scholarly journals Ice Stream C, Antarctica, sticky spots detected by microearthquake monitoring

1994 ◽  
Vol 20 ◽  
pp. 183-186 ◽  
Author(s):  
S. Anandakrishnan ◽  
R. B. Alley
Keyword(s):  
Shear Stress ◽  
Stress Drop ◽  
Fast Moving ◽  
Fault Plane ◽  
Spatial Density ◽  
Ice Stream ◽  
Slow Moving ◽  
Ice Stream B ◽  
Basal Shear ◽  
Plane Area

Microearthquakes at the base of slow-moving Ice Stream C occur many times more frequently than at the base of fast-moving Ice Stream B. We suggest that the microearthquake source sites are so-called “sticky spots”, defined as limited zones of stronger Subglacial material interspersed within a weaker matrix. The fault-plane area of the microearthquakes (O(102m2)) is therefore a measure of the size of the sticky spots. The spatial density of the microearthquakes (O(10 km-2)) is a measure of the distribution of sticky spots.The average stress drop associated with these microearthquakes is consistent with an ice-stream bed model of weak subglacial till interspersed with stronger zones that support much or all of the basal shear stress. We infer a weak inter-sticky-spot material by the large distances (O(103m)), relative to fault radius, to which the microearthquake stress change is transmitted.

scholarly journals Ice Stream C, Antarctica, sticky spots detected by microearthquake monitoring

1994 ◽  
Vol 20 ◽  
pp. 183-186 ◽  
Author(s):  
S. Anandakrishnan ◽  
R. B. Alley
Keyword(s):  
Shear Stress ◽  
Stress Drop ◽  
Fast Moving ◽  
Fault Plane ◽  
Spatial Density ◽  
Ice Stream ◽  
Slow Moving ◽  
Ice Stream B ◽  
Basal Shear ◽  
Plane Area

Microearthquakes at the base of slow-moving Ice Stream C occur many times more frequently than at the base of fast-moving Ice Stream B. We suggest that the microearthquake source sites are so-called “sticky spots”, defined as limited zones of stronger Subglacial material interspersed within a weaker matrix. The fault-plane area of the microearthquakes (O (102m2)) is therefore a measure of the size of the sticky spots. The spatial density of the microearthquakes (O (10 km-2)) is a measure of the distribution of sticky spots.The average stress drop associated with these microearthquakes is consistent with an ice-stream bed model of weak subglacial till interspersed with stronger zones that support much or all of the basal shear stress. We infer a weak inter-sticky-spot material by the large distances (O (103 m)), relative to fault radius, to which the microearthquake stress change is transmitted.

scholarly journals Acoustic impedance and basal shear stress beneath four Antarctic ice streams

2003 ◽  
Vol 36 ◽  
pp. 225-232 ◽  
Author(s):  
David G. Vaughan ◽  
Andrew M. Smith ◽  
P. Chandrika Nath ◽  
Emmanuel Le Meur
Keyword(s):  
Shear Stress ◽  
Acoustic Impedance ◽  
Dynamic State ◽  
Ice Thickness ◽  
Deformation History ◽  
Ice Streams ◽  
Ice Stream ◽  
Wide Range ◽  
Slow Moving ◽  
Basal Shear

AbstractThe acoustic impedance of the subglacial material beneath 7.2 km profiles on four ice streams in Antarctica has been measured using a seismic technique. The ice streams span a wide range of dynamic conditions with flow rates of 35–464 m a–1. The acoustic impedance indicates that poorly lithified or dilated sedimentary material is ubiquitous beneath these ice streams. Meanacoustic impedance across each profile correlates well with basal shear stress and the slipperiness of the bed, indicating that acoustic impedance is a good diagnostic not only for the porosity of the subglacial material, but also for its dynamic state (deforming or non-deforming). Beneath two of the ice streams, lodged (non-deforming) and dilated (deforming) sediment coexist but their distribution is not obviously controlled by basal topography or ice thickness. Their distribution may be controlled by complex material properties or the deformation history. Beneath Rutford Ice Stream, lodged and dilated sediment coexist and are distributed in broad bands several kilometres wide, whileon Talutis Inlet there is considerable variability over much shorter distances; this may reflect differences in the mechanism of drainage beneath the ice streams. The material beneath the slow-moving Carlson Inlet is probably lodged but unlithified sediment; this is consistent with the hypothesis that Carlson Inlet was once a fast-flowing ice stream but is now in a stagnant phase, which could possibly be revivedby raised basal water content. The entire bed beneath fast-flowing Evans Ice Stream is dilated sediment.

