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Thomas S. James - One of the best experts on this subject based on the ideXlab platform.

  • long term monitoring by absolute gravimetry tides to Postglacial Rebound
    Journal of Geodynamics, 2006
    Co-Authors: A Lambert, N Courtier, Thomas S. James
    Abstract:

    Abstract Absolute gravity measurements over nearly a decade at 10 field sites in North America have provided insights on a range of phenomena from tides to Postglacial Rebound. In this overview we demonstrate the potential of long-term, terrestrial gravity measurements at selected sites to assist in the interpretation of temporal variations in the global gravity field from the Gravity Recovery and Climate Experiment (GRACE) mission. Absolute gravity measurements can be used to calibrate annual soil moisture effects observed by GRACE and to complement GRACE results over periods longer than the expected mission lifetime. Although the preferred terrestrial gravity monitoring system is a global network of superconducting gravimeters (Global Geodynamics Project) in combination with regular absolute gravity measurements, major advances can be made using absolute gravimetry alone. Operating the FG5 gravimeter in continuous mode at a near-shore site shows that special attention must be paid to possible sea-level related biases on gravity values as a result of near-shore hydrological effects. Where soil becomes saturated annually, simple soil moisture models can be used to remove the annual soil moisture effect from gravity, or to invert gravity observations for the soil moisture effect on space missions. Our measurements show that the phenomenon of “episodic tremor and slip” (ETS) in the Cascadia Subduction Zone is accompanied by gravity change that is most likely caused by mass redistribution and not height change. An inter-annual variation of unknown origin with a “period” of around 7 years is present at 10 North American field sites, as well as in data from Table Mountain Gravity Observatory, Boulder, Colorado. Correcting the long-term gravity trends for the inter-annual variation brings the trends into line with GPS vertical rates from four, co-located, continuous, GPS sites in the mid-continent and allows conclusions to be drawn on the thickness of the Laurentide ice-sheet.

  • gps crustal strain Postglacial Rebound and seismic hazard in eastern north america the saint lawrence valley example
    Journal of Geophysical Research, 2005
    Co-Authors: Stephane Mazzotti, Thomas S. James, Joe Henton, J Adams
    Abstract:

    [1] We present Global Positioning System (GPS) measurements that constrain the amplitude, pattern, and origin of crustal deformation in the Saint Lawrence valley, Quebec, one of the most seismically active regions of eastern North America. The GPS network shows coherent southeastward motion of 0.6 ± 0.2 mm yr−1, relative to North America, and uplift of 2.6 ± 0.4 mm yr−1. Network average horizontal strain rates are mostly ESE-WNW shortening at (1.7 ± 1.0) × 10−9 yr−1. The shortening rate across the Charlevoix seismic zone is about twice as big as the regional average. These measurements are consistent with both Postglacial Rebound (PGR) models and the deformation style indicated by earthquake focal mechanisms. Although the GPS data do not discriminate between various models of crustal deformation, they provide important constraints on large earthquake recurrence. Assuming that the GPS strain estimates are representative of seismic moment release, they constrain the maximum magnitude of truncated Gutenberg-Richter recurrence statistics in the Charlevoix seismic zone to MX = 7.8 ± 0.6 (one M ≥ 7 earthquake per 400–1300 years). The remarkable agreement between the GPS strain rates, seismic catalogue statistics, and PGR predictions suggests that in Charlevoix, most of the PGR-driven crustal strain may be released by large (M ≥ 7) earthquakes. In the rest of the Saint Lawrence valley, PGR strain rates are significantly larger than seismic strain rates, suggesting either that PGR deformation remains mostly elastic or that large events are more frequent than indicated by small earthquake statistics (i.e., characteristic earthquakes).

  • gps crustal strain Postglacial Rebound and seismic hazard in eastern north america the saint lawrence valley example
    Journal of Geophysical Research, 2005
    Co-Authors: Stephane Mazzotti, Thomas S. James, Joe Henton, John H Adams
    Abstract:

