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Myer, D, Sandwell D, Brooks B, Foster J, Shimada M.  2008.  Inflation along Kilauea's Southwest Rift Zone in 2006. Journal of Volcanology and Geothermal Research. 177:418-424.   10.1016/j.jvolgeores.2008.06.006   AbstractWebsite

We report on InSAR and GPS results showing the first crustal inflation along the southwest rift zone at Kilauea volcano in over 20 years. Two independent interferograms (May 2-August 2, 2006 and June 22-Nov 7, 2006) from the ALOS PALSAR instrument reveal domal uplift located southwest of the main caldera. The uplift is bounded on the northeast by the caldera and follows the southwest rift zone for about 12 km. It is approximately 8 km wide. We use data derived from permanent GPS stations to calibrate the InSAR displacement data and estimate uplift of 7.7 cm during the first interferogram and 8.9 cm during the second with line-of-sight volumes of 2.8 x 10(6) m(3) and 3.0 X 10(6) m(3) respectively. The earthquake record for the periods before, during, and after inflation shows that a swarm of shallow earthquakes (z<5 km) signaled the beginning of the uplift and that elevated levels of shallow seismicity along the rift zones occurred throughout the uplift period. GPS data indicate that the inflation occurred steadily over nine months between mid-January and mid-October, 2006 making injection of a sill unlikely. We attribute the inflation to recharge of a shallow ductile area under the SWRZ. (c) 2008 Elsevier B.V. All rights reserved.

Muller, RD, Qin XD, Sandwell DT, Dutkiewicz A, Williams SE, Flament N, Maus S, Seton M.  2016.  The GPlates Portal: Cloud-based interactive 3D visualization of global geophysical and geological data in a web browser. Plos One. 11   10.1371/journal.pone.0150883   AbstractWebsite

The pace of scientific discovery is being transformed by the availability of 'big data' and open access, open source software tools. These innovations open up new avenues for how scientists communicate and share data and ideas with each other and with the general public. Here, we describe our efforts to bring to life our studies of the Earth system, both at present day and through deep geological time. The GPlates Portal (portal.gplates.org) is a gateway to a series of virtual globes based on the Cesium Javascript library. The portal allows fast interactive visualization of global geophysical and geological data sets, draped over digital terrain models. The globes use WebGL for hardware-accelerated graphics and are cross-platform and cross-browser compatible with complete camera control. The globes include a visualization of a high-resolution global digital elevation model and the vertical gradient of the global gravity field, highlighting small-scale seafloor fabric such as abyssal hills, fracture zones and seamounts in unprecedented detail. The portal also features globes portraying seafloor geology and a global data set of marine magnetic anomaly identifications. The portal is specifically designed to visualize models of the Earth through geological time. These space-time globes include tectonic reconstructions of the Earth's gravity and magnetic fields, and several models of long-wavelength surface dynamic topography through time, including the interactive plotting of vertical motion histories at selected locations. The globes put the on-the-fly visualization of massive data sets at the fingertips of end-users to stimulate teaching and learning and novel avenues of inquiry.

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.

Mellors, RJ, Sichoix L, Sandwell DT.  2002.  Lack of precursory slip to the 1999 Hector Mine, California, earthquake as constrained by InSAR. Bulletin of the Seismological Society of America. 92:1443-1449.   10.1785/0120010244   AbstractWebsite

We looked for evidence of interseismic strain occurring between the 1992 Landers earthquake and the 1999 Hector Mine earthquake near the Lavic Lake and Bullion faults by using interferometric synthetic aperture radar (InSAR). Interferograms covering the Hector Mine epicentral region were studied for possible slip along the Bullion and Lavic Lake faults by both visual inspection and a matched filter technique intended to emphasize slip located at the nucleation point. Some indications of possible deformation associated with the 5 July 1992 M-L 5.4 Pisgah event was observed, but high decorrelation prevented a conclusive determination. We have seen no evidence for precursory slip in the epicentral region up to 30 days before the Hector Mine event. We estimated that the slip equivalent to a M-w 4.5 event would have been observable in the months before the Hector Mine event, and this places an upper bound on the long-term precursory slip, had it occurred. We have noted that InSAR is well suited for detecting precursory slip in general due to the high spatial resolution and the lack of ground instrumentation required but that the detection level depends on the depth and orientation of the slip.

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.