scholarly journals Water-pressure Coupling of Sliding and Bed Deformation: III. Application to Ice Stream B, Antarctica

1989 ◽  
Vol 35 (119) ◽  
pp. 130-139 ◽  
Author(s):  
R.B. Alley ◽  
D.D. Blankenship ◽  
S.T. Rooney ◽  
C.R. Bentley
Keyword(s):  
Shear Stress ◽  
Strain Rate ◽  
Water Pressure ◽  
Effective Pressure ◽  
West Antarctica ◽  
Bed Deformation ◽  
Ice Stream ◽  
Basal Sliding ◽  
Ice Stream B ◽  
Basal Shear

AbstractGeophysical studies and glaciological analyses suggest strongly that Ice Stream B, West Antarctica, moves primarily by pervasive deformation of a meters thick subglacial till. Analysis of the longitudinal profile of the ice stream up-stream of the ice plain suggests that basal sliding is slow everywhere, that effective pressure decreases slowly down-stream, and that the strain-rate of pervasive shear is proportional to the basal shear stress and inversely proportional to the square or cube of the effective pressure. Discrete shearing may occur beneath the pervasively deforming zone. These and other hypotheses, which build on the analyses of the first two papers in this series, can be tested in the field.

scholarly journals Sensitivity of Pine Island Glacier, West Antarctica, to changes in ice-shelf and basal conditions: a model study

2002 ◽  
Vol 48 (163) ◽  
pp. 552-558 ◽  
Author(s):  
Marjorie Schmeltz ◽  
Eric Rignot ◽  
Todd K. Dupont ◽  
Douglas R. MacAyeal
Keyword(s):  
Shear Stress ◽  
Element Model ◽  
Shelf Area ◽  
West Antarctica ◽  
Ice Shelf ◽  
Ice Stream ◽  
Model Study ◽  
Grounding Line ◽  
Glacier Velocity ◽  
Basal Shear

AbstractWe use a finite-element model of coupled ice-stream/ice-shelf flow to study the sensitivity of Pine Island Glacier, West Antarctica, to changes in ice-shelf and basal conditions. By tuning a softening coefficient of the ice along the glacier margins, and a basal friction coefficient controlling the distribution of basal shear stress underneath the ice stream, we are able to match model velocity to that observed with interferometric synthetic aperture radar (InSAR). We use the model to investigate the effect of small perturbations on ice flow. We find that a 5.5–13% reduction in our initial ice-shelf area increases the glacier velocity by 3.5–10% at the grounding line. The removal of the entire ice shelf increases the grounding-line velocity by > 70%. The changes in velocity associated with ice-shelf reduction are felt several tens of km inland. Alternatively, a 5% reduction in basal shear stress increases the glacier velocity by 13% at the grounding line. By contrast, softening of the glacier side margins would have to be increased a lot more to produce a comparable change in ice velocity. Hence, both the ice-shelf buttressing and the basal shear stress contribute significant resistance to the flow of Pine Island Glacier.

scholarly journals Ribbed bedforms on palaeo-ice stream beds resemble regular patterns of basal shear stress (‘traction ribs’) inferred from modern ice streams

2016 ◽  
Vol 62 (234) ◽  
pp. 696-713 ◽  
Author(s):  
CHRIS R. STOKES ◽  
MARTIN MARGOLD ◽  
TIMOTHY T. CREYTS
Keyword(s):  
Shear Stress ◽  
Ice Sheets ◽  
Western Canada ◽  
Coupled Flow ◽  
Ice Streams ◽  
Ice Stream ◽  
Stream Beds ◽  
Basal Shear ◽  
Subglacial Meltwater ◽  
Regular Patterns