    [1] We present Global Positioning System (GPS) measurements that constrain the amplitude, pattern, and origin of crustal deformation in the Saint Lawrence valley, Quebec, one of the most seismically active regions of eastern North America. The GPS network shows coherent southeastward motion of 0.6 ± 0.2 mm yr−1, relative to North America, and uplift of 2.6 ± 0.4 mm yr−1. Network average horizontal strain rates are mostly ESE-WNW shortening at (1.7 ± 1.0) × 10−9 yr−1. The shortening rate across the Charlevoix seismic zone is about twice as big as the regional average. These measurements are consistent with both Postglacial Rebound (PGR) models and the deformation style indicated by earthquake focal mechanisms. Although the GPS data do not discriminate between various models of crustal deformation, they provide important constraints on large earthquake recurrence. Assuming that the GPS strain estimates are representative of seismic moment release, they constrain the maximum magnitude of truncated Gutenberg-Richter recurrence statistics in the Charlevoix seismic zone to MX = 7.8 ± 0.6 (one M ≥ 7 earthquake per 400–1300 years). The remarkable agreement between the GPS strain rates, seismic catalogue statistics, and PGR predictions suggests that in Charlevoix, most of the PGR-driven crustal strain may be released by large (M ≥ 7) earthquakes. In the rest of the Saint Lawrence valley, PGR strain rates are significantly larger than seismic strain rates, suggesting either that PGR deformation remains mostly elastic or that large events are more frequent than indicated by small earthquake statistics (i.e., characteristic earthquakes).

  • new constraints on laurentide Postglacial Rebound from absolute gravity measurements
    Geophysical Research Letters, 2001
    Co-Authors: A Lambert, Thomas S. James, N Courtier, Glenn Sasagawa, Fred J Klopping, Daniel Winester, J Liard
    Abstract:

    Repeated absolute gravity measurements have been made over a period of several years at six sites along a 3000 km-long, mid-continental, North American profile from the coast of Hudson Bay southward to Iowa. With the exception of the southern-most site, the observed rates of change of gravity are significantly higher than rates predicted by current models, such as ICE-3G and a laterally homogeneous, standard Earth. The observed gravity change rates suggest significant modifications, such as a 2 to 3-fold increase in lower mantle viscosity or a 50% increase in Laurentide ice sheet thickness west of Lake Superior.

  • Postglacial Rebound at the northern cascadia subduction zone
    Quaternary Science Reviews, 2000
    Co-Authors: Thomas S. James, John J Clague, Kelin Wang, Ian Hutchinson
    Abstract:

    Postglacial Rebound is the response of the Earth to the decay of ice-sheets. A Postglacial Rebound model explains crustal tilting and rapid uplift at the northern Cascadia subduction zone that occurred during retreat of the Cordilleran ice-sheet. Observations explained by the model include the shoreline tilts of two proglacial lakes that formed at 13.5}14 ka (14C yr ago) and rapid sea level fall (land uplift) at 12}12.5 ka. Modelled mantle viscosity values range from 5]1018 to 5]1019 Pa s, and are consistent with previous viscosity inferences from observations of crustal deformation following subduction zone earthquakes (1018}1019 Pa s). No lower limit to subduction zone mantle viscosity is apparent from our model, but viscosity values equal to or larger than 1020 Pa s are de"nitely ruled out. Our modelled subduction zone viscosity values are smaller than most upper-mantle viscosity estimates derived from Postglacial Rebound studies of tectonically less-active regions (1020}1021 Pa s). The rapid observed uplift at 12 ka requires, in addition to a low mantle viscosity, rapid unloading from a sudden collapse of remaining coastal portions of the southern Cordilleran ice-sheet. The sudden collapse provides 0.18 m of global eustatic sea level rise, approximately 0.7% of the sea level rise associated with melt-water pulse IA. Predictions of a global Postglacial Rebound model (ICE-3G) with a 1021 Pa s upper-mantle viscosity were previously applied to geodetic data from this region to isolate signals associated with the earthquake cycle. Owing to the low-viscosity values, and resulting rapid recovery of glacial deformation, our model predicts present-day Postglacial Rebound uplift rates at least 10 times smaller than ICE-3G (less than about 0.1 mm/yr). As the ICE-3G adjustments were substantial, this indicates the need for re-evaluation of the geodetic data. ( 2000 Elsevier Science Ltd. All rights reserved.

John Wahr - One of the best experts on this subject based on the ideXlab platform.