McAdoo, DC, Sandwell DT.  1989.  On the Source of Cross-Grain Lineations in the Central Pacific Gravity-Field. Journal of Geophysical Research-Solid Earth and Planets. 94:9341-9352.   10.1029/JB094iB07p09341   AbstractWebsite

Subtle lineations in the marine gravity field of the central Pacific derived from Seasat altimeter data were observed by Haxby and Weissel (1986). They suggested that these “cross-grain” lineations were evidence of small-scale convection beneath the Pacific plate. We have examined these features by comparing multiple, collinear gravity and bathymetry profiles in the Fourier transform domain. Our nine gravity profiles were each obtained by stacking (averaging) three or more individual, repeat Geosat/ERM altimeter passes. Prior to stacking, the individual Geosat passes were fit to a cubic spline and then differentiated along track to produce along-track deflections of the vertical (or horizontal gravity). Corresponding bathymetric profiles were produced by projecting, onto Geosat ground tracks, bathymetric observations from six R/V Thomas Washington legs and three R/V Conrad legs that virtually coincide with these Geosat tracks. After Fourier transforming the resulting gravity and bathymetry profiles, we estimate admittances of gravity to bathymetry. These admittances are generally low; they also tend to be negative at very short wavelengths (λ<50 km). They are consistent with models of flexural isostatic compensation by a very thin lithosphere (approximately 2 km). They are not consistent with models of dynamic compensation. We suggest, therefore, that either (1) these cross-grain lineations began to form very near the East Pacific Rise or (2) they formed on older, anomalously weak lithosphere. We also suggest that the gravity lineations result primarily from loads beneath the seafloor in combination with, secondarily, loads on the seafloor. Depths of these subseafloor loads appear not to exceed significantly typical Moho depths.

McAdoo, DC, Sandwell DT.  1985.  Folding of Oceanic Lithosphere. Journal of Geophysical Research-Solid Earth and Planets. 90:8563-8569.   10.1029/JB090iB10p08563   AbstractWebsite

Folding of the lithosphere just south of the Bay of Bengal appears as (1) undulations in acoustic basement topography and (2) as linear geoid undulations in the Seasat altimeter data. From the Seasat data we find that the east-west trending folds have wavelengths ranging from 130 to 250 km and clustering about 190 km. The horizontal gravity disturbances due to the folds range in amplitude from 15 to 50 mGal. Elastic models of oceanic lithosphere have, in the past, been used to demonstrate the implausibility of lithosphere buckling, or folding, in response to compression. These elastic models typically predict that compressive stresses of about 5 GPa are required to buckle oceanic lithosphere with an age comparable to that of the northeastern Indian Ocean (40–70 Ma). These stresses exceed the strength of lithospheric rock. We use an elastic-plastic model to show that oceanic lithosphere of this age should have a net compressive strength equal to about 12% of the elastic buckling stress. We further demonstrate that loads approaching the net compressive strength cause the lithosphere to fold with a wavelength about 200 km, i.e., the wavelength observed from Seasat. Our results reinforce earlier speculation that this folding may be related to the Himalayan orogeny.

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.

Matthews, KJ, Mullner RD, Sandwell DT.  2016.  Oceanic microplate formation records the onset of India-Eurasia collision. Earth and Planetary Science Letters. 433:204-214.   10.1016/j.epsl.2015.10.040   AbstractWebsite

Mapping of seafloor tectonic fabric in the Indian Ocean, using high-resolution satellite-derived vertical gravity gradient data, reveals an extinct Pacific-style oceanic microplate ('Mammerickx Microplate') west of the Ninetyeast Ridge. It is one of the first Pacific-style microplates to be mapped outside the Pacific basin, suggesting that geophysical conditions during formation probably resembled those that have dominated at eastern Pacific ridges. The microplate formed at the Indian-Antarctic ridge and is bordered by an extinct ridge in the north and pseudofault in the south, whose conjugate is located north of the Kerguelen Plateau. Independent microplate rotation is indicated by asymmetric pseudofaults and rotated abyssal hill fabric, also seen in multibeam data. Magnetic anomaly picks and age estimates calculated from published spreading rates suggest formation during chron 21o (similar to 47.3 Ma). Plate reorganizations can trigger ridge propagation and microplate development, and we propose that Mammerickx Microplate formation is linked with the India-Eurasia collision (initial 'soft' collision). The collision altered the stress regime at the Indian-Antarctic ridge, leading to a change in segmentation and ridge propagation from an establishing transform. Fast Indian-Antarctic spreading that preceded microplate formation, and Kerguelen Plume activity, may have facilitated ridge propagation via the production of thin and weak lithosphere; however both factors had been present for tens of millions of years and are therefore unlikely to have triggered the event. Prior to the collision, the combination of fast spreading and plume activity was responsible for the production of a wide region of undulate seafloor to the north of the extinct ridge and 'W' shaped lineations that record back and forth ridge propagation. Microplate formation provides a precise means of dating the onset of the India-Eurasia collision, and is completely independent of and complementary to timing constraints derived from continental geology or convergence histories. (C) 2015 Elsevier B.V. All rights reserved.

