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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, Smith WHF.  1997.  Marine gravity anomaly from Geosat and ERS 1 satellite altimetry. Journal of Geophysical Research-Solid Earth. 102:10039-10054.   10.1029/96jb03223   AbstractWebsite

Closely spaced satellite altimeter profiles collected during the Geosat Geodetic Mission (similar to 6 km) and the ERS 1 Geodetic Phase (8 km) are easily converted to grids of vertical gravity gradient and gravity anomaly. The long-wavelength radial orbit error is suppressed below the noise level of the altimeter by taking the along-track derivative of each profile. Ascending and descending slope profiles are then interpolated onto separate uniform grids. These four grids are combined to form comparable grids of east and north vertical deflection using an iteration scheme that interpolates data gaps with minimum curvature. The vertical gravity gradient is calculated directly from the derivatives of the vertical deflection grids, while Fourier analysis is required to construct gravity anomalies from the two vertical deflection grids. These techniques are applied to a combination of high-density data from the dense mapping phases of Geosat and ERS 1 along with lower-density but higher-accuracy profiles from their repeat orbit phases. A comparison with shipboard gravity data shows the accuracy of the satellite-derived gravity anomaly is about 4-7 mGal for random skip tracks. The accuracy improves to 3 mGal when the ship track follows a Geosat Exact Repeat Mission track line. These data provide the first view of the ocean floor structures in many remote areas of the Earth. Some applications include inertial navigation, prediction of seafloor depth, planning shipboard surveys, plate tectonics, isostasy of volcanoes and spreading ridges, and petroleum exploration.

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, D, Smith-Konter B.  2018.  Maxwell: A semi-analytic 4D code for earthquake cycle modeling of transform fault systems. Computers & Geosciences. 114:84-97.   10.1016/j.cageo.2018.01.009   AbstractWebsite

We have developed a semi-analytic approach (and computational code) for rapidly calculating 3D time-dependent deformation and stress caused by screw dislocations imbedded within an elastic layer overlying a Maxwell viscoelastic half-space. The maxwell model is developed in the Fourier domain to exploit the computational advantages of the convolution theorem, hence substantially reducing the computational burden associated with an arbitrarily complex distribution of force couples necessary for fault modeling. The new aspect of this development is the ability to model lateral variations in shear modulus. Ten benchmark examples are provided for testing and verification of the algorithms and code. One final example simulates interseismic deformation along the San Andreas Fault System where lateral variations in shear modulus are included to simulate lateral variations in lithospheric structure.

Smith, WHF, Sandwell DT.  1997.  Measured and estimated seafloor topography : [world]. Research publication / World Data Center-A for Marine Geology and Geophysics RP-1. , Boulder, COLa Jolla, CA: National Geophysical Data Center, NOAA ;Geological Data Center, Scripps Institution of Oceanography, Abstract
Levitt, DA, Sandwell DT.  1996.  Modal depth anomalies from multibeam bathymetry: Is there a south Pacific superswell? Earth and Planetary Science Letters. 139:1-16.   10.1016/0012-821x(95)00247-a   AbstractWebsite

A region west of the southern East Pacific Rise (SEPR), between the Marquesas and Austral Fracture Zones has previously been found to exhibit anomalous depth-age behavior, based on gridded bathymetry and single-beam soundings. Since gridded bathymetry has been shown to be unsuitable for some geophysical analysis and since the area is characterized by unusually robust volcanism, the magnitude and regional extent of depth anomalies over the young eastern flank of the so called 'South Pacific Superswell' are re-examined using a mode-seeking estimation procedure on data obtained from several recent multibeam surveys. The modal technique estimates a representative seafloor depth, based on the assumption that bathymetry from non-edifice and edifice-populated seafloor has a low and a high standard deviation, respectively. Flat seafloor depth values are concentrated in a few bins which correspond to the mode. This method estimates a representative seafloor value even on seafloor for which more than 90% of coverage is dominated by ridge and seamount clusters, where the mean and median estimates may be shallow by hundreds of meters. Where volcanism-related bias is moderate, the mode, mean and median estimates are close. Depth-age results indicate that there is only a small anomaly (< 200 m) over 15-35 Ma Pacific Plate seafloor with little age-dependent shallowing, suggesting that the lithosphere east of the main hot-spot locations on the 'superswell' is normal. An important implication is that, in sparsely surveyed areas, depths from ETOPO-5 are significantly different from true depths even at large scales (similar to 1000 km) and thus are unsuitable for investigations of anomalies associated with depth-age regressions. We find that seafloor slopes on conjugate profiles of the Pacific and Nazca Plates from 15 to 35 Ma are both slightly lower than normal, but are within the global range. Proximate to the SEPR, seafloor slopes are very low (218 m Myr(-1/2)) on the Pacific Plate (0-22 Ma) and slightly high (similar to 410 m Myr(-1/2)) on the Nazca Plate (0-8 Ma); slopes for older Pacific seafloor (22-37 Ma) are near normal (399 m Myr(-1/2)). Seafloor slopes are even lower north of the Marquesas Fracture Zone but are highly influenced by the Marquesas Swell. We find that the low subsidence rate on young Pacific seafloor cannot be explained by a local hot-spot or a small-scale convective model exclusively and a stretching/thickening model requires implausible crustal thickness variation (similar to 30%).

Smith, BR, Sandwell DT.  2006.  A model of the earthquake cycle along the San Andreas Fault System for the past 1000 years. Journal of Geophysical Research-Solid Earth. 111   10.1029/2005jb003703   AbstractWebsite

[1] We simulate 1000 years of the earthquake cycle along the San Andreas Fault System by convolving best estimates of interseismic and coseismic slip with the Green's function for a point dislocation in an elastic plate overlying a viscoelastic half-space. Interseismic slip rate is based on long-term geological estimates while fault locking depths are derived from horizontal GPS measurements. Coseismic and postseismic deformation is modeled using 70 earthquake ruptures, compiled from both historical data and paleoseismic data. This time-dependent velocity model is compared with 290 present-day geodetic velocity vectors to place bounds on elastic plate thickness and viscosity of the underlying substrate. Best fit models (RMS residual of 2.46 mm/yr) require an elastic plate thickness greater than 60 km and a substrate viscosity between 2 x 10(18) and 5 x 10(19) Pa s. These results highlight the need for vertical velocity measurements developed over long time spans (> 20 years). Our numerical models are also used to investigate the 1000-year evolution of Coulomb stress. Stress is largely independent of assumed rheology, but is very sensitive to the slip history on each fault segment. As expected, present-day Coulomb stress is high along the entire southern San Andreas because there have been no major earthquakes over the past 150 - 300 years. Animations S1 and S2 of the time evolution of vector displacement and Coulomb stress are available as auxiliary material.

Luttrell, K, Sandwell D, Smith-Konter B, Bills B, Bock Y.  2007.  Modulation of the earthquake cycle at the southern San Andreas fault by lake loading. Journal of Geophysical Research-Solid Earth. 112   10.1029/2006jb004752   AbstractWebsite

Changes in the level of ancient Lake Cahuilla over the last 1500 years in the Salton Trough alter the state of stress by bending the lithosphere in response to the applied lake load and by varying the pore pressure magnitude within the crust. The recurrence interval of the lake is similar to the recurrence interval of rupture on the southern San Andreas and San Jacinto faults, both of which are partially covered by the lake at its highstand. Furthermore, four of the last five ruptures on the southern San Andreas fault have occurred near a time of substantial lake level change. We investigate the effect of Coulomb stress perturbations on local faults due to changing level of Lake Cahuilla to determine a possible role for the lake in affecting the timing of fault rupture. Coulomb stress is calculated with a three-dimensional model of an elastic plate overlying a viscoelastic half-space. Plate thickness and half-space relaxation time are adjusted to match observed vertical deformation since the last lake highstand. The lake cycle causes positive and negative Coulomb stress perturbations of 0.2-0.6 MPa on the southern San Andreas within the lake and 0.1-0.2 MPa on the southern San Andreas outside the lake. These Coulomb stress perturbations are comparable to stress magnitudes known to have triggered events at other faults along the North America-Pacific plate boundary.