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Smith-Konter, BR, Thornton GM, Sandwell DT.  2014.  Vertical crustal displacement due to interseismic deformation along the San Andreas fault: Constraints from tide gauges. Geophysical Research Letters. 41:3793-3801.   10.1002/2014gl060091   AbstractWebsite

Interseismic motion along complex strike-slip fault systems such as the San Andreas Fault System (SAFS) can produce vertical velocities that are similar to 10 times smaller than horizontal velocities, caused by along-strike variations in fault orientation and locking depth. Tide gauge stations provide a long (50-100 year) recording history of sea level change due to several oceanographic and geologic processes, including vertical earthquake cycle deformation. Here we compare relative sea level displacements with predictions from a 3-D elastic/viscoelastic earthquake cycle model of the SAFS. We find that models with lithospheric structure reflecting a thick elastic plate (> 50km) and moderate viscosities produce vertical motions in surprisingly good agreement with the relative tide gauge uplift rates. These results suggest that sea level variations along the California coastline contain a small but identifiable tectonic signal reflecting the flexure of the elastic plate caused by bending moments applied at the ends of locked faults.

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.

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.

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.

Lyons, SN, Sandwell DT, Smith WHF.  2000.  Three-dimensional estimation of elastic thickness under the Louisville Ridge. Journal of Geophysical Research-Solid Earth. 105:13239-13252.   10.1029/2000jb900065   AbstractWebsite

A three-dimensional approach to estimating elastic thickness is presented which uses dense satellite altimetry and sparse ship bathymetry. This technique is applied to the Louisville Ridge system to study the tectonic history of the region. The inversion is performed as both a first-order approximation and a nonlinear relationship between gravity and topography based on Parker's [1973] equation. While the higher-order effect on the gravity anomaly is nearly zero for most of the region, the magnitude is significant over the summits of the ridge. Nevertheless, the inclusion of the nonlinear terms has only a minor influence on the elastic thickness estimate within each region, lowering the value by similar to 1-2 km compared with the linear result. The incorrect assumption of two dimensionality for circular features exhibits a marked effect on the gravitational anomaly, resulting in false sidelobe structure of nearly 20 mGal for large seamounts. Our elastic thickness estimates are compared with the contradictory values obtained in previous studies by Cazenave and Dominh [1984] and Watts et al. [1988]. We find an increasing elastic thickness along the chain from southeast to northwest, with a discontinuity along the Wishbone scarp. The jump in elastic thickness values northwest of the scarp appears to be an indication of an age discontinuity caused by an extinct spreading center north of the ridge.

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.

Johnson, CL, Sandwell DT.  1994.  Lithospheric Flexure on Venus. Geophysical Journal International. 119:627-647.   10.1111/j.1365-246X.1994.tb00146.x   AbstractWebsite