Marks, KM, Sandwell DT, Vogt PR, Hall SA.  1991.  Mantle Downwelling beneath the Australian-Antarctic Discordance Zone - Evidence from Geoid Height Versus Topography. Earth and Planetary Science Letters. 103:325-338.   10.1016/0012-821x(91)90170-m   AbstractWebsite

The Australian-Antarctic discordance zone (AAD) is an anomalously deep and rough segment of the Southeast Indian Ridge between 120-degrees and 128-degrees-E. A large, negative (deeper than predicted) depth anomaly is centered on the discordance, and a geoid low is evident upon removal of a low-order geoid model and the geoid height-age relation. We investigate two models that may explain these anomalies: a deficiency in ridge-axis magma supply that produces thin oceanic crust (i.e. shallow Airy compensation), and a downwelling and/or cooler mantle beneath the AAD that results in deeper convective-type compensation. To distinguish between these models, we have calculated the ratio of geoid height to topography from the slope of a best line fit by functional analysis (i.e. non-biased linear regression), a method that minimizes both geoid height and topography residuals. Geoid/topography ratios of 2.1 +/- 0.9 m/km for the entire study area (38-degrees-60-degrees-S, 105-degrees-140-degrees-E), 2.3 +/- 1.8 m/km for a subset comprising crust less-than-or-equal-to 25 Ma, and 2.7 +/- 2.0 m/km for a smaller area centered on the AAD were obtained. These ratios are significantly larger than predicted for thin oceanic crust (0.4 m/km), and 2.7 m/km is consistent with downwelling convection beneath young lithosphere. Average compensation depths of 27, 29, and 34 km, respectively, estimated from these ratios suggest a mantle structure that deepens towards the AAD. The deepest compensation (34 km) of the AAD is below the average depth of the base of the young lithosphere (approximately 30 km), and a downwelling of asthenospheric material is implied. The observed geoid height-age slope over the discordance is unusually gradual at -0.133 m/m.y. We calculate that an upper mantle 170-degrees-C cooler and 0.02 g/cm3 denser than normal can explain the shallow slope. Unusually fast shear velocities in the upper 200 km of mantle beneath the discordance, and major-element geochemical trends consistent with small amounts of melting at shallow depths, provide strong evidence for cooler temperatures beneath the AAD.

Marks, KM, Smith WHF, Sandwell DT.  2013.  Significant improvements in marine gravity from ongoing satellite missions. Marine Geophysical Research. 34:137-146.   10.1007/s11001-013-9190-8   AbstractWebsite

Incorporating new altimeter data from CryoSat-2 (30 months), Envisat (18 months), and Jason-1 (7 months) satellites into an updated marine gravity field yields significant reduction in noise and improved resolution. Compared to an older gravity field that did not include the new altimeter data, incoherent power is reduced globally by approximately 2.9 dB at 15 km, 1.6 dB at 20 km, and 1.0 dB at 25 km wavelengths. Coherence analyses between the updated gravity and recent multibeam surveys distributed throughout the world's oceans shows an average increase of similar to 0.023 in mean coherence in the 20-160 km waveband, with the biggest increase (> 0.08) over fast spreading ridges and smallest (< 0.02) over slow spreading ridges and continental shelves. The shortest wavelength at which coherence is above 0.5 decreased globally by similar to 2 km wavelength, with the biggest decrease (> 3.5 km) over fast spreading ridges and smallest (< 1.5 km) over slow spreading ridges and continental shelves. In the Clipperton fracture zone area these improvements result in seamounts that are more accurately located, the detection of smaller seamounts, and the expression of north-south trending abyssal hill fabric. As more altimeter data from the ongoing satellite missions are incorporated into future gravity field updates, finer-scale details of the seafloor will continue to emerge.

Marks, KM, Sandwell DT.  1991.  Analysis of Geoid Height Versus Topography for Oceanic Plateaus and Swells Using Nonbiased Linear-Regression. Journal of Geophysical Research-Solid Earth and Planets. 96:8045-8055.   10.1029/91jb00240   AbstractWebsite

We have investigated the relationship between geoid height and topography for 53 oceanic plateaus and swells to determine the mode of compensation. The ratio of geoid height to topography was obtained from the slope of a best line fit by functional analysis (i.e. nonbiased linear regression), a method that minimizes both geoid height and topography residuals. This method is more appropriate than traditional least squares analysis that minimizes only geoid height residuals, because uncertainties are present in both data types. We find that approximately half of the oceanic and continental plateaus analyzed have low ratios that are consistent with Airy-compensated crustal thickening. The remaining plateaus, however, have higher geoid/topography ratios than predicted by the simple Airy model, and the seismically determined Moho depths beneath some of these features are too shallow for crustal thickening alone. A two-layer Airy compensation model, composed of thickened crust underlain by an anomalously low density "mantle root", is used to explain these observations. The Walvis Ridge, and the Agulhas, Crozet, and north Kerguelen plateaus have geoid/topography ratios and Moho depths that are consistent with the two-layer Airy model. The proximity of the Agulhas Plateau to a RRR triple junction during its early development, and the excessive volcanism at active spreading ridges that created the Crozet and north Kerguelen plateaus and the Walvis Ridge, may have produced regions of enhanced depletion and hence the low-density mantle anomalies. If this explanation is correct, then the low-density mantle anomaly persists over time and remains embedded in the lithosphere beneath the oceanic feature.

