scholarly journals Evaluation of strain rates on Ice Stream B, Antarctica, obtained using GPS phase measurements

1994 ◽  
Vol 20 ◽  
pp. 254-262 ◽  
Author(s):  
Hulbe Christina L.
Keyword(s):  
Vertical Velocity ◽  
Stationary Waves ◽  
Strain Rates ◽  
Phase Measurements ◽  
Velocity Gradients ◽  
Ice Stream ◽  
Stop And Go ◽  
Global Positioning ◽  
Horizontal Strain ◽  
Ice Stream B

The “stop-and-go” kinematic Global Positioning System (GPS) technique was used to survey 270 stations twice within a 25 km by 10 km strain grid on the surface of Ice Stream B. One or two geodetic quality receivers operated as reference pivots, while two similar receivers traveled to grid stations. Each station was occupied for 25s. The method is rapid and reliable. Each survey was completed within 2 weeks. Calculated horizontal strain rates are accurate to 1%. Relative vertical velocities are accurate to 20 mm km1a-1. Maps of the four horizontal velocity gradients, relative vertical velocity and surface elevation are presented. The vertical velocity pattern is used to identify the part of the topography that forms stationary waves and that which is migrating. No strong quantitative link is found between the pattern in horizontal strain rate and surface topography. In particular, there is no evidence that the topography is relaxing toward isostasy. None of the “weir-type” sticky spots, which are commonly observed with other glaciers, is found but there could be two of the “submerged-boulder type”, which cause lateral flow diversion. Evidence for “hot stripes” and zones of preferred crystal orientation is not found.

scholarly journals Evaluation of strain rates on Ice Stream B, Antarctica, obtained using GPS phase measurements

1994 ◽  
Vol 20 ◽  
pp. 254-262
Author(s):  
Christina L. Hulbe ◽  
Ian M. Whillans
Keyword(s):  
Vertical Velocity ◽  
Stationary Waves ◽  
Strain Rates ◽  
Phase Measurements ◽  
Velocity Gradients ◽  
Ice Stream ◽  
Stop And Go ◽  
Global Positioning ◽  
Horizontal Strain ◽  
Ice Stream B

The “stop-and-go” kinematic Global Positioning System (GPS) technique was used to survey 270 stations twice within a 25 km by 10 km strain grid on the surface of Ice Stream B. One or two geodetic quality receivers operated as reference pivots, while two similar receivers traveled to grid stations. Each station was occupied for 25s. The method is rapid and reliable. Each survey was completed within 2 weeks. Calculated horizontal strain rates are accurate to 1%. Relative vertical velocities are accurate to 20 mm km1 a-1. Maps of the four horizontal velocity gradients, relative vertical velocity and surface elevation are presented. The vertical velocity pattern is used to identify the part of the topography that forms stationary waves and that which is migrating. No strong quantitative link is found between the pattern in horizontal strain rate and surface topography. In particular, there is no evidence that the topography is relaxing toward isostasy. None of the “weir-type” sticky spots, which are commonly observed with other glaciers, is found but there could be two of the “submerged-boulder type”, which cause lateral flow diversion. Evidence for “hot stripes” and zones of preferred crystal orientation is not found.

scholarly journals Velocity pattern in a transect across Ice Stream B, Antarctica

1993 ◽  
Vol 39 (133) ◽  
pp. 562-572 ◽  
Author(s):  
I. M. Whillans ◽  
M. Jackson ◽  
Y-H. Tseng
Keyword(s):  
Aerial Photography ◽  
Lateral Shear ◽  
Ice Flow ◽  
Measured Velocity ◽  
Velocity Gradients ◽  
Spreading Ridges ◽  
Ice Stream ◽  
Surface Slopes ◽  
Fast Ice ◽  
Ice Stream B