Topographic flexural signatures on Venus are generally associated with the outer edges of coronae, with some chasmata and with rift zones. Using Magellan altimetry profiles and grids of venusian topography, we identified 17 potential flexure sites. Both 2-D cartesian, and 2-D axisymmetric, thin-elastic plate models were used to establish the flexural parameter and applied load/bending moment. These parameters can be used to infer the thickness, strength and possibly the dynamics of the venusian lithosphere. Numerical simulations show that the 2-D model provides an accurate representation of the flexural parameter as long as the radius of the feature is several times the flexural parameter. However, an axisymmetric model must be used to obtain a reliable estimate of load/bending moment. 12 of the 17 areas were modelled with a 2-D thin elastic plate model, yielding best-fit effective elastic thicknesses in the range 12 to 34 km. We find no convincing evidence for flexure around smaller coronae, though five possible candidates have been identified. These five features show circumferential topographic signatures which, if interpreted as flexure, yield mean elastic thicknesses ranging from 6 to 22 km. We adopt a yield strength envelope for the venusian lithosphere based on a dry olivine rheology and on the additional assumption that strain rates on Venus are similar to, or lower than, strain rates on Earth. Many of the flexural signatures correspond to relatively high plate-bending curvatures so the upper and lower parts of the lithosphere should theoretically exhibit brittle fracture and flow, respectively. For areas where the curvatures are not too extreme, the estimated elastic thickness is used to estimate the larger mechanical thickness of the lithosphere. The large amplitude flexures in Aphrodite Terra predict complete failure of the plate, rendering mechanical thickness estimates from these features unreliable. One smaller corona also yielded an unreliable mechanical thickness estimate based on the marginal quality of the profile data. Reliable mechanical thicknesses found by forward modelling in this study are 21 km-37 km, significantly greater than the 13 km-20 km predictions based on heat-flow scaling arguments and chondritic thermal models. If the modelled topography is the result of lithospheric flexure, then our results for mechanical thickness, combined with the lack of evidence for flexure around smaller features, are consistent with a venusian lithosphere somewhat thicker than predicted. Dynamical models for bending of a viscous lithosphere at low strain rates predict a thick lithosphere, also consistent with low temperature gradients. Recent laboratory measurements indicate that dry crustal materials are much stronger than previously believed. Corresponding time-scales for gravitational relaxation are 10(8)-10(9) yr, making gravitational relaxation an unlikely mechanism for the generation of the few inferred flexural features. If dry olivine is also found to be stronger than previously believed, the mechanical thickness estimates for Venus will be reduced, and will be more consistent with the predictions of global heat scaling models.

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.

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

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

Johnson, CL, Sandwell DT.  1992.  Joints in Venusian Lava Flows. Journal of Geophysical Research-Planets. 97:13601-13610. AbstractWebsite

Venusian plains regions, as imaged by the Magellan spacecraft, display many styles of tectonic and volcanic deformation. Radar images of several areas of the volcanic plains reveal polygonal patterns of bright lineations, Intersection geometries of the lineations defining the polygonal patterns are typical of those found in tensile networks. In addition, the polygonal patterns generally exhibit no preferred orientation, implying that they are the result of horizontally isotropic stress fields. Such stress fields usually arise on the Earth as a consequence of desiccation, freeze-thaw cycles, or cooling and produce mud cracks, ice-wedge polygons, and columnar joints, respectively. We propose that the polygonal patterns seen in the Magellan images of some of the volcanic plains are the result of thermal stresses. We consider two alternative scenarios which would generate sufficient tensile thermal stresses Lo cause failure. The first scenario is that of a cooling lava flow; the residual thermal stress which would develop (assuming no failure of the rock) is tensional and of the order of 400 MPa. This is much greater than the strength of unfractured terrestrial basalt (approximately 10 MPa), so we can expect joints to form during cooling of Venusian lava flows. However, the spacing of the polygonal lineations seen in Magellan images is typically 1-2 km, much larger than the largest spacings of decimeters for joints in terrestrial lavas. The second scenario involves an increased heat flux to the base of the lithosphere; the resulting thermal stresses cause the upper lithosphere to be in tension and the lower lithosphere to be in compression. Brittle tensile failure occurs near the surface due to the finite yield strength of the lithosphere. The maximum depth to which failure occurs increases with increasing elevation of the temperature gradient. For an initially 25-km-thick lithosphere and temperature gradient of ll-degrees/km, this maximum depth varies from 0.5 km to 2 km as the temperature gradient is increased to 12-degrees/km and 22-degrees/km, respectively. Both the cooling flow scenario and the heated lithosphere scenario produce isotropic tensile surface stress patterns, but the heated lithosphere model is more compatible with the kilometer scale of the polygonal patterns seen in Magellan images.

Cheney, RE, Douglas BC, McAdoo DC, Sandwell DT.  1986.  Geodetic and oceanographic applications of satellite altimetry. Space geodesy and geodynamics. ( Anderson A, Cazenave A, Eds.)., London, United Kingdom (GBR): Academic Press, London AbstractWebsite