29, and 30 km for TOPEX, Geosat, and ERS 1, respectively. TOPEX 31-cycle stacks (22 km) resolve slightly shorter wavelengths than Geosat 31-cycle stacks (24 km). The stacked data, which are publicly available, will be used in future global gravity grids, and for detailed studies of mid-ocean ridge axes, fracture zones, sea mounts, and seafloor roughness.

Neumann, GA, Forsyth DW, Sandwell D.  1993.  Comparison of Marine Gravity from Shipboard and High-Density Satellite Altimetry Along the Mid-Atlantic Ridge, 30.5-Degrees-35.5-Degrees-S. Geophysical Research Letters. 20:1639-1642.   10.1029/93gl01487   AbstractWebsite

We compare new marine gravity fields derived from satellite altimetry with shipboard measurements over a region of more than 120,000 square kilometers in the central South Atlantic. Newly declassified satellite data were employed to construct free-air anomaly maps on 0.05 degree grids [Sandwell and Smith, 1992; Marks et al., 1993]. An extensive gravity and bathymetry dataset from four cruises along the Mid-Atlantic Ridge from 30.5-35.5-degrees-S provides a benchmark for testing the two-dimensional resolution and accuracy of the satellite measurements where their crosstrack spacing is near their widest. The satellite gravity signal is coherent with bathymetry in this region down to wavelengths of 26 km (gamma2=0.5), compared to 12.5 km for shipboard gravity. Residuals between the shipboard and satellite datasets have a roughly normal distribution. The standard deviation of satellite gravity with respect to shipboard measurements is nearly 7 mGal in a region of 140 mGal total variation, whereas the internal standard deviation at crossovers for GPS-navigated shipboard data is 1.8 mGal. The differences between shipboard and satellite data are too large to use satellite gravity to determine crustal thickness variations within a typical ridge segment.

Small, C, Sandwell DT.  1992.  A Comparison of Satellite and Shipboard Gravity Measurements in the Gulf-of-Mexico. Geophysics. 57:885-893.   10.1190/1.1443301   AbstractWebsite

Satellite altimeters have mapped the marine geoid over virtually all of the world's oceans. These geoid height measurements may be used to compute free air gravity anomalies in areas where shipboard measurements are scarce. Two-dimensional (2-D) transformations of geoid height to gravity are limited by currently available satellite track spacing and usually sacrifice short wavelength resolution. Full resolution may be retained along widely spaced satellite tracks if a one dimensional (1-D) transformation is used. Although the 1-D transform retains full resolution, it assumes that the gravity field is lineated perpendicular to the profile and is therefore limited by the orientation of the profile relative to the field. We investigate the resolution and accuracy of the 1-D transform method in the Northern Gulf of Mexico by comparing satellite gravity profiles with high quality shipboard data provided by Edcon Inc. The long wavelength components of the gravity field are constrained by a low degree reference field while the short wavelength components are computed from altimeter profiles. We find that rms misfit decreases with increasing spherical harmonic degree of the reference field up to 180 degrees (lambda > 220 km) with negligible improvement for higher degrees. The average rms misfit for the 17 profiles used in this study was 6.5 mGal with a 180 degree reference field. Spectral coherence estimates indicate that the satellite data resolve features with wavelengths as short as 25 km.

Sandwell, DT, Poehls KA.  1980.  A Compensation Mechanism for the Central Pacific. Journal of Geophysical Research. 85:3751-3758.   10.1029/JB085iB07p03751   AbstractWebsite

Geos 3 derived geoid heights and sea floor topography were averaged into 256 square areas (203 km on a side) for a region in the central Pacific containing a large portion of the Hawaiian Island chain. The whole region is about 500 m shallower than normal sea floor of the same age. The major portion of the depth anomaly is the Hawaiian swell. Data were analyzed using a two-dimensional fast Fourier transform. A transfer function was computed to determine the part of the observed geoid height that is coherent and in phase with the topography. A number of compensation models were tested against this function. Of these models no single physically reasonable model was found to have an acceptable fit. Accordingly, two models were introduced, one compensating short-wavelength topography at a shallow depth (14 km) and the other compensating the longer wavelengths by a deep mechanism. Acceptable deep compensation models include Airy-Heiskanen type compensation at depths between 40 and 80 km. Using the transition wavelength between the two models (1100 km), an estimate is made of the amplitude and shape of the heat anomaly needed to uplift the Hawaiian swell. The peak of the anomaly has an amplitude of 530 mW m−2 and is located 275 km east of Hawaii.

