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Royer, J-Y, Gahagan LM, Lawver LA, Mayes CL, Nuernberg D, Sandwell DT, Scotese CR.  1990.  A tectonic chart for the Southern Ocean derived from Geosat altimetry data. AAPG Studies in Geology. 31( St. John B, Ed.).:89-99., Tulsa, OK, United States (USA): American Association of Petroleum Geologists, Tulsa, OK AbstractWebsite

Presented is a new tectonic fabric map of the southern ocean south of 45S, derived from Geosat altimeter profiles and published bathymetric charts and magnetic anomaly picks. The interpretation of the Geosat data is based on an analysis of the first derivative of the geoid profiles (i.e., vertical deflection profiles). To improve the accuracy and resolution of the vertical deflection profiles, 22 repeat cycles from the first year of the Geosat/Exact Repeat Mission (Geosat/ERM) were averaged. At wavelengths less than about 200 km, the vertical deflection is highly correlated with sea-floor topography and thus reveals major features in areas that were previously unsurveyed. The density of the Geosat data is greatest in the high latitudes where lineated bathymetric features such as fracture zones, spreading ridges, trenches, and rifted margins stand out. To construct the tectonic fabric chart, the Geosat data are analyzed in combination with available shipboard bathymetric data and magnetic anomaly identifications. (Auth.)

Gahagan, LM, Scotese CR, Royer JY, Sandwell DT, Winn JK, Tomlins RL, Ross MI, Newman JS, Muller RD, Mayes CL, Lawver LA, Heubeck CE.  1988.  Tectonic Fabric Map of the Ocean Basins from Satellite Altimetry Data. Tectonophysics. 155:1-&.   10.1016/0040-1951(88)90258-2   AbstractWebsite

Satellite altimetry data provide a new source of information on the bathymetry of the ocean floor. The tectonic fabric of the oceans (i.e., the arrangement of fracture zones, ridges, volcanic plateaus and trenches) is revealed by changes in the horizontal gravity gradient as recorded by satellite altimetry measurements. SEASAT and GEOSAT altimetry data have been analyzed and a global map of the horizontal gravity gradient has been produced that can be used to identify a variety of marine tectonic features. The uniformity of the satellite coverage provides greater resolution and continuity than maps based solely on ship-track data. This map is also the first global map to incorporate the results of the GEOSAT mission, and as a result, new tectonic features are revealed at high southerly latitudes.This map permits the extension of many tectonic features well beyond what was previously known. For instance, various fracture zones, such as the Ascension, Tasman, and Udintsev fracture zones, can be extended much closer to adjacent coninental margins. The tectonic fabric map also reveals many features that have not been previously mapped. These features include extinct ridges, minor fracture zone lineations and seamounts. In several areas, especially across aseismic plateaus or along the margins of the continents, the map displays broad gravity anomalies whose origin may be related to basement structures.

Mayes, CL, Lawver LA, Sandwell DT.  1990.  Tectonic History and New Isochron Chart of the South-Pacific. Journal of Geophysical Research-Solid Earth and Planets. 95:8543-8567.   10.1029/JB095iB06p08543   AbstractWebsite

We have developed an internally consistent isochron chart and a tectonic history of the South Pacific using a combination of new satellite altimeter data and shipboard magnetic and bathymetric data. Highly accurate, vertical deflection profiles (1–2 μrad), derived from 22 repeat cycles of Geosat altimetry, reveal subtle lineations in the gravity field associated with the South Pacific fracture zones. These fracture zone lineations are correlated with sparse shipboard bathymetric identifications of fracture zones and thus can be used to determine paleospreading directions in uncharted areas. The high density of Geosat altimeter profiles reveals previously unknown details in paleospreading directions on all of the major plates. Magnetic anomaly identifications and magnetic lineation interpretations from published sources were combined with these fracture zone lineations to produce a tectonic fabric map. The tectonic fabric was then used to derive new poles of rotation for 12 selected times in the Late Cretaceous and Cenozoic. From our reconstructions, we estimated the former location of the spreading centers in order to derive a new set of isochrons (interpreted unes of equal age on the ocean floor). We believe that the use of new Geosat altimeter data in combination with a multi-plate reconstruction has led to an improvement in our understanding of South Pacific tectonics.