AbstractRepeat aerial photography is used to obtain closely spaced measurements of velocity and elevation over a complete transect of Ice Stream tributary B2, including the shear margins, the fast ice of the ice stream and several unusual features, as well as the UpB camp. Persistent features, mainly crevasses, are tracked to provide 1541 values of velocity and 1933 values of elevation. These are used to describe ice flow in the ice stream. Within the ice stream, the dominant velocity gradient is lateral shear. Crevasse patterns are studied in relation to measured velocity gradients. Crevasses intersect one another at acute angles, indicating that their origin is deeper than the depth to which crevasses penetrate. One feature within the ice stream seems to be a raft of stiff ice. Others are crevasse trains. Also, there are spreading ridges, perhaps due to upwelling ice. There is no evidence of large sticky spots within the studied transect, i.e. no steep surface slopes with associated surface stretching just up-glacier and surface compression down-glacier.

scholarly journals Crevasse Deformation and Examples From Ice Stream B, West Antarctica (Abstract)

1988 ◽  
Vol 11 ◽  
pp. 211-211
Author(s):  
P. L. Vorriberger ◽  
I. M. Whillans
Keyword(s):  
Velocity Field ◽  
Strain Rate ◽  
Velocity Fields ◽  
Normal Strain ◽  
Lateral Shear ◽  
The Third ◽  
Velocity Gradients ◽  
Ice Stream ◽  
Wide Range ◽  
Ice Stream B

Crevasses are subject to rotation and bending according to the velocity field through which they travel. The objective of this study is to determine to what extent the velocity field can be inferred from measurements of the resulting shapes of crevasses.A quantitative model of crevasse deformation is developed, based on the following assumptions: (1) each crevasse is assumed to open perpendicularly to the principal extensional regional strain-rate, (2) the crevasse forms when the principal extensional strain-rate exceeds some specified critical value, and (3) velocity gradients are constant over the area of interest. The first two assumptions are reasonable and the third is necessary for an analytic solution of flow trajectories. The crevasse is carried along, rotated, and bent, and may continue to increase in length. Calculations are made for different velocity fields, and velocity fields are sought that produce crevasses similar to those found in three different areas of Ice Stream B.Hook-shaped crevasses occur just outside the chaotic zone at the ice-stream margin. These are similar to the curved marginal crevasses often found in the accumulation zone of valley glaciers. They are successfully modelled by combining strong lateral shear with slow flow of ice from the ice ridge into the ice stream. The curvature at the most sharply bent part of the crevasse is found to be a useful measure and, together with measurements of ice flow from the ridge, can be used to infer the rate of lateral shear. This rate compares favorably with the single measurement obtained so far (Bindschadler and others 1987).A pattern of splaying crevasses develops on the ice stream down-glacier of its narrowest part. These crevasses are similar to longitudinal crevasses found in the ablation zone of many valley glaciers. Models with linear variation in velocity cannot reproduce the observed pattern. However, we have been able to simulate higher-order variations by joining together successive linear models. The observed crevasse pattern is successfully produced if the side shearing varies as the third power of distance from the center of symmetry of the crevasse pattern. Such a variation is expected for a linear gradient in side-drag stress and a third-power constitutive relation for ice. The observed crevasse pattern is thus consistent with side drag varying linearly across the ice stream.The third example is the rotation of transverse crevasses, which occur in trains on the main part of the ice stream. This rotation is due to side shearing but its magnitude is also affected by turning of the flow line and by normal strain-rates. It is therefore possible to reproduce the observed pattern for a wide range of velocity fields, and so measurements of the orientation of transverse crevasses provide only an upper limit on side shearing within the main body of the ice stream.There are many other examples of crevasse patterns on Ice Stream Β and on other glaciers that can be studied in this way. We propose that important constraints can be placed on velocity gradients and on the flow dynamics by using quantitative modelling of crevasse shapes.

scholarly journals Velocity pattern in a transect across Ice Stream B, Antarctica

1993 ◽  
Vol 39 (133) ◽  
pp. 562-572 ◽  
Author(s):  
I. M. Whillans ◽  
M. Jackson ◽  
Y-H. Tseng
Keyword(s):  
Aerial Photography ◽  
Lateral Shear ◽  
Ice Flow ◽  
Measured Velocity ◽  
Velocity Gradients ◽  
Spreading Ridges ◽  
Ice Stream ◽  
Surface Slopes ◽  
Fast Ice ◽  
Ice Stream B

