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Tong, XP, Sandwell DT, Fialko Y.  2010.  Coseismic slip model of the 2008 Wenchuan earthquake derived from joint inversion of interferometric synthetic aperture radar, GPS, and field data. Journal of Geophysical Research-Solid Earth. 115   10.1029/2009jb006625   AbstractWebsite

We derived a coseismic slip model for the M(w) 7.9 2008 Wenchuan earthquake on the basis of radar line-of-sight displacements from ALOS interferograms, GPS vectors, and geological field data. Available interferometric synthetic aperture radar (InSAR) data provided a nearly complete coverage of the surface deformation along both ascending (fine beam mode) and descending orbits (ScanSAR to ScanSAR mode). The earthquake was modeled using four subfaults with variable geometry and dip to capture the simultaneous rupture of both the Beichuan fault and the Pengguan fault. Our model misfits show that the InSAR and GPS data are highly compatible; the combined inversion yields a 93% variance reduction. The best fit model has fault planes that rotate from shallow dip in the south (35 degrees) to nearly vertical dip toward the north (70 degrees). Our rupture model is complex with variations in both depth and rake along two major fault strands. In the southern segment of the Beichuan fault, the slip is mostly thrust (<13 m) and occurred principally in the upper 10 km of the crust; the rupture progressively transformed to right-lateral strike slip as it propagated northeast (with maximum offsets of 7 m). Our model suggests that most of the moment release was limited to the shallow part of the crust (depth less than 10 km). We did not find any "shallow slip deficit" in the slip depth distribution of this mixed mechanism earthquake. Aftershocks were primarily distributed below the section of the fault that ruptured coseismically.

Smith, B, Sandwell D.  2003.  Coulomb stress accumulation along the San Andreas Fault system. Journal of Geophysical Research-Solid Earth. 108   10.1029/2002jb002136   AbstractWebsite

[1] Stress accumulation rates along the primary segments of the San Andreas Fault system are computed using a three-dimensional (3-D) elastic half-space model with realistic fault geometry. The model is developed in the Fourier domain by solving for the response of an elastic half-space due to a point vector body force and analytically integrating the force from a locking depth to infinite depth. This approach is then applied to the San Andreas Fault system using published slip rates along 18 major fault strands of the fault zone. GPS-derived horizontal velocity measurements spanning the entire 1700 x 200 km region are then used to solve for apparent locking depth along each primary fault segment. This simple model fits remarkably well (2.43 mm/yr RMS misfit), although some discrepancies occur in the Eastern California Shear Zone. The model also predicts vertical uplift and subsidence rates that are in agreement with independent geologic and geodetic estimates. In addition, shear and normal stresses along the major fault strands are used to compute Coulomb stress accumulation rate. As a result, we find earthquake recurrence intervals along the San Andreas Fault system to be inversely proportional to Coulomb stress accumulation rate, in agreement with typical coseismic stress drops of 1-10 MPa. This 3-D deformation model can ultimately be extended to include both time-dependent forcing and viscoelastic response.

Lyons, SN, Bock Y, Sandwell DT.  2002.  Creep along the imperial fault, southern California, from GPS measurements. Journal of Geophysical Research-Solid Earth. 107   10.1029/2001jb000763   AbstractWebsite

[1] In May of 1999 and 2000, we surveyed with Global Positioning System (GPS) 46 geodetic monuments established by Imperial College, London, in a dense grid (half-mile spacing) along the Imperial Fault, with three additional National Geodetic Survey sites serving as base stations. These stations were previously surveyed in 1991 and 1993. The Imperial College sites were surveyed in rapid-static mode (15-20 min occupations), while the NGS sites continuously received data for 10 h d(-1). Site locations were calculated using the method of instantaneous positioning, and velocities were determined relative to one of the NGS base stations. Combining our results with far-field velocities from the Southern California Earthquake Center (SCEC), we fit the data to a simple elastic dislocation model with 35 mm yr(-1) of right-lateral slip below 10 km and 9 mm yr(-1) of creep from the surface down to 3 km. The velocity field is asymmetrical across the fault and could indicate a dipping fault plane to the northeast or a viscosity contrast across the fault.