Sandwell, DT.  1982.  Thermal isostasy; response of a moving lithosphere to a distributed heat source. Journal of Geophysical Research. 87:1001-1014., Washington, DC, United States (USA): American Geophysical Union, Washington, DC   10.1029/JB087iB02p01001   AbstractWebsite

Spreading ridges and hot spot swells are identified by their high surface heat flow, shallow seafloor, and high geopotential. To understand these and other thermotectonic features, the oceanic lithosphere is modeled as a thermomechanical boundary layer moving through a three-dimensional, time-independent heat source. The heat source mimics the heat advection associated with a spreading ridge or hot spot without introducing the nonlinearities of these flow processes. The Fourier transforms of three Green's functions (response functions), which relate the three observable fields to their common heat source, are determined analytically. Each of these reponse functions is highly anisotropic because the lithosphere is moving with respect to the source. However, the ratio of the gravity response function to the topography response function (i.e., gravity/topography transfer function) is nearly isotropic and has a maximum lying between the flexural wavelength and 2pi times the thickness of the thermal boundary layer. The response functions are most useful for determining the surface heat flow, seafloor topography, and geopotential for complex lithospheric thermal structures. In practice, these three observables are calculated by multiplying the Fourier transform of the heat source by the appropriate response function and inverse transforming the products. Almost any time-independent thermotectonic feature can be modeled using this technique. Included in this report are examples of spreading ridges and thermal swells, although more complex geometries such as ridges offset by transform faults and RRR-type triple junctions can also be modeled. Because forward modeling is both linear and computationally simple, the inverse of this technique could be used to infer some basic characteristics of the heat source directly from the observed fields.

Sandwell, DT.  1986.  Thermal-Stress and the Spacings of Transform Faults. Journal of Geophysical Research-Solid Earth and Planets. 91:6405-6417.   10.1029/JB091iB06p06405   AbstractWebsite

Bathymetric charts are used with satellite altimeter profiles to locate major ridge-transform intersections along five spreading ridges. The ridges are the Mid-Atlantic Ridge, the East Pacific Rise, the Chile Rise, the Pacific-Antarctic Rise, and the Southeast Indian Ridge. Analysis of these data show spacings between transform faults W increase linearly with spreading rate ν (W/ν = 6.28 m.y.). This linear correlation is explained by a thermoelastic model of a cooling strip of lithosphere spreading at a rate ν. The traction-free boundaries of the thin elastic strip simulate cracks in the lithosphere at transform faults. A two-dimensional thermoelastic solution for the in-plane stress shows the largest stress component is tensional and parallel to the ridge. Stresses are zero at the ridge and increase as (age)½ to a maximum value at an age of W/4ν. All stress components are small for ages greater than W/ν. When the transform spacing is large compared with the spreading rate (W/ν > 100 m.y.) thermal stresses exceed the strength of the lithosphere for ages between 0 and 30 Ma. The observed maximum ratio of transform spacing to spreading rate (W/ν = 10 m.y.) results in low thermal stresses that only exceed the strength of the lithosphere for ages less than 1 Ma. Thus transform faults relieve most of the thermal stress. Model predictions also agree with earthquake studies showing that normal faults in young lithosphere have tensional axes aligned with the ridge. Moreover, oceanic intraplate earthquakes rarely occur in lithosphere older than 30 Ma as predicted by the model. These and other geophysical observations confirm Turcotte's hypothesis that transform faults are thermal contraction cracks.

Sandwell, DT.  1984.  Thermomechanical Evolution of Oceanic Fracture-Zones. Journal of Geophysical Research. 89:1401-1413.   10.1029/JB089iB13p11401   AbstractWebsite

A fracture zone (FZ) model is constructed from existing models of the thermal and mechanical evolution of the oceanic lithosphere. As the lithosphere cools by conduction, thermal and mechanical boundary layers develop and increase in thickness as (age)1/2. Surface expressions of this development, such as topography and deflection of the vertical (i.e., gravity field), are most apparent along major FZ's because of the sharp age contrast. A simple model, including the effects of lateral heat transport but with no elastic layer, predicts that variations in seafloor depth and deflection of the vertical will become increasingly smooth and ultimately disappear as the FZ ages. Observations, however, show that both the FZ topography and the deflection of the vertical remain sharp as the FZ evolves. These two observations, as well as the observed asymmetry in deflection of the vertical profiles across the Udintsev, Romanche, and Mendocino FZ's, are explained by including a continuous elastic layer in the model. The asymmetry in deflection of the vertical is a consequence of elastic thickness variations across the FZ. Modeling also shows that the evolution of the FZ topography is extremely sensitive to the initial thermal structure near the ridge-transform intersection. Model geoid steps and their development with age are used to access techniques for measuring geoid offsets across FZ's. Reasonable step estimation techniques will underestimate the overall step amplitude by up to 50%. This implies that abnormally thin thermal boundary layers, derived from studies of geoid height versus age, are not required by the data.