  • bedrock displacements in greenland manifest ice mass variations climate cycles and climate change
    Proceedings of the National Academy of Sciences of the United States of America, 2012
    Co-Authors: Michael Bevis, John Wahr, Eric Kendrick, Abel Brown, Shfaqat Abbas Khan, Finn Bo Madsen, Michael J Willis, Per Knudsen, D Caccamise, Bjorn Johns
    Abstract:

    The Greenland GPS Network (GNET) uses the Global Positioning System (GPS) to measure the displacement of bedrock exposed near the margins of the Greenland ice sheet. The entire network is uplifting in response to past and present-day changes in ice mass. Crustal displacement is largely accounted for by an annual oscillation superimposed on a sustained trend. The oscillation is driven by earth’s elastic response to seasonal variations in ice mass and air mass (i.e., atmospheric pressure). Observed vertical velocities are higher and often much higher than predicted rates of Postglacial Rebound (PGR), implying that uplift is usually dominated by the solid earth’s instantaneous elastic response to contemporary losses in ice mass rather than PGR. Superimposed on longer-term trends, an anomalous ‘pulse’ of uplift accumulated at many GNET stations during an approximate six-month period in 2010. This anomalous uplift is spatially correlated with the 2010 melting day anomaly.

  • inference of mantle viscosity from grace and relative sea level data
    Geophysical Journal International, 2007
    Co-Authors: Archie Paulson, Shijie Zhong, John Wahr
    Abstract:

    SUMMARY Gravity Recovery And Climate Experiment (GRACE) satellite observations of secular changes in gravity near Hudson Bay, and geological measurements of relative sea level (RSL) changes over the last 10 000 yr in the same region, are used in a Monte Carlo inversion to infer-mantle viscosity structure. The GRACE secular change in gravity shows a significant positive anomaly over a broad region (>3000 km) near Hudson Bay with a maximum of ∼2.5 μGal yr−1 slightly west of Hudson Bay. The pattern of this anomaly is remarkably consistent with that predicted for Postglacial Rebound using the ICE-5G deglaciation history, strongly suggesting a Postglacial Rebound origin for the gravity change. We find that the GRACE and RSL data are insensitive to mantle viscosity below 1800 km depth, a conclusion similar to that from previous studies that used only RSL data. For a mantle with homogeneous viscosity, the GRACE and RSL data require a viscosity between 1.4 × 1021 and 2.3 × 1021 Pa s. An inversion for two mantle viscosity layers separated at a depth of 670 km, shows an ensemble of viscosity structures compatible with the data. While the lowest misfit occurs for upper- and lower-mantle viscosities of 5.3 × 1020 and 2.3 × 1021 Pa s, respectively, a weaker upper mantle may be compensated by a stronger lower mantle, such that there exist other models that also provide a reasonable fit to the data. We find that the GRACE and RSL data used in this study cannot resolve more than two layers in the upper 1800 km of the mantle.

  • limitations on the inversion for mantle viscosity from Postglacial Rebound
    Geophysical Journal International, 2007
    Co-Authors: Archie Paulson, Shijie Zhong, John Wahr
    Abstract:

    SUMMARY Observations of Postglacial Rebound (PGR) can provide important constraints on mantle viscosity structure. In this study, we investigate how well PGR observations are able to constrain the spherically symmetric (1-D) viscosity structure of the Earth. We generate synthetic PGR data by calculating the response of an earth model with realistic 3-D viscosity. The viscosity model is constructed starting from seismic tomography models. We generate synthetic PGR data from this model including relative sea levels, exponential relaxation times, ˙ J2, polar wander, and GRACE time-variable gravity measurements, where most of the data is concentrated in the Laurentide region. We then attempt an inversion for a 1-D (spherically symmetric) viscosity structure based on minimizing the misfit to these PGR data. Using a Monte Carlo algorithm to invert for two layers of viscosity [upper and lower mantle (UM and LM)], we obtain well-constrained values which correspond to the two-layer average of the logarithm of the 3-D viscosity structure in the vicinity of Laurentia. We then attempt to invert for four layers of viscosity. In this case we find a ‘trade-off effect’ in which neighbouring layers may have highly variable viscosities while maintaining a constant average between them. Since the PGR data are insensitive to this trade-off in neighbouring layers, the viscosity of any one of the four layers cannot be well constrained. By repeating the inversion with synthetic data derived from an Earth model which itself is 1-D with four viscosity layers, we demonstrate that it is the insensitivity of the PGR data, not complications due to 3-D structure, that allow the trade-off effect and the resulting failure of the inversion. We also perform a resolution test to study the extent of the insensitivity of PGR to these viscosity trade-offs, finding that the limit of resolution in the UM is about the size of the entire UM, and similarly for the LM. This reinforces our findings that only two layers are obtainable in an inversion for viscosity from PGR.