Baer, G, Shamir G, Sandwell D, Bock Y.  2001.  Crustal deformation during 6 years spanning the M (sub w) = 7.2 1995 Nuweiba earthquake, analyzed by Interferometric Synthetic Aperture Radar. Israel Journal of Earth-Sciences. 50( Baer G, Wdowinski S, Eds.).:9-22., Jerusalem, Israel (ISR): Laser Pages Publishing, Jerusalem AbstractWebsite

The November 22, 1995, M (sub w) = 7.2 Nuweiba earthquake occurred along one of the left-stepping segments of the Dead Sea Transform in the Gulf of Elat (Aqaba). We examine the surface deformation patterns in the region by Interferometric Synthetic Aperture Radar (InSAR) for the period 1993 to 1999, which includes the end of one seismic cycle and the beginning of the next. Because the main rupture was under water, ERS coverage is limited to distances of approximately 5 km or more away from the rupture. Pre-earthquake interferograms do not show any detectable deformation along the Gulf. Coseismic interferograms show deformation at distances of up to 50 km from the main rupture, with the highest fringe rate (strain) NW of the rupture termination. Coseismic phase gradient maps show triggered slip along faults parallel to the main rupture (sinistral or normal with the Gulf side down) along the western shore of the Gulf, and in a belt of extensional faults along the eastern shore, striking at angles of about 30 degrees to the major rupture. Postseismic deformation is observed only in a time window of up to 6 months following the mainshock. It was concentrated in the region of the high coseismic strain, and seems to be related to the M (sub L) <4.5 aftershocks in the respective time window.

Schubert, G, Sandwell D.  1989.  Crustal Volumes of the Continents and of Oceanic and Continental Submarine Plateaus. Earth and Planetary Science Letters. 92:234-246.   10.1016/0012-821x(89)90049-6   AbstractWebsite

Global topographic data and the assumption of Airy isostasy have been used to estimate the crustal volumes of the continents and the oceanic and continental submarine plateaus. The calculated crustal volumes are 7182 × 10^6 km^3 for the continents, 242 × 10^6 km^3 for continental submarine plateaus, and 369 × 10^6 km^3 for oceanic plateaus. The Falkland Plateau and the Lord Howe Rise are the two largest continental submarine plateaus with volumes of 48 × 10^6 km^3 and 47 × 10^6 km^3, respectively. Total continental crustal volume is 7581 × 10^6 km^3 (including the volume of continental sediments on the ocean floor 160 × 10^6 km^3), in good agreement with previous estimates. Continental submarine plateaus on the seafloor comprise 3.2% of the total continental crustal volume. The largest oceanic plateaus in order of decreasing size are the Ontong-Java Plateau, the Kerguelen Plateau, the Caribbean, the Chagos Laccadive Ridge, the Ninetyeast Ridge, and the Mid-Pacific Mountains. Together they comprise 54% of the total anomalous crustal volume in oceanic plateaus. An upper bound to the continental crust addition rate by the accretion of oceanic plateaus is 3.7 km^3/yr, a value that assumes accretion of all oceanic plateaus, with a total volume of 4.9% of the continental crustal volume, on a 100 Myr time scale. Even if a substantial fraction of the crustal volume in oceanic plateaus is subducted, accretion of oceanic plateaus could make a contribution to continental growth since the upper bound to the addition rate exceeds recent estimates of the island arc addition rate. Subduction of continental submarine plateaus with the oceanic lithosphere on a 100 Myr time scale gives an upper bound to the continental crustal subtraction rate of 2.4 km^3/yr, much larger than recent estimates of crustal subtraction by subduction of seafloor sediments. Effective subduction of all oceanic plateaus implies equally effective subduction of continental submarine plateaus. A potentially important way to recycle continental crust back into the mantle may be the break off of small fragments from the continents, entrapment of the continental fragments in the seafloor, and subduction of the fragments with the oceanic lithosphere. This process may be occurring in the Mediterranean for Corsica and Sardinia.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Barbot, S, Fialko Y, Sandwell D.  2008.  Effect of a compliant fault zone on the inferred earthquake slip distribution. Journal of Geophysical Research-Solid Earth. 113   10.1029/2007jb005256   AbstractWebsite

We present a new semi-analytic method to evaluate the deformation due to a screw dislocation in arbitrarily heterogeneous and/or anisotropic elastic half plane. The method employs integral transformations to reduce the governing partial differential equations to the integral Fredholm equation of the second kind. Dislocation sources, as well as spatial perturbations in the elastic properties are modeled using equivalent body forces. The solution to the Fredholm equation is obtained in the Fourier domain using a method of successive over-relaxation, and is mapped into the spatial domain using the inverse Fast Fourier Transform. We apply this method to investigate the effect of a soft damage zone around an earthquake fault on the co-seismic displacement field, and on the earthquake slip distribution inferred from inversions of geodetic data. In the presence of a kilometer-wide damage zone with a reduction of the effective shear modulus of a factor of 2, inversions that assume a laterally homogeneous model tend to underestimate the amount of slip in the middle of the seismogenic layer by as much as 20%. This bias may accentuate the inferred maxima in the seismic moment release at depth between 3-6 km suggested by previous studies of large strike-slip earthquakes.