AbstractRepeat aerial photography is used to obtain closely spaced measurements of velocity and elevation over a complete transect of Ice Stream tributary B2, including the shear margins, the fast ice of the ice stream and several unusual features, as well as the UpB camp. Persistent features, mainly crevasses, are tracked to provide 1541 values of velocity and 1933 values of elevation. These are used to describe ice flow in the ice stream. Within the ice stream, the dominant velocity gradient is lateral shear. Crevasse patterns are studied in relation to measured velocity gradients. Crevasses intersect one another at acute angles, indicating that their origin is deeper than the depth to which crevasses penetrate. One feature within the ice stream seems to be a raft of stiff ice. Others are crevasse trains. Also, there are spreading ridges, perhaps due to upwelling ice. There is no evidence of large sticky spots within the studied transect, i.e. no steep surface slopes with associated surface stretching just up-glacier and surface compression down-glacier.

scholarly journals Crevasse Deformation and Examples from Ice Stream B, Antarctica

1990 ◽  
Vol 36 (122) ◽  
pp. 3-10 ◽  
Author(s):  
P.L. Vornberger ◽  
I.M. Whillans
Keyword(s):  
Velocity Field ◽  
Strain Rate ◽  
Critical Value ◽  
Velocity Fields ◽  
The Third ◽  
Velocity Gradients ◽  
Ice Stream ◽  
Lateral Shearing ◽  
Extensional Strain ◽  
Ice Stream B

AbstractCrevasses, once formed, are subject to rotation and bending according to the velocity field through which they travel. Because of this, crevasse shapes can be used to infer something about the velocity field of a glacier. This is done using a model in which each crevasse opens perpendicularly to the principal extensional strain-rate, when that strain-rate exceeds some specified critical value, and is then deformed according to the same velocity gradients that formed the crevasse. This model describes how crevasses are formed, translated, rotated, bent, and lengthened.Velocity fields are sought for which calculations produce crevasses approximating those found in three example areas on Ice Stream B, Antarctica. The first example is the hook-shaped crevasses that occur just outside the chaotic shear zone at the ice-stream margin. They are used to infer a rate of lateral shearing, and side drag. The second example, a pattern of splaying crevasses, is satisfactorily simulated by a model with side-drag stress varying linearly across the ice stream. This confirms that this region is restrained almost entirely by side drag. The third example is transverse crevasses and their change in orientation, but many different velocity fields can produce the observed pattern. Of these three examples, the shapes of hook-shaped marginal crevasses and splaying crevasses can provide useful information whereas transverse crevasses are less helpful.

scholarly journals Crevasse Deformation and Examples from Ice Stream B, Antarctica

1990 ◽  
Vol 36 (122) ◽  
pp. 3-10 ◽  
Author(s):  
P.L. Vornberger ◽  
I.M. Whillans
Keyword(s):  
Velocity Field ◽  
Strain Rate ◽  
Critical Value ◽  
Velocity Fields ◽  
The Third ◽  
Velocity Gradients ◽  
Ice Stream ◽  
Lateral Shearing ◽  
Extensional Strain ◽  
Ice Stream B

AbstractCrevasses, once formed, are subject to rotation and bending according to the velocity field through which they travel. Because of this, crevasse shapes can be used to infer something about the velocity field of a glacier. This is done using a model in which each crevasse opens perpendicularly to the principal extensional strain-rate, when that strain-rate exceeds some specified critical value, and is then deformed according to the same velocity gradients that formed the crevasse. This model describes how crevasses are formed, translated, rotated, bent, and lengthened.Velocity fields are sought for which calculations produce crevasses approximating those found in three example areas on Ice Stream B, Antarctica. The first example is the hook-shaped crevasses that occur just outside the chaotic shear zone at the ice-stream margin. They are used to infer a rate of lateral shearing, and side drag. The second example, a pattern of splaying crevasses, is satisfactorily simulated by a model with side-drag stress varying linearly across the ice stream. This confirms that this region is restrained almost entirely by side drag. The third example is transverse crevasses and their change in orientation, but many different velocity fields can produce the observed pattern. Of these three examples, the shapes of hook-shaped marginal crevasses and splaying crevasses can provide useful information whereas transverse crevasses are less helpful.