  • Postglacial Rebound and earth s viscosity structure from grace
    Journal of Geophysical Research, 2002
    Co-Authors: I Velicogna, John Wahr
    Abstract:

    [1] The Gravity Recovery and Climate Experiment (GRACE) satellite mission was launched in March 2002 and has an expected 5-year lifetime. One potential application of GRACE measurements of time-variable gravity will be to isolate the Postglacial Rebound signal, which can then be used to estimate the Earth's viscosity structure. In this paper we present a sensitivity analysis of simulated GRACE data, designed to assess the accuracy with which those data can be used to recover a simple model of Earth viscosity. We find that without combining with any other data type, but ignoring complications caused by uncertainties in the global ice loading history, GRACE data alone would allow us to determine the viscosity of a uniform lower mantle layer and an upper mantle/transition zone layer to within ±30–40% and to estimate lithospheric thickness to within ±15–20%. GRACE will have a harder time differentiating between the separate viscosities of the transition zone and upper mantle, but accuracies of within a factor of 2 might still be achievable for those parameters. Errors in the ice loading history could significantly degrade these viscosity estimates, particularly for the transition zone and upper mantle. The accuracy of recovery of the true Earth viscosity will depend in part on how well the model parameterization used for the grid search can represent the true Earth structure. However, combining GRACE data with data from other more traditional measurements of Postglacial Rebound has the potential of dramatically improving viscosity estimates throughout the Earth, particularly in the lower mantle.

  • a method for separating antarctic Postglacial Rebound and ice mass balance using future icesat geoscience laser altimeter system gravity recovery and climate experiment and gps satellite data
    Journal of Geophysical Research, 2002
    Co-Authors: Isabella Velicogna, John Wahr
    Abstract:

    5.3 and 19.9 mm yr � 1 for Postglacial Rebound and ice mass trend, respectively, when smoothed over 250 km scales. The largest source of error in the combined signals is the effect of the unknown time-variable accumulation on the density of the ice column. To estimate this contribution and so obtain better estimates of ice mass trend and Postglacial Rebound, we add Global Positioning System (GPS) measurements of vertical velocities as additional constraints. Using an empirical relation between the errors in Postglacial Rebound and ice mass trend that result from the unknown density variation within the ice column, we are able to solve for all three unknowns in the problem: ice mass trend, Postglacial Rebound, and the snow compaction trend. The addition of a plausible distribution of GPS measurements reduces the errors in estimates of Postglacial Rebound and ice mass trend to 3.4 and 15.9 mm yr � 1 , respectively. INDEX TERMS: 1863 Hydrology:

Donald F Argus - One of the best experts on this subject based on the ideXlab platform.

  • constraining models of Postglacial Rebound using space geodesy a detailed assessment of model ice 5g vm2 and its relatives
    Geophysical Journal International, 2010
    Co-Authors: Donald F Argus, Richard W Peltier
    Abstract:

    SUMMARY Using global positioning system, very long baseline interferometry, satellite laser ranging and Doppler Orbitography and Radiopositioning Integrated by Satellite observations, including the Canadian Base Network and Fennoscandian BIFROST array, we constrain, in models of Postglacial Rebound, the thickness of the ice sheets as a function of position and time and the viscosity of the mantle as a function of depth. We test model ICE-5G VM2 T90 Rot, which well fits many hundred Holocene relative sea level histories in North America, Europe and worldwide. ICE-5G is the deglaciation history having more ice in western Canada than ICE-4G; VM2 is the mantle viscosity profile having a mean upper mantle viscosity of 0.5 × 1021 Pa s and a mean uppermost-lower mantle viscosity of 1.6 × 1021 Pa s; T90 is an elastic lithosphere thickness of 90 km; and Rot designates that the model includes (rotational feedback) Earth's response to the wander of the North Pole of Earth's spin axis towards Canada at a speed of ≈1° Myr−1. The vertical observations in North America show that, relative to ICE-5G, the Laurentide ice sheet at last glacial maximum (LGM) at ≈26 ka was (1) much thinner in southern Manitoba, (2) thinner near Yellowknife (Northwest Territories), (3) thicker in eastern and southern Quebec and (4) thicker along the northern British Columbia–Alberta border, or that ice was unloaded from these areas later (thicker) or earlier (thinner) than in ICE-5G. The data indicate that the western Laurentide ice sheet was intermediate in mass between ICE-5G and ICE-4G. The vertical observations and GRACE gravity data together suggest that the western Laurentide ice sheet was nearly as massive as that in ICE-5G but distributed more broadly across northwestern Canada. VM2 poorly fits the horizontal observations in North America, predicting places along the margins of the Laurentide ice sheet to be moving laterally away from the ice centre at 2 mm yr−1 in ICE-4G and 3 mm yr−1 in ICE-5G, in disagreement with the observation that the interior of the North American Plate is deforming more slowly than 1 mm yr−1. Substituting VM5a T60 for VM2 T90, that is, introducing into the lithosphere at its base a layer with a high viscosity of 10 × 1021 Pa s, greatly improves the fit of the horizontal observations in North America. ICE-4G VM5a T60 Rot predicts most of the North American Plate to be moving horizontally more slowly than ≈1 mm yr−1, in agreement with the data. ICE-5G VM5a T60 Rot well fits both the vertical and horizontal observations in Europe. The space geodetic data cannot distinguish between models with and without rotational feedback, in the vertical because the velocity of Earth’ centre is uncertain, and in the horizontal because the areas of the plate interiors having geodetic sites is not large enough to detect the small differences in the predictions of rotational feedback going across the plate interiors.