Fialko, Y, Sandwell D, Simons M, Rosen P.  2005.  Three-dimensional deformation caused by the Bam, Iran, earthquake and the origin of shallow slip deficit. Nature. 435:295-299.   10.1038/nature03425   AbstractWebsite

Our understanding of the earthquake process requires detailed insights into how the tectonic stresses are accumulated and released on seismogenic faults. We derive the full vector displacement field due to the Bam, Iran, earthquake of moment magnitude 6.5 using radar data from the Envisat satellite of the European Space Agency. Analysis of surface deformation indicates that most of the seismic moment release along the 20-km-long strike-slip rupture occurred at a shallow depth of 4 - 5 km, yet the rupture did not break the surface. The Bam event may therefore represent an end-member case of the 'shallow slip deficit' model, which postulates that coseismic slip in the uppermost crust is systematically less than that at seismogenic depths ( 4 - 10 km). The InSAR-derived surface displacement data from the Bam and other large shallow earthquakes suggest that the uppermost section of the seismogenic crust around young and developing faults may undergo a distributed failure in the interseismic period, thereby accumulating little elastic strain.

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.

Barbot, S, Fialko Y, Sandwell D.  2009.  Three-dimensional models of elastostatic deformation in heterogeneous media, with applications to the Eastern California Shear Zone. Geophysical Journal International. 179:500-520.   10.1111/j.1365-246X.2009.04194.x   AbstractWebsite

P>We present a semi-analytic iterative procedure for evaluating the 3-D deformation due to faults in an arbitrarily heterogeneous elastic half-space. Spatially variable elastic properties are modelled with equivalent body forces and equivalent surface traction in a 'homogenized' elastic medium. The displacement field is obtained in the Fourier domain using a semi-analytic Green function. We apply this model to investigate the response of 3-D compliant zones (CZ) around major crustal faults to coseismic stressing by nearby earthquakes. We constrain the two elastic moduli, as well as the geometry of the fault zones by comparing the model predictions to Synthetic Aperture Radar inferferometric (InSAR) data. Our results confirm that the CZ models for the Rodman, Calico and Pinto Mountain faults in the Eastern California Shear Zone (ECSZ) can explain the coseismic InSAR data from both the Landers and the Hector Mine earthquakes. For the Pinto Mountain fault zone, InSAR data suggest a 50 per cent reduction in effective shear modulus and no significant change in Poisson's ratio compared to the ambient crust. The large wavelength of coseismic line-of-sight displacements around the Pinto Mountain fault requires a fairly wide (similar to 1.9 km) CZ extending to a depth of at least 9 km. Best fit for the Calico CZ, north of Galway Dry Lake, is obtained for a 4 km deep structure, with a 60 per cent reduction in shear modulus, with no change in Poisson's ratio. We find that the required effective rigidity of the Calico fault zone south of Galway Dry Lake is not as low as that of the northern segment, suggesting along-strike variations of effective elastic moduli within the same fault zone. The ECSZ InSAR data is best explained by CZ models with reduction in both shear and bulk moduli. These observations suggest pervasive and widespread damage around active crustal faults.

Smith, B, Sandwell D.  2004.  A three-dimensional semianalytic viscoelastic model for time-dependent analyses of the earthquake cycle. Journal of Geophysical Research-Solid Earth. 109   10.1029/2004jb003185   AbstractWebsite

[ 1] Exploring the earthquake cycle for large, complex tectonic boundaries that deform over thousands of years requires the development of sophisticated and efficient models. In this paper we introduce a semianalytic three-dimensional (3-D) linear viscoelastic Maxwell model that is developed in the Fourier domain to exploit the computational advantages of the convolution theorem. A new aspect of this model is an analytic solution for the surface loading of an elastic plate overlying a viscoelastic half-space. When fully implemented, the model simulates ( 1) interseismic stress accumulation on the upper locked portion of faults, ( 2) repeated earthquakes on prescribed fault segments, and ( 3) the viscoelastic response of the asthenosphere beneath the plate following episodic ruptures. We verify both the analytic solution and computer code through a variety of 2-D and 3-D tests and examples. On the basis of the methodology presented here, it is now possible to explore thousands of years of the earthquake cycle along geometrically complex 3-D fault systems.