Neves, MC, Cabral J, Luttrell K, Figueiredo P, Rockwell T, Sandwell D.  2015.  The effect of sea level changes on fault reactivation potential in Portugal. Tectonophysics. 658:206-220.   10.1016/j.tecto.2015.07.023   AbstractWebsite

The aim of this study is to assess the impact of sea level changes on both the stress field and the potential of fault reactivation in west Iberia. The analysis is applied to a set of five active faults distributed across Portugal, selected for representing predominant fault directions and for being seismically active. The results show that the rise of sea level since the Last Glacial Maximum has produced flexural effects with distinct impacts on different faults. The Coulomb stress changes induced by the sea level rise along the S. Marcos-Quarteira (south Portugal) and the Horseshoe (offshore SW Iberia) faults are found to be extremely small, independently of the elastic plate thickness. These faults are thus unaffected by flexural effects related to ocean loading, and are unlikely to possess any paleoseismic record of this phenomenon. In contrast, the eustatic sea level rise during the late Pleistocene could have raised the Coulomb stress by 0.5-1 MPa along the Manteigas-Vilarica-Braganca (north Portugal) and Lower Tagus Valley (Lisbon area) fault systems. Such stress perturbations are probably sufficient to impact the seismic cycle of the Manteigas-Vilarica-Braganca fault, bringing it closer to failure and possibly triggering the earthquake clusters that have been observed in previous paleoseismologic studies. (C) 2015 Elsevier B.V. All rights reserved.

Gonzalez-Ortega, A, Fialko Y, Sandwell D, Nava-Pichardo FA, Fletcher J, Gonzalez-Garcia J, Lipovsky B, Floyd M, Funning G.  2014.  El Mayor-Cucapah ( M-w 7.2) earthquake: Early near-field postseismic deformation from InSAR and GPS observations. Journal of Geophysical Research-Solid Earth. 119:1482-1497.   10.1002/2013jb010193   AbstractWebsite

El Mayor-Cucapah earthquake occurred on 4 April 2010 in northeastern Baja California just south of the U.S.-Mexico border. The earthquake ruptured several previously mapped faults, as well as some unidentified ones, including the Pescadores, Borrego, Paso Inferior and Paso Superior faults in the Sierra Cucapah, and the Indiviso fault in the Mexicali Valley and Colorado River Delta. We conducted several Global Positioning System (GPS) campaign surveys of preexisting and newly established benchmarks within 30km of the earthquake rupture. Most of the benchmarks were occupied within days after the earthquake, allowing us to capture the very early postseismic transient motions. The GPS data show postseismic displacements in the same direction as the coseismic displacements; time series indicate a gradual decay in postseismic velocities with characteristic time scales of 669days and 203days, assuming exponential and logarithmic decay, respectively. We also analyzed interferometric synthetic aperture radar (InSAR) data from the Envisat and ALOS satellites. The main deformation features seen in the line-of-sight displacement maps indicate subsidence concentrated in the southern and northern parts of the main rupture, in particular at the Indiviso fault, at the Laguna Salada basin, and at the Paso Superior fault. We show that the near-field GPS and InSAR observations over a time period of 5months after the earthquake can be explained by a combination of afterslip, fault zone contraction, and a possible minor contribution of poroelastic rebound. Far-field data require an additional mechanism, most likely viscoelastic relaxation in the ductile substrate.

Wei, M, Sandwell D.  2006.  Estimates of heat flow from Cenozoic seafloor using global depth and age data. Tectonophysics. 417:325-335.   10.1016/j.tecto.2006.02.004   AbstractWebsite