  • defining the translational velocity of the reference frame of earth
    Geophysical Journal International, 2007
    Co-Authors: Donald F Argus
    Abstract:

    SUMMARY Earth’s centre is fundamental to geodesy and geoscience because motions of sites on the surface are estimated relative to it. International Terrestrial Reference Frames ITRF2000 and ITRF2005 are defined by the centre of mass of Earth’s system (CM), consisting of solid Earth, the ice sheets, the oceans, and the atmosphere. Satellite LAGEOS rotates about CM; satellite laser ranging (SLR) is used to estimate the velocity of CM relative to sites on the surface. However, ITRF2000 and ITRF2005 differ by 1.8 mm yr −1 , suggesting that the velocity of CM is constrained poorly by SLR. In this study, we define Earth’s reference frame with the centre of mass of solid Earth (CE). Site velocities estimated using SLR, VLBI, GPS and DORIS are corrected for a Postglacial Rebound model and inverted for the rotational velocities of the plates and the rotational and translational velocities of the four space techniques. Because the Postglacial Rebound predictions are relative to CE, the velocity of CE relative to sites on the surface is estimated. Because the input SLR site velocities are relative to CM, the output SLR translational velocity is the velocity of CM relative to CE. The estimated velocity of CE does not depend strongly on the Postglacial Rebound model corrected for. Equal within uncertainties and having a root mean square of 0.5 mm yr −1 are estimates of the velocity of CE determined assuming that plate interiors are deforming radially as predicted by three Postglacial Rebound models and an estimate of the velocity of CE determined assuming that parts of plate interiors neither beneath nor along the margins of the late Pleistocene ice sheets are not deforming laterally. The velocity of CE equals within uncertainties (probability greater than 5 per cent) the velocity of CM in ITRF2000. The velocity of CE differs significantly (0.05 per cent probability) from the velocity of CM in ITRF2005. Earth’s reference frame (and, we believe, ITRF’s) should be defined with the tightly constrained velocity of CE, not with the poorly constrained velocity of CM. Because CE is believed to be moving relative to CM no faster than 0.5 mm yr −1 , the velocity of CE estimated in this study is likely to be nearer the true velocity of CM than is the velocity of CM estimated using SLR.

  • glacial isostatic adjustment observed using very long baseline interferometry and satellite laser ranging geodesy
    Journal of Geophysical Research, 1999
    Co-Authors: Donald F Argus, Richard W Peltier, M M Watkins
    Abstract:

    In global space geodetic solutions, radial site motions are usually estimated relative to the geocenter (the center of figure of the solid Earth). Most geodesists estimate the motion of the geocenter assuming both that sites do not move radially and that sites move laterally as predicted by plate motion model NUVEL-1A [DeMets et al., 1990, 1994]. Here we estimate the motion of the geocenter assuming that the plate interiors deform radially and laterally as predicted by the Postglacial Rebound model of Peltier [1994] or that of Peltier [1996] without assuming a priori knowledge about relative plate motion. Radial site motions estimated relative to this Rebound-adjusted geocenter are in the same reference frame as the Rebound model predictions, whereas site motions estimated without adjusting for Rebound are not. We further constrain the motion of the Rebound-adjusted geocenter using satellite laser ranging's sensitivity to the center of mass (of the solid Earth, the oceans, and the atmosphere) by assuming that the mean velocity between the Rebound-adjusted geocenter and the center of mass is negligible over the time period of geodetic measurement. Twenty years of observation with satellite laser ranging and very long baseline interferometry record the isostatic response of the solid Earth to the unloading of the late Pleistocene ice sheets. The misfits of the Postglacial Rebound model of Peltier [1994] and that of Peltier [1996] are 34% and 16% less, respectively, than the misfit of the rigid plate model. Sites at Onsala (Sweden) and Algonquin Park (Ontario) are observed to be rising at 3 mm/yr and 2 mm/yr, respectively, reflecting unloading of the Fennoscandian and Laurentide ice sheets. Sites along the east coast of the United States are subsiding at <2 mm/yr, indicating that the forebulge produced by the Laurentide ice sheet is currently collapsing very slowly. Sites beneath the margins of the ice sheets during the last glacial maximum are currently moving laterally away from the ice sheet centers at <1.5 mm/yr, in disagreement with the moderately fast outward motion predicted by the model of Peltier [1996].