Sandwell, DT, Sichoix L.  2000.  Topographic phase recovery from stacked ERS interferometry and a low-resolution digital elevation model. Journal of Geophysical Research-Solid Earth. 105:28211-28222.   10.1029/2000jb900340   AbstractWebsite

A hybrid approach to topographic recovery from ERS interferometry is developed and assessed. Tropospheric/ionospheric artifacts, imprecise orbital information, and layover are key issues in recovering topography and surface deformation from repeat-pass interferometry. Previously, we developed a phase gradient approach to stacking interferograms to reduce these errors and also to reduce the short-wavelength phase noise (see Sandwell and Pi-ice [1998] and Appendix A). Here the method is extended to use a low-resolution digital elevation model to constrain long-wavelength phase errors and an iteration scheme to minimize errors in the computation of phase gradient. We demonstrate the topographic phase recovery on 16-m postings using 25 ERS synthetic aperture radar images from an area of southern California containing 2700 m of relief. On the basis of a comparison with 81 GPS monuments, the ERS-derived topography has a typical absolute accuracy of better than 10 m except in areas of layover. The resulting topographic phase enables accurate two-pass, real-time interferometry even in mountainous areas where traditional phase unwrapping schemes fail. As an example, we form a topography-free (127-m perpendicular baseline) interferogram spanning 7.5 years; fringes from two major earthquakes and aseismic slip on the San Andreas Fault are clearly isolated.

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Bassett, D, Sandwell DT, Fialko Y, Watts AB.  2016.  Upper-plate controls on co-seismic slip in the 2011 magnitude 9.0 Tohoku-oki earthquake. Nature. 531:92-96.: Nature Publishing Group   10.1038/nature16945   Abstract

The March 2011 Tohoku-oki earthquake was only the second giant (moment magnitude Mw ≥ 9.0) earthquake to occur in the last 50 years and is the most recent to be recorded using modern geophysical techniques. Available data place high-resolution constraints on the kinematics of earthquake rupture, which have challenged prior knowledge about how much a fault can slip in a single earthquake and the seismic potential of a partially coupled megathrust interface. But it is not clear what physical or structural characteristics controlled either the rupture extent or the amplitude of slip in this earthquake. Here we use residual topography and gravity anomalies to constrain the geological structure of the overthrusting (upper) plate offshore northeast Japan. These data reveal an abrupt southwest–northeast-striking boundary in upper-plate structure, across which gravity modelling indicates a south-to-north increase in the density of rocks overlying the megathrust of 150–200 kilograms per cubic metre. We suggest that this boundary represents the offshore continuation of the Median Tectonic Line, which onshore juxtaposes geological terranes composed of granite batholiths (in the north) and accretionary complexes (in the south). The megathrust north of the Median Tectonic Line is interseismically locked, has a history of large earthquakes (18 with Mw > 7 since 1896) and produced peak slip exceeding 40 metres in the Tohoku-oki earthquake. In contrast, the megathrust south of this boundary has higher rates of interseismic creep, has not generated an earthquake with MJ > 7 (local magnitude estimated by the Japan Meteorological Agency) since 1923, and experienced relatively minor (if any) co-seismic slip in 20111. We propose that the structure and frictional properties of the overthrusting plate control megathrust coupling and seismogenic behaviour in northeast Japan.

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Shum, CK, Werner RA, Sandwell DT, Zhang BH, Nerem RS, Tapley BD.  1990.  Variations of Global Mesoscale Eddy Energy Observed from GEOSAT. Journal of Geophysical Research-Oceans. 95:17865-&.   10.1029/JC095iC10p17865   AbstractWebsite

The global distribution of eddy kinetic energy has been synoptically observed from analysis of the Geosat Exact Repeat Mission (ERM) altimeter data collected for a 2-year period from November 1986 through November 1988. Using a technique developed by Sandwell and Zhang (1989), altimeter data from forty-four 17-day repeat cycles (2 years) were processed into sea surface slopes along the satellite ground track, averaged, and filtered to produce a mean sea surface slope profile having an estimated accuracy of 0.2 μrad (2 cm sea level change over 100 km distance). A series of global eddy kinetic energy maps, each averaged over 3 months, and their mean were then generated. The maximum mean eddy kinetic energy per unit mass exceeds 2000 cm^2/s^2 for most of the western boundary currents; however, it only reaches approximately 500 cm^2/s^2 for the Antarctic Circumpolar Current (ACC). More than 65% of the world ocean has relatively low variability with an eddy kinetic energy of less than 300 cm^2/s^2. Results obtained from this study are in general agreement with other Geosat ocean variability studies (e.g., Zlotnicki et al., 1989). However, significantly higher variability is found when compared with either Seasat or ship drift data. Significant seasonal variations were found in the Gulf Stream and Kuroshio currents. The ACC system exhibits no apparent seasonal variation.