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.

Mammerickx, J, Sandwell D.  1986.  Rifting of Old Oceanic Lithosphere. Journal of Geophysical Research-Solid Earth and Planets. 91:1975-1988.   10.1029/JB091iB02p01975   AbstractWebsite

Geophysical data from five regions in the Pacific and Indian oceans reveal that long distance (>400 km) spreading center jumps have occurred in the past. The present-day seafloor morphology is used to develop a scenario for a spreading center jump. The major events are (1) thinning and weakening of the lithosphere at the future rifting site, (2) rifting of the weakened lithosphere (during rifting, the crack is filled from above by normal faulting and wedge subsidence; viscous upwelling fills the crack from below), (3) spreading at the rift site results in a ridge bounded by two troughs (spreading ceases at the dying spreading center, resulting in a deep central graben surrounded by flexural ridges; periods of slow spreading at both spreading centers produce rough topography), (4) ageing and cooling that produce a general deepening of the abandoned spreading ridge and also reduce the thermal contrast across the fossil rifting site. The new spreading center develops into a normal spreading rift. The major topographic expressions apparent in the seafloor today are the deep trough of the abandoned spreading center and the proximal and distal troughs which formed when the emerging spreading center bisected the fossil rifting site. The proximal trough (nearer the new spreading ridge) and the distal trough (farther from the new ridge) are first-order topographic features, 100–1000 km long and 300 km wide, resembling fracture zones with which they are often confused. They share with fracture zones the characteristic of bringing together fragments of lithosphere of different ages, but unlike fracture zones they are generally parallel to magnetic lineations.

Malinverni, ES, Sandwell DT, Tassetti AN, Cappelletti L.  2014.  InSAR decorrelation to assess and prevent volcanic risk. European Journal of Remote Sensing. 47:537-556.   10.5721/EuJRS20144730   AbstractWebsite

SAR can be invaluable describing pre-eruption surface deformation and improving the understanding of volcanic processes. This work studies correlation of pairs of SAR images focusing on the influence of surface, climate conditions and acquisition band. Chosen L-band and C-band images (ENVISAT, ERS and ALOS) cover most of the Yellowstone caldera (USA) over a span of 4 years, sampling all the seasons. Interferograms and correlation maps are generated and studied in relation to snow depth and temperature. To isolate temporal decorrelation pairs of images with the shortest baseline are chosen. Results show good performance during winter, bad attitude towards wet snow and good coherence during summer with L-band performing better over vegetation.

Maia, M, Ackermand D, Dehghani GA, Gente P, Hekinian R, Naar D, O'Connor J, Perrot K, Morgan JP, Ramillien G, Revillon S, Sabetian A, Sandwell D, Stoffers P.  2000.  The Pacific-Antarctic Ridge-Foundation hotspot interaction: a case study of a ridge approaching a hotspot. Marine Geology. 167:61-84.   10.1016/s0025-3227(00)00023-2   AbstractWebsite

The Foundation hotspot-Pacific-Antarctic Ridge (PAI) system is the best documented case of a fast spreading ridge approaching a hotspot and interacting with it. The morphology, crustal structure inferred from gravity anomalies and the chemical composition of the lavas of the axial area of the PAR show evidence of the influence of the hotspot, that is presently located roughly 35 km west of the spreading ridge axis. Along-axis variation in the Mantle Bouguer anomaly is about 28 mGal, corresponding to a crustal thickening of 1.5 km where the hotspot is nearer to the PAR. Anomalous ridge elevation is 650 m and the along-axis width of the chemical anomaly is 200 km. A comparison of these axial parameters with those derived for other ridge-hotspot systems, suggests that the amount of plume material reaching the ridge axis is smaller for the Foundation-PAR system. This implies a weaker connection between the plume and the ridge. Cumulative effects of a fast spreading rate and of a fast ridge-hotspot relative motion can be responsible for this weak plume-ridge flow. The how from the hotspot may be less efficiently channelled towards the ridge axis when a fast ridge is rapidly moving towards a hotspot. (C) 2000 Elsevier Science B.V. All rights reserved.