scholarly journals Crevasse Deformation and Examples From Ice Stream B, West Antarctica (Abstract)

1988 ◽  
Vol 11 ◽  
pp. 211
Author(s):  
P. L. Vorriberger ◽  
I. M. Whillans
Keyword(s):  
Velocity Field ◽  
Strain Rate ◽  
Velocity Fields ◽  
Normal Strain ◽  
Lateral Shear ◽  
The Third ◽  
Velocity Gradients ◽  
Ice Stream ◽  
Wide Range ◽  
Ice Stream B

Crevasses are subject to rotation and bending according to the velocity field through which they travel. The objective of this study is to determine to what extent the velocity field can be inferred from measurements of the resulting shapes of crevasses. A quantitative model of crevasse deformation is developed, based on the following assumptions: (1) each crevasse is assumed to open perpendicularly to the principal extensional regional strain-rate, (2) the crevasse forms when the principal extensional strain-rate exceeds some specified critical value, and (3) velocity gradients are constant over the area of interest. The first two assumptions are reasonable and the third is necessary for an analytic solution of flow trajectories. The crevasse is carried along, rotated, and bent, and may continue to increase in length. Calculations are made for different velocity fields, and velocity fields are sought that produce crevasses similar to those found in three different areas of Ice Stream B. Hook-shaped crevasses occur just outside the chaotic zone at the ice-stream margin. These are similar to the curved marginal crevasses often found in the accumulation zone of valley glaciers. They are successfully modelled by combining strong lateral shear with slow flow of ice from the ice ridge into the ice stream. The curvature at the most sharply bent part of the crevasse is found to be a useful measure and, together with measurements of ice flow from the ridge, can be used to infer the rate of lateral shear. This rate compares favorably with the single measurement obtained so far (Bindschadler and others 1987). A pattern of splaying crevasses develops on the ice stream down-glacier of its narrowest part. These crevasses are similar to longitudinal crevasses found in the ablation zone of many valley glaciers. Models with linear variation in velocity cannot reproduce the observed pattern. However, we have been able to simulate higher-order variations by joining together successive linear models. The observed crevasse pattern is successfully produced if the side shearing varies as the third power of distance from the center of symmetry of the crevasse pattern. Such a variation is expected for a linear gradient in side-drag stress and a third-power constitutive relation for ice. The observed crevasse pattern is thus consistent with side drag varying linearly across the ice stream. The third example is the rotation of transverse crevasses, which occur in trains on the main part of the ice stream. This rotation is due to side shearing but its magnitude is also affected by turning of the flow line and by normal strain-rates. It is therefore possible to reproduce the observed pattern for a wide range of velocity fields, and so measurements of the orientation of transverse crevasses provide only an upper limit on side shearing within the main body of the ice stream. There are many other examples of crevasse patterns on Ice Stream Β and on other glaciers that can be studied in this way. We propose that important constraints can be placed on velocity gradients and on the flow dynamics by using quantitative modelling of crevasse shapes.

scholarly journals Observations of a reversal in vertical and horizontal strain-rate regime during a motion event on Unteraargletscher, Bernese Alps, Switzerland

2002 ◽  
Vol 48 (163) ◽  
pp. 566-574 ◽  
Author(s):  
G. Hilmar Gudmundsson
Keyword(s):  
Strain Rate ◽  
Global Positioning System ◽  
Ice Thickness ◽  
Strain Rates ◽  
Mean Values ◽  
Vertical Strain ◽  
Motion Event ◽  
Global Positioning ◽  
Measuring Site ◽  
Horizontal Strain