Sandwell, DT, Renkin ML.  1988.  Compensation of Swells and Plateaus in the North Pacific - No Direct Evidence for Mantle Convection. Journal of Geophysical Research-Solid Earth and Planets. 93:2775-2783.   10.1029/JB093iB04p02775   AbstractWebsite

At intermediate and long wavelengths the ratio of geoid height to topography is sensitive to the depth and mode of compensation. A low geoid/topography ratio (<2 m/km) signifies shallow Airy compensation. A higher ratio (∼6 m/km) signifies thermal isostasy and/or dynamic uplift from a mantle plume. A very high geoid/topography ratio (>8 m/km) in conjunction with a poor correlation between geoid height and topography is evidence of mantle convection. After subtracting a reference geoid from the observed geoid, previous studies have found a regular pattern of geoid highs and lows with a characteristic wavelength of 3000–4000 km. Since these geoid highs and lows were poorly correlated with topography and resulted in very high geoid/topography ratios (10–20 m/km), they were believed to reflect the planform of mantle convection. We show that the regular pattern of geoid highs and lows is an artifact caused by truncating the reference geoid at spherical harmonic degree 10. Since the geoid spectrum is “red,” the residual geoid is dominated by degree 11. When the harmonics of the reference geoid are rolled off gradually, the regular pattern of geoid highs and lows disappears. In the Northeast Pacific, the new residual geoid reflects the lithosphere age offsets across the major fracture zones. In the Northwest Pacific, the residual geoid corresponds to isostatically compensated swells and plateaus. We have calculated the geoid/topography ratio for 10 swells and plateaus and have found a range of compensation depths. The highest geoidAopography ratio of 5.5 m/km occurs on the flanks of the Hawaiian Swell. Intermediate ratios occur in four areas, including the Midway Swell. These intermediate ratios reflect a linear combination of the decaying thermal swell and the increasing volume of Airy-compensated seamounts. Low geoid/topography ratios occur over the remaining five areas (e.g., Emperor Seamounts), reflecting the absence of a thermal swell. Our findings do not support the hypothesis that the planform of mantle convection is evident in the geoid. We see only indirect evidence of thermal plumes reheating the lower lithosphere.

Luttrell, K, Sandwell D.  2012.  Constraints on 3-D stress in the crust from support of mid-ocean ridge topography. Journal of Geophysical Research-Solid Earth. 117   10.1029/2011jb008765   AbstractWebsite

The direction of crustal stresses acting at mid-ocean ridges is well characterized, but the magnitude of these stresses is poorly constrained. We present a method by which the absolute magnitude of these stresses may be constrained using seafloor topography and gravity. The topography is divided into a short-wavelength portion, created by rifting, magmatism, and transform faulting, and a long-wavelength portion associated with the cooling and subsidence of the oceanic lithosphere. The short-wavelength surface and Moho topography are used to calculate the spatially varying 3-D stress tensor in the crust by assuming that in creating this topography, the deviatoric stress reached the elastic-plastic limiting stress; the Moho topography is constrained by short-wavelength gravity variations. Under these assumptions, an incompressible elastic material gives the smallest plastic failure stress associated with this topography. This short-wavelength topographic stress generally predicts the wrong style of earthquake focal mechanisms at ridges and transform faults. However, the addition of an in-plane regional stress field is able to reconcile the combined crustal stress with both the ridge and transform focal mechanisms. By adjusting the magnitude of the regional stress, we determine a lower bound for in situ ridge-perpendicular extension of 25-40 MPa along the slow spreading mid-Atlantic ridge, 40-50 MPa along the ultra-slow spreading ridges in the western Indian Ocean, and 10-30 MPa along the fast spreading ridges of the southeastern Indian and Pacific Oceans. Furthermore, we constrain the magnitude of ridge-parallel extension to be between 4 and 8 MPa in the Atlantic Ocean, between -1 and 7 MPa in the western Indian Ocean, and between -1 and 3 MPa in the southeastern Indian and Pacific Oceans. These observations suggest that a deep transform valley is an essential feature of the ridge-transform spreading center.