  • Postglacial Rebound from vlbi geodesy on establishing vertical reference
    Geophysical Research Letters, 1996
    Co-Authors: Donald F Argus
    Abstract:

    Difficulty in establishing a reference frame fixed to the earth's interior complicates the measurement of the vertical (radial) motions of the surface. I propose that a useful reference frame for vertical motions is that found by minimizing differences between vertical motions observed with VLBI [Ma and Ryan, 1995] and predictions from Postglacial Rebound predictions [Peltier, 1995]. The optimal translation of the geocenter is 1.7 mm/yr toward 36°N, 11l°E when determined from the motions of 10 VLBI sites. This translation gives a better fit of observations to predictions than does the VLBI reference frame used by Ma and Ryan [1995], but the improvement is statistically insignificant. The root mean square of differences decreases 20% to 0.73 mm/yr and the correlation coefficient increases from 0.76 to 0.87. Postglacial Rebound is evident in the uplift of points in Sweden and Ontario that were beneath the ancient ice sheets of Fennoscandia and Canada, and in the subsidence of points in the northeastern U.S., Germany, and Alaska that were around the periphery of the ancient ice sheets.

  • Postglacial Rebound from vlbi geodesy on establishing vertical reference
    Geophysical Research Letters, 1996
    Co-Authors: Donald F Argus
    Abstract:

    Difficulty in establishing a reference frame fixed to the earth's interior complicates the measurement of the vertical (radial) motions of the surface. I propose that a useful reference frame for vertical motions is that found by minimizing differences between vertical motions observed with VLBI [Ma and Ryan] and predictions from Postglacial Rebound predictions [Peltier]. The optimal translation of the geocenter is 1.7mm/year toward 36degN, 111degE when determined from the motions of 10 VLBI sites. This translation gives a better fit of observations to predictions than does the VLBI reference frame used by Ma and Ryan, but the improvement is statistically insignificant. The root mean square of differences decreases 20% to 0.73 mm/yr and the correlation coefficient increases from 0.76 to 0.87. Postglacial Rebound is evident in the uplift of points in Sweden and Ontario that were beneath the ancient ice sheets of Fennoscandia and Canada, and in the subsidence of points in the northeastern U.S., Germany, and Alaska that were around the periphery of the ancient ice sheets.

Kurt Lambeck - One of the best experts on this subject based on the ideXlab platform.

  • the search for Postglacial Rebound near the lambert glacier antarctica
    Earth Planets and Space, 2000
    Co-Authors: Paul Tregoning, Andrew Welsh, Herbert Mcqueen, Kurt Lambeck
    Abstract:

    A GPS network has been installed to monitor vertical crustal movement in the Lambert Glacier region, East Antarctica. The program commenced in January 1998 with a solar-powered GPS system installed at Beaver Lake. Solar-powered observations were also made late in the Antarctic summer of 1999. In January 2000, two new solar-powered sites will be installed to expand the monitoring network. In addition, we will be installing a hydrogen fuel cell power system at Beaver Lake to enable the equipment to operate throughout the winter months when solar power is not available. In this paper we outline the equipment which has been developed in order to operate remote GPS equipment in Antarctica, provide predictions of the expected rate of Rebound and comment on preliminary results from the data collected to date.

  • mantle dynamics Postglacial Rebound and the radial viscosity profile
    Physics of the Earth and Planetary Interiors, 2000
    Co-Authors: Georg Kaufmann, Kurt Lambeck
    Abstract:

    Abstract We infer the radial viscosity structure of the Earth’s mantle from observations of long-wavelength geoid, glacially-induced sea-level changes, and changes in the Earth’s rotation and gravitational field. We employ a combination of forward and formal inverse modeling of long-term mantle circulation driven by large-scale density differences deduced from seismic tomography. Based on the resulting unscaled mantle viscosity profiles, we model the time-dependent glacial isostatic adjustment of the Earth related to past and present changes in the ice-ocean mass imbalance and we deduce scaled mantle viscosity profiles, which simultaneously fit the long-wavelength geoid constraint and glacially-induced changes of the Earth’s shape. Three mantle viscosity profiles are fitting the observational data equally well. All profiles are characterized by a two order of magnitude variation of viscosity within the Earth’s mantle. Variations of viscosity in the upper mantle are less than one order of magnitude. In the lower mantle, the viscosity differs significantly with depth for all models. Average viscosities in the upper and lower mantle are around (2−5)×1020 and (1−3)×1022 Pa s, respectively.