AbstractDuring a motion event on Unteraargletscher, Bernese Alps, Switzerland, in spring 1996, surface velocities were measured up to eight times a day at four different locations along the central flowline using global positioning system equipment. In addition, accumulated vertical strains over the uppermost 50 and 100 m were measured at a location where the total ice thickness is 260 m. The motion event was accompanied by high horizontal and vertical strain rates as compared to annual mean values. A reversal in strain regime was observed, with horizontal strain rates changing to extension while vertical strain rates became compressive. This strain-rate reversal coincided, within the temporal resolution of the data, with a maximum in vertical ice displacement at the surface.Within a day, variations in vertical strain from 0.04 a−1 to −0.06 a−1 were observed over the uppermost 100 m. Vertical stretching is estimated to have contributed to at least 20% of the anomalous vertical ice movement at the surface. There were significant differences between measured longitudinal strain, averaged over a distance corresponding to a few ice thicknesses, and measured vertical strain. In spring 1997 a similar, but more detailed, set of measurements was collected at the same measuring site, and vertical strain rates were found to vary non-uniformly with depth, with the largest values closest to the surface.

Dense GPS derived deformation and rotation of intraplate blocks in Central Europe - comparison to seismicity and volcanism

2021 ◽  
Author(s):  
Zhiguo Deng ◽  
Torsten Dahm
Keyword(s):  
High Precision ◽  
Velocity Fields ◽  
Shear Stresses ◽  
Intraplate Seismicity ◽  
Strain Rates ◽  
Horizontal Shear ◽  
Regional Patterns ◽  
Quaternary Volcanism ◽  
Intraplate Deformation ◽  
Horizontal Strain

<p>Intraplate deformation is often small but can nowdays be resolved from high precision GNSS velocity fields derived from decade-long time series and high precision network or point wise  solutions if uncertainties are smaller than ~0.2 mm/a.</p><p>If local effects are discarded, dense velocity fields may resolve regional patterns of intraplate deformation and motion, which are related to the bending of lithospheric plates, to mantle upwelling, the diffuse or zoned deformation along structural weaknesses or faults, and the rotation of rigid blocks within a plate. </p><p>We derive for the first time, dense high precision network solutions at 323 GNSS stations in Germany and adjacent areas and resolve regions experiencing uplift with velocities of up to ~2 mm/a, rotational relative motions with angular velocities of ~0.7±0.3 mas/a, and horizontal shear along an extended,  NS trending zone with strain rates in the range of 10-8 1/a. </p><p>We integrate European dense velocity solutions into our dataset to discuss the geodynamic context to European microplate motions, the Alpine collision, the structure of the European mantle, Quaternary volcanism and historical seismicity. </p><p>Unexpectedly, the zones of high horizontal strain rates only partly correlate to seismicity. Such a non-correlation between ongoing horizontal strain and seismicity has been recognized before. We discuss possible reasons for the absence of intraplate seismicity in regions experiencing recent strain, including the stress shadow effects if the strain buildup is reducing shear stresses from plate tectonics. The combination of GNSS derived dense velocity fields with time dependent seismicity models may change our current understanding of intraplate seismicity and impact the assessment of intraplate seismic hazard in future. </p>

scholarly journals First point measurements of ice-sheet thickness change in Antarctica

1998 ◽  
Vol 27 ◽  
pp. 125-129 ◽  
Author(s):  
Gordon S. Hamilton ◽  
Ian M. Whillans ◽  
Peter J. Morgan
Keyword(s):  
Ice Sheet ◽  
Sheet Thickness ◽  
Snow Accumulation ◽  
Repeated Measurements ◽  
Occupation Times ◽  
Thickness Change ◽  
Global Positioning ◽  
First Results ◽  
Ice Stream B

Ice-sheet thickening or thinning rates in Antarctica are measured using the “coffee-can” or “submergence velocity” method. in this, repeated measurements of the positions of firn anchors are obtained using the global positioning system (GPS). The thickness change is (lie difference between vertical velocity so obtained and long-term rate of snow accumulation. Minor corrections for firn settling and downslopc motion are made. The technique avoids difficulties of short-term fluctuations in snowfall or snow den-sification. The result for Byrd Station is near balance, -0.004 (0.022) ma−1, and for the Dragon, just outboard of Ice Stream B, thinning at -0.096 (0.044) ma−1. Uncertainties with these first results are mainly due to the short occupation times during the first GPS surveys.

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