Rapidly-flowing ice streams are an important mechanism through which ice sheets lose mass, and much work has been focussed on elucidating the processes that increase or decrease their velocity. Recent work using standard inverse methods has inferred previously-unrecognised regular patterns of high basal shear stress (‘sticky spots’ >200 kPa) beneath a number of ice streams in Antarctica and Greenland, termed ‘traction ribs’. They appear at a scale intermediate between smaller ribbed moraines and much larger mega-ribs observed on palaeo-ice sheet beds, but it is unclear whether they have a topographic expression at the bed. Here, we report observations of rib-like bedforms from DEMs along palaeo-ice stream beds in western Canada that resemble both the pattern and dimensions of traction ribs. Their identification suggests that traction ribs may have a topographic expression that lies between, and partly overlaps with, ribbed moraines and much larger mega-ribs. These intermediate-sized bedforms support the notion of a ribbed bedform continuum. Their formation remains conjectural, but our observations from palaeo-ice streams, coupled with those from modern ice masses, suggest they are related to wave-like instabilities occurring in the coupled flow of ice and till and modulated by subglacial meltwater drainage. Their form and pattern may also involve glaciotectonism of subglacial sediments.

scholarly journals Micro-earthquakes beneath Ice Streams Β and C, West Antarctica: observations and implications

1993 ◽  
Vol 39 (133) ◽  
pp. 455-462 ◽  
Author(s):  
S. Anandakrishnan ◽  
C. R. Bentley
Keyword(s):  
Fault Plane ◽  
Source Parameters ◽  
West Antarctica ◽  
Ice Streams ◽  
Rapid Movement ◽  
Plane Analysis ◽  
Ice Stream ◽  
The Difference ◽  
Ice Stream B ◽  
Active Flow

Abstract Micro-earthquakes have been monitored at two locations on Ice Stream Β and one on Ice Stream C using a seismographic array built specifically for that purpose. Subglacial micro-earthquakes arc 20 times more abundant beneath Ice Stream C than beneath Ice Stream B, despite the 100 times more rapid movement of Ice Stream B. Triangulation shows the foci beneath Ice Stream C, like those beneath Ice Stream B, to be within a few meters of the base of the ice, presumably within the uppermost part of the bed, and fault-plane analysis indicates slips on horizontal planes at about a 30° angle to the presumed direction of formerly active flow. Source parameters, computed from spectra of the arrivals, confirmed that the speed of slip is three orders of magnitude faster beneath Ice Stream C than beneath Ice Stream Β which means that a five orders-of-magnitude greater fraction of the velocity of Ice Stream C is contributed by the faulting, although that fraction is still small. We attribute the difference in activity beneath the two ice streams to the loss of dilatancy in the till beneath Ice Stream C in the process that led to its stagnation.

scholarly journals Non-linear responses of Rutford Ice Stream, Antarctica, to semi-diurnal and diurnal tidal forcing

2010 ◽  
Vol 56 (195) ◽  
pp. 167-176 ◽  
Author(s):  
Matt A. King ◽  
Tavi Murray ◽  
Andy M. Smith
Keyword(s):  
Shear Stress ◽  
Linear Response ◽  
Small Variation ◽  
Model Parameters ◽  
Similar Response ◽  
Mean Flow ◽  
Tidal Forcing ◽  
Ice Stream ◽  
Non Linear ◽  
Basal Shear