  • Postglacial Rebound and fault instability in Fennoscandia
    Geophysical Journal International, 1999
    Co-Authors: Patrick Wu, Paul Johnston, Kurt Lambeck
    Abstract:

    SUMMARY The best available Rebound model is used to investigate the role that Postglacial Rebound plays in triggering seismicity in Fennoscandia. The salient features of the model include tectonic stress due to spreading at the North Atlantic Ridge, overburden pressure, gravitationally self-consistent ocean loading, and the realistic deglaciation history and compressible earth model which best fits the sea-level and ice data in Fennoscandia. The model predicts the spatio-temporal evolution of the state of stress, the magnitude of fault instability, the timing of the onset of this instability, and the mode of failure of lateglacial and Postglacial seismicity. The consistency of the predictions with the observations suggests that Postglacial Rebound is probably the cause of the large Postglacial thrust faults observed in Fennoscandia. The model also predicts a uniform stress field and instability in central Fennoscandia for the present, with thrust faulting as the predicted mode of failure. However, the lack of spatial correlation of the present seismicity with the region of uplift, and the existence of strike-slip and normal modes of current seismicity are inconsistent with this model. Further unmodelled factors such as the presence of high-angle faults in the central region of uplift along the Baltic coast would be required in order to explain the pattern of seismicity today in terms of Postglacial Rebound stress. The sensitivity of the model predictions to the eVects of compressibility, tectonic stress, viscosity and ice model is also investigated. For sites outside the ice margin, it is found that the mode of failure is sensitive to the presence of tectonic stress and that the onset timing is also dependent on compressibility. For sites within the ice margin, the eVect of Earth rheology is shown to be small. However, ice load history is shown to have larger eVects on the onset time of earthquakes and the magnitude of fault instability.

  • Postglacial Rebound and sea level contributions to changes in the geoid and the earth s rotation axis
    Geophysical Journal International, 1999
    Co-Authors: Paul Johnston, Kurt Lambeck
    Abstract:

    SUMMARY The rate of change of the spherical harmonic degree 2 component of the Earth’s gravitational potential (CoA 20) and polar wander velocity are two signals which are sensitive to Late Pleistocene deglaciation, current changes in sea level and deep mantle viscosity. Diierent load and earth models have been used in earlier papers to predict the component of these geophysical signals caused by the collapse of the last great ice sheets and recent melting of polar ice caps. In this paper, we present a systematic analysis of the dependence of the predictions on parameters of the ice and earth model. We show that the key parameters of the ice model which govern the predictions are the mass, the location of the centre of mass and the midpoint of the deglaciation phase. Of secondary importance is the length of the deglaciation phase and the mean ice load prior to the Last Glacial Maximum.These conclusions enable us to make a more robust inference of mantle viscosity than has been made before, allowing for the uncertainties in the model of Late Pleistocene and present deglaciation. As previous authors have shown, the lower-mantle viscosity is the most important rheological parameter and therefore theCoA 20 observation complements sea level observations, which are primarily sensitive to lithospheric thickness and the viscosity of the upper part of the mantle. Using realistic constraints on the sizes, locations and timing of deglaciation of the Late Pleistocene ice sheets and current changes in polar ice caps, the observation of CoA 20 is used to infer lower-mantle viscosity as a function of the present rate of sea level change. If the present rate of non-steric sea level change is 1 mm yr {1 and that change has been occurring for less than 1000 years, then the lower-mantle viscosity satis¢es log10 glm~21:82+0:15, which is consistent with inferences drawn from recent sea level analyses and con¢rms other analyses of theCoA 20 observation. If the polar wander signal is produced entirely by Postglacial Rebound and current sea level change, no more than 20 per cent of the present contribution to global sea level change comes from Greenland. The above conclusions also hold if the density discontinuities at 420 and 670 km are modelled as phase boundaries rather than material (chemical) boundaries.