Sandwell, D, Schubert G.  2010.  A contraction model for the flattening and equatorial ridge of Iapetus. Icarus. 210:817-822.   10.1016/j.icarus.2010.06.025   AbstractWebsite

Others have explained the excess flattening of Iapetus by a model in which the moon formed at a high spin rate, achieved isostatic equilibrium by very rapid interior heating caused by short-lived radioactive isotopes (SLRI), and subsequently cooled, locking in the excess flattening with respect to an equilibrium shape at its present spin rate. Here we propose an alternate model that does not require an unusually high initial spin rate or the SLRI. The initial formation of Iapetus results in a slightly oblate spheroid with porosity >10%. Radioactive heating by long-lived isotopes warms the interior to about 200 K, at which point it becomes ductile and the interior compacts by 10%, while the 120 km-thick exterior shell remains strong. The shell must deform to match the reduced volume of the ductile interior, and we propose that this deformation occurs along the equator, perhaps focused by a thinner equatorial shell. The final shape of the collapsed sphere matches the observed shape of Iapetus today, described as an oblate ellipse, except along the equator where strain concentration forms a broad ridge. To maintain this non-equilibrium shape, the thickness of the shell must exceed 120 km. Testing the equatorial focusing hypothesis will require a model that includes non-linear processes to account for the finite yield strength of the thick lithosphere. Nevertheless, we show that the stress in the lithosphere generated by the contraction of the interior is about 3 times greater than the stress needed to deform the lithosphere, so some type of lithospheric deformation is expected. (C) 2010 Elsevier Inc. All rights reserved.

Baer, G, Sandwell D, Williams S, Bock Y, Shamir G.  1999.  Coseismic deformation associated with the November 1995, M-w=7.1 Nuweiba earthquake, Gulf of Elat (Aqaba), detected by synthetic aperture radar interferometry. Journal of Geophysical Research-Solid Earth. 104:25221-25232.   10.1029/1999jb900216   AbstractWebsite

The November 22, 1995, M-w=7.1 Nuweiba earthquake occurred along one of the left-stepping segments of the Dead Sea Transform in the Gulf of flat (Aqaba). Although it was the largest earthquake along this fault in the last few centuries, little is yet known about the geometry of the rupture, the slip distribution along it, and the nature of postseismic deformation following the main shock. In this study we examine the surface deformation pattern during the coseismic phase of the earthquake in an attempt to better elucidate the earthquake rupture process. As the entire rupture zone was beneath the waters of the Gulf, and there is very little Global Positioning System (GPS) data available in the region for the period spanning the earthquake, interferometric synthetic aperture radar (INSAR) provides the only source of information of surface deformation associated with this earthquake. We chose four synthetic aperture radar (SAR) scenes of about 90x90 km each spanning the rupture area, imaged by the ERS-1 and ERS-2 satellites. The coseismic interferograms show contours of equal satellite-to-ground range changes that correspond to surface displacements due to the earthquake rupture. Interferograms that span the earthquake by 1 week show similar fringe patterns' as those that span the earthquake by 6 months, suggesting that postseismic deformation is minor or confined to the first week after the earthquake. A high displacement gradient is seen on the western side of the Gulf, 20-40 km south of flat and Aqaba, where the total satellite-to-ground range changes are at least 15 cm. The displacement gradient is relatively uniform on the eastern side of the Gulf and the range changes are less than 10 cm. To interpret these results, we compare them to synthetic interferograms generated by elastic dislocation models with a variety of fault parameters. Although selecting the best fit fault parameters is nonunique, we are able to generate a group of simplified model interferograms that provide a reasonable fit to the coseismic interferogram and serve to constrain the location of the fault. The present analysis shows that if the rupture reached the Gulf-bottom surface, the mean sinistral slip along the fault is constrained to about 1.4 m. If surface rupture did not occur, the average sinistral slip is constrained to the range of 1.4-3 m for a fault patch buried 0-4 km below the Gulf-bottom Surface, respectively, with a minor normal component.