AbstractModulation of the flow of Rutford Ice Stream, Antarctica, has been reported previously at semi-diurnal, diurnal, fortnightly and semi-annual periods. A model that includes non-linear response to tidal forcing has been shown to fit closely observations at fortnightly frequencies. Here we examine that model further and test its fit at a larger set of observed frequencies, including the large semi-annual displacement. We show analytically that, when forced by major tidal terms, the model (using a basal shear stress exponent m = 3) predicts several discrete response periods from 4 hours to 0.5 years. We examine a 1.5 year GPS record from Rutford Ice Stream and find that the model, when forced by a numerical tide model, is able to reproduce the reported semi-annual signal. We confirm that about 5% of the mean flow is due solely to the (m = 3) non-linear response to tidally varying basal shear stress. Our best-fitting set of model parameters is similar to those originally reported using a much shorter data record, although with noticeably improved fit, suggesting these parameters are robust. We find that m ≈ 3 fits the data well, but that m ≈ 2 does not. Furthermore, we find that a small variation in flow over the 18.6 year lunar node tide cycle is expected. Fits to semi-diurnal and diurnal terms remain relatively poor, possibly due to viscoelastic effects that are not included in the model and reduced GPS data quality at some discrete periods. For comparison, we predict the response of Bindschadler Ice Stream and Lambert Glacier and show, given identical model parameters, a similar response pattern but with ∼1–2 orders of magnitude smaller variability; these may still be measurable and hence useful in testing the applicability of this model to other locations.

scholarly journals The marginal shear stress of Ice Stream B, West Antarctica

1997 ◽  
Vol 43 (145) ◽  
pp. 415-426 ◽  
Author(s):  
Miriam Jackson ◽  
Barclay Kamb
Keyword(s):  
Shear Stress ◽  
Strain Rate ◽  
Time Scale ◽  
Shear Zones ◽  
Hot Water ◽  
West Antarctica ◽  
Ice Stream ◽  
Flow Law ◽  
Ice Stream B

AbstractTo ascertain whether the velocity of Ice Stream B, West Antarctica, may be controlled by the stresses in its marginal shear zones (the “Snake” and the “Dragon”), we undertook a determination of the marginal shear stress in the Dragon near Camp Up B by using ice itself as a stress meter. The observed marginal shear strain rate of 0.14 a−1is used to calculate the marginal shear stress from the flow law of ice determined by creep tests on ice cores from a depth of 300 m in the Dragon, obtained by using a hot-water ice-coring drill. The test-specimen orientation relative to the stress axes in the tests is chosen on the basis ofc-axis fabrics so that the test applies horizontal shear across vertical planes parallel to the margin. The resulting marginal shear stress is (2.2 ± 0.3) × 105Pa. This implies that 63–100% of the ice stream’s support against gravitational loading comes from the margins and only 37–0% from the base, so that the margins play an important role in controlling the ice-stream motion. The marginal shear-stress value is twice that given by the ice-stream model of Echelmeyer and others (1994) and the corresponding strain-rate enhancement factors differ greatly (E≈ 1–2 vs 10–12.5). This large discrepancy could be explained by recrystallization of the ice during or shortly after coring. Estimates of the expected recrystallization time-scale bracket the ∼1 h time-scale of coring and leave the likelihood of recrystallization uncertain. However, the observed two-maximum fabric type is not what is expected for annealing recrystallization from the sharp single-maximum fabric that would be expected in situ at the high shear strains involved (γ ∼ 20). Experimental data from Wilson (1982) suggest that, if the core did recrystallize, the prior fabric was a two-maximum fabric not substantially different from the observed one, which implies that the measured flow law and derived marginal shear stress are applicable to the in situ situation. These issues need to be resolved by further work to obtain a more definitive observational assessment of the marginal shear stress.

scholarly journals The role of the margins in the dynamics of an active ice stream

1994 ◽  
Vol 40 (136) ◽  
pp. 527-538 ◽  
Author(s):  
K. A. Echelmeyer ◽  
W. D. Harrison ◽  
C. Larsen ◽  
J. E. Mitchell
Keyword(s):  
Numerical Models ◽  
Force Balance ◽  
Shear Stresses ◽  
List Type ◽  
Ice Stream ◽  
Strain Heating ◽  
Ice Stream B ◽  
Basal Shear ◽  
Marginal Zones

AbstractA transverse profile of velocity was measured across Ice