  • material versus isobaric internal boundaries in the earth and their influence on Postglacial Rebound
    Geophysical Journal International, 1997
    Co-Authors: Paul Johnston, Kurt Lambeck, Detlef Wolf
    Abstract:

    SUMMARY Most previous earth models used to calculate viscoelastic relaxation after the removal of the Late Pleistocene ice loads implicitly assume that there is no exchange of mass across the mantle density discontinuities on periods of tens of thousands of years (the material boundary formulation). In the present study, simple incompressible models are used to determine the Earth's behaviour in the case where the density discontinuity remains at a constant pressure rather than deforming with the material (the isobaric boundary formulation). The calculation of the movement of the boundary is more rigorous than in earlier studies and uses the local incremental pressure calculated at the depth of the boundary and allows for the vertical deformation caused by the change in volume as material changes phase. It is shown that the buoyancy modes associated with the density discontinuities decrease in strength and increase in relaxation time analogous to what results when the density contrast is reduced. Also, two viscoelastic modes arise from an isobaric boundary, which is also predicted when there is a contrast in rigidity or viscosity across a material boundary. The difference in predicted radial deformation between the isobaric boundary model and the material boundary model is largest for long-wavelength loads for which the material incremental pressure at depth is largest. If the isobaric boundary model is appropriate for the treatment of the mineral phase changes in the mantle on glacial Rebound timescales, then previous inferences of the deep-mantle to shallow-mantle viscosity ratio based on large-scale deformation (spherical harmonic degree

Georg Kaufmann - One of the best experts on this subject based on the ideXlab platform.

  • effects of lateral viscosity variations on Postglacial Rebound an analytical approach
    Geophysical Journal International, 2002
    Co-Authors: Georg Kaufmann, Detlef Wolf
    Abstract:

    Summary So far, interpretations of Postglacial Rebound have usually been based on the assumption of laterally homogeneous earth models. However, in view of the increasing evidence of the 3-D structure of the Earth’s mantle, several studies on the effects of lateral variations of the viscosity on Postglacial Rebound have been completed during the last decade. In the present study, we consider load-induced viscoelastic  and viscous  perturbations of incompressible and vertically homogeneous flat earth models with lateral variations  of the shear modulus and viscosity. The solution is obtained using a perturbation method and given in analytical  form. Our results show that sinusoidal variations of the parameters of less than one order of magnitude can modify the vertical surface displacement associated with a model of the Fennoscandian ice sheet by several tens of metres. The modifications increase approximately in proportion to the amplitude of the lateral variations. The overall increase of the modifications with increasing wavelength is complicated in detail and also results in changes of the spatial and temporal distributions of the vertical surface displacement.

  • mantle dynamics Postglacial Rebound and the radial viscosity profile
    Physics of the Earth and Planetary Interiors, 2000
    Co-Authors: Georg Kaufmann, Kurt Lambeck
    Abstract:

    Abstract We infer the radial viscosity structure of the Earth’s mantle from observations of long-wavelength geoid, glacially-induced sea-level changes, and changes in the Earth’s rotation and gravitational field. We employ a combination of forward and formal inverse modeling of long-term mantle circulation driven by large-scale density differences deduced from seismic tomography. Based on the resulting unscaled mantle viscosity profiles, we model the time-dependent glacial isostatic adjustment of the Earth related to past and present changes in the ice-ocean mass imbalance and we deduce scaled mantle viscosity profiles, which simultaneously fit the long-wavelength geoid constraint and glacially-induced changes of the Earth’s shape. Three mantle viscosity profiles are fitting the observational data equally well. All profiles are characterized by a two order of magnitude variation of viscosity within the Earth’s mantle. Variations of viscosity in the upper mantle are less than one order of magnitude. In the lower mantle, the viscosity differs significantly with depth for all models. Average viscosities in the upper and lower mantle are around (2−5)×1020 and (1−3)×1022 Pa s, respectively.

  • lateral asthenospheric viscosity variations and Postglacial Rebound a case study for the barents sea
    Geophysical Research Letters, 1998
    Co-Authors: Georg Kaufmann
    Abstract:

    The effect of lateral asthenospheric viscosity variations on observable signatures related to Postglacial Rebound in the Barents Sea is studied. Using a finite-element approach to discretize the problem, a comparison between a laterally homogeneous reference earth model and a laterally heterogeneous earth model is performed. The results indicate that a change in asthenospheric viscosity of about three orders of magnitude influences predictions of land uplift up to 10–20 m, present-day velocities up to 0.5–1.5 mm/a, and present-day gravity anomalies up to 0.4–0.8 mGal in the northwestern part of the Barents Sea region.