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.

Howell, S, Smith-Konter B, Frazer N, Tong XP, Sandwell D.  2016.  The vertical fingerprint of earthquake cycle loading in southern California. Nature Geoscience. 9:611-+.   10.1038/ngeo2741   AbstractWebsite

The San Andreas Fault System, one of the best-studied transform plate boundaries on Earth, is well known for its complex network of locked faults that slowly deform the crust in response to large-scale plate motions(1-8). Horizontal interseismic motions of the fault system are largely predictable, but vertical motions arising from tectonic sources remain enigmatic. Here we show that when carefully treated for spatial consistency, global positioning system-derived vertical velocities expose a small-amplitude (+/- 2mmyr(-1)), but spatially considerable (200 km), coherent pattern of uplift and subsidence straddling the fault system in southern California. We employ the statistical method of model selection to isolate this vertical velocity field fromnon-tectonic signals that induce velocity variations in both magnitude and direction across small distances (less than tens of kilometres; ref. 9), and find remarkable agreement with the sense of vertical motions predicted by physical earthquake cycle models spanning the past few centuries(6,10). We suggest that these motions reveal the subtle, but identifiable, tectonic fingerprint of far-field flexure due to more than 300 years of fault locking and creeping depth variability. Understanding this critical component of interseismic deformation at a complex strike-slip plate boundary will better constrain regional mechanics and crustal rheology, improving the quantification of seismic hazards in southern California and beyond.

Kilb, D, Keen CS, Newman RL, Kent GM, Sandwell DT, Vernon FL, Johnson CL, Orcutt JA.  2003.  The Visualization Center at Scripps Institution of Oceanography: Education and Outreach. Seismological Research Letters. 74:641-648. AbstractWebsite

The immersive environment of the Visualization Center at Scripps, coupled with the presentation of current seismological research, has great education and outreach potential. Since its March 2002 opening, the Visualization Center at Scripps has had more than 2,500 visitors, and numerous virtual visitors have explored our Web pages, which include streaming QuickTime movies of geophysical data, tutorials on how to use SGI/Mac/Windows registered visualization software, and examples of visualizations developed by SIO researchers and faculty members (http://siovizcenter.ucsd.edu/library.shtml). We will continue to expand the use of the Visualization Center at Scripps for K-12 and informal education, and to use the center to supply geophysical data sets, movies, and research results to as large a group of educators as possible. Our goal is to develop more sustained K-12 educational programs and to generate assessments of the center's programs and the educational products created at the Center.

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Sandwell, D, Fialko Y.  2004.  Warping and cracking of the Pacific plate by thermal contraction. Journal of Geophysical Research-Solid Earth. 109   10.1029/2004jb003091   AbstractWebsite

Lineaments in the gravity field and associated chains of volcanic ridges are widespread on the Pacific plate but are not yet explained by plate tectonics. Recent studies have proposed that they are warps and cracks in the plate caused by uneven thermal contraction of the cooling lithosphere. We show that the large thermoelastic stress produced by top-down cooling is optimally released by lithospheric flexure between regularly spaced parallel cracks. Both the crack spacing and approximate gravity amplitude are predicted by elastic plate theory and variational principle. Cracks along the troughs of the gravity lineaments provide conduits for the generation of volcanic ridges in agreement with new observations from satellite-derived gravity. Our model suggests that gravity lineaments are a natural consequence of lithospheric cooling so that convective rolls or mantle plumes are not required.

Yale, M, Sandwell D, Herring A.  1998.  What are the limitations of satellite altimetry? The Leading Edge. 17:73-76.: Society of Exploration Geophysicists   10.1190/1.1437832   AbstractWebsite

Radar altimeter measurements of the marine geoid collected during the Seasat altimeter mission gave geophysicists hope of uncovering the gravity field over all the ocean basins. However, because of insufficient track density, it has taken 16 years for the full potential of the satellite altimeter to be realized. The high‐density coverage obtained by ERS-1 during its geodetic mapping phase (April 1994–March 1995) prompted the U.S. Navy to declassify all of the Geosat altimeter data in June 1995. The combination of these two high‐density data sets provided the first global view of all the ocean basins at a wavelength resolution of 20–30 km.