The total heat output of the Earth constrains models of mantle and core dynamics. Previously published estimates (42-44 TW) have recently been questioned because the measured conductive heat flow on young oceanic lithosphere is about a factor of 2 less than the expected heat flow based on half-space cooling models. Taking the conductive ocean heat flow values at face value reduces the global heat flow from 44 to 31 TW, which has major implications for geodynamics and Earth history. To help resolve this issue, we develop a new method of estimating total oceanic heat flow from depth and age data. The overall elevation of the global ridge system, relative to the deep ocean basins, provides an independent estimate of the total heat content of the lithosphere. Heat flow is proportional to the measured subsidence rate times the heat capacity divided by the thermal expansion coefficient. The largest uncertainty in this method is due to uncertainties in the thermal expansion coefficient and heat capacity. Scalar subsidence rate is computed from gradients of depth and age grids. The method cannot be applied over very young seafloor (< 3 Ma) where age gradient is discontinuous and the assumption of isostasy is invalid. Between 3 and 66 Ma, the new estimates are in agreement with half-space cooling model. Our rnodel-independent estimate of the total heat output of Cenozoic seafloor is 18.6 to 20.5 TW, which leads to a global output of 42 to 44 TW in agreement with previous studies. (c) 2006 Elsevier B.V. All rights reserved.

Luttrell, KM, Tong XP, Sandwell DT, Brooks BA, Bevis MG.  2011.  Estimates of stress drop and crustal tectonic stress from the 27 February 2010 Maule, Chile, earthquake: Implications for fault strength. Journal of Geophysical Research-Solid Earth. 116   10.1029/2011jb008509   AbstractWebsite

The great 27 February 2010 M(w) 8.8 earthquake off the coast of southern Chile ruptured a similar to 600 km length of subduction zone. In this paper, we make two independent estimates of shear stress in the crust in the region of the Chile earthquake. First, we use a coseismic slip model constrained by geodetic observations from interferometric synthetic aperture radar (InSAR) and GPS to derive a spatially variable estimate of the change in static shear stress along the ruptured fault. Second, we use a static force balance model to constrain the crustal shear stress required to simultaneously support observed fore-arc topography and the stress orientation indicated by the earthquake focal mechanism. This includes the derivation of a semianalytic solution for the stress field exerted by surface and Moho topography loading the crust. We find that the deviatoric stress exerted by topography is minimized in the limit when the crust is considered an incompressible elastic solid, with a Poisson ratio of 0.5, and is independent of Young's modulus. This places a strict lower bound on the critical stress state maintained by the crust supporting plastically deformed accretionary wedge topography. We estimate the coseismic shear stress change from the Maule event ranged from -6 MPa (stress increase) to 17 MPa (stress drop), with a maximum depth-averaged crustal shear-stress drop of 4 MPa. We separately estimate that the plate-driving forces acting in the region, regardless of their exact mechanism, must contribute at least 27 MPa trench-perpendicular compression and 15 MPa trench-parallel compression. This corresponds to a depth-averaged shear stress of at least 7 MPa. The comparable magnitude of these two independent shear stress estimates is consistent with the interpretation that the section of the megathrust fault ruptured in the Maule earthquake is weak, with the seismic cycle relieving much of the total sustained shear stress in the crust.

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.

Sandwell, DT, Schubert G.  1992.  Evidence for Retrograde Lithospheric Subduction on Venus. Science. 257:766-770.   10.1126/science.257.5071.766   AbstractWebsite

Annular moats and outer rises around large Venus coronae such as Artemis, Latona, and Eithinoha are similar in arcuate planform and topography to the trenches and outer rises of terrestrial subduction zones. On Earth, trenches and outer rises are modeled as the flexural response of a thin elastic lithosphere to the bending moment of the subducted slab; this lithospheric flexure model also accounts for the trenches and outer rises outboard of the major coronae on Venus. Accordingly, it is proposed that retrograde lithospheric subduction may be occurring on the margins of the large Venus coronae while compensating back-arc extension is occurring in the expanding coronae interiors. Similar processes may be taking place at other deep arcuate trenches or chasmata on Venus such as those in the Dali-Diana chasmata area of eastern Aphrodite Terra.

Winterer, EL, Sandwell DT.  1987.  Evidence from EN-Echelon Cross-Grain Ridges for Tensional Cracks in the Pacific Plate. Nature. 329:534-537.   10.1038/329534a0   AbstractWebsite

Sea-floor topography in the Pacific is mainly aligned with original spreading directions1, but is overprinted by alignments created by mid-plate processes. Spreading produces abyssal hills and fracture zones, and mid-plate volcanism generates seamounts, isolated or in chains. A different category of topography, the 'Cross-grain', discovered in geoid-height data collected by the Seasat radar altimeter2, comprises linear troughs and swells spaced ~200 km apart, oblique to fracture zones and abyssal hills but parallel to the Hawaiian chain. Three models have been proposed for the Cross-grain: small-scale convection, organized into longitudinal rolls by the shear of the Pacific Plate2; compressive buckling3; and lithospheric boudinage resulting from plate-wide tensile stresses4,5. None of the previously available data ruled out any of these models. Here we report multi-beam bathymetric data revealing long, narrow en-echelon ridges along the Cross-grain, interpreted as evidence of tension cracks in the Pacific plate.

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.

Royer, JY, Sandwell DT.  1989.  Evolution of the Eastern Indian-Ocean since the Late Cretaceous - Constraints from Geosat Altimetry. Journal of Geophysical Research-Solid Earth and Planets. 94:13755-13782.   10.1029/JB094iB10p13755   AbstractWebsite

We propose a new model for the tectonic evolution of the eastern Indian Ocean from the Late Cretaceous to the present. Two types of data are used to improve previously published reconstructions. First, recent reinterpretations of seafloor magnetic anomalies, between Australia and Antarctica and in the Wharton Basin, provide new constraints on spreading rates and the timing of major reorganizations. Second, vertical deflection profiles (i.e., horizontal gravity anomaly), derived from 22 repeat cycles of Geosat altimeter data, reveal the tectonic fabric associated with fracture zones. These new Geosat data provide tight constraints on paleospreading directions. For example, three prominent fracture zones can be traced from south of Tasmania to the George V Basin, Antarctica, providing an important constraint on the relative motions of Australia and Antarctica through the Late Eocene. In addition, the Geosat profiles are used to locate the conjugate continental margins and continent-ocean boundaries of Australia and Antarctica, as well as the conjugate rifted margins of Kerguelen Plateau and Broken Ridge. Based on a compilation of magnetic anomaly data from the Crozet Basin, the Central Indian Basin, the Wharton Basin and the Australian-Antarctic Basin, ten plate tectonic reconstructions are proposed. Reconstructions at chrons 5 (11 Ma), 6 (21 Ma), 13 (36 Ma) and 18 (43 Ma) confirm that the Southeast Indian Ridge behaved as a single plate boundary since chron 18. The constraints from the Geosat data provide an improvement in the fit of the Kerguelen Plateau and Broken Ridge at chron 20 (46 Ma). To avoid overlaps between Broken Ridge and the Kerguelen Plateau prior to their breakup, our model includes relative motions between the northern and southern provinces of the Kerguelen Plateau. Finally, we examine the implications of our model for the relative motions of India, Australia and Antarctica on the tectonic evolution of the Kerguelen Plateau and Broken Ridge, and the adjacent Labuan Basin and Diamantina Zone, as well as the emplacement of the Ninetyeast Ridge and the Kerguelen Plateau over a fixed hot spot.

Sandwell, DT.  1996.  Exploration of the remote ocean basins with satellite altimeters. McGraw-Hill 1996 yearbook of science & technology. :178-182., Maidenhead: McGraw-Hill Abstract
Lyons, S, Sandwell D.  2003.  Fault creep along the southern San Andreas from interferometric synthetic aperture radar, permanent scatterers, and stacking. Journal of Geophysical Research-Solid Earth. 108   10.1029/2002jb001831   AbstractWebsite

[1] Interferometric synthetic aperture radar (InSAR) provides a practical means of mapping creep along major strike-slip faults. The small amplitude of the creep signal (<10 mm/yr), combined with its short wavelength, makes it difficult to extract from long time span interferograms, especially in agricultural or heavily vegetated areas. We utilize two approaches to extract the fault creep signal from 37 ERS SAR images along the southern San Andreas Fault. First, amplitude stacking is utilized to identify permanent scatterers, which are then used to weight the interferogram prior to spatial filtering. This weighting improves correlation and also provides a mask for poorly correlated areas. Second, the unwrapped phase is stacked to reduce tropospheric and other short-wavelength noise. This combined processing enables us to recover the near-field (&SIM;200 m) slip signal across the fault due to shallow creep. Displacement maps from 60 interferograms reveal a diffuse secular strain buildup, punctuated by localized interseismic creep of 4-6 mm/yr line of sight (LOS, 12-18 mm/yr horizontal). With the exception of Durmid Hill, this entire segment of the southern San Andreas experienced right-lateral triggered slip of up to 10 cm during the 3.5-year period spanning the 1992 Landers earthquake. The deformation change following the 1999 Hector Mine earthquake was much smaller (<1 cm) and broader than for the Landers event. Profiles across the fault during the interseismic phase show peak-to-trough amplitude ranging from 15 to 25 mm/yr (horizontal component) and the minimum misfit models show a range of creeping/locking depth values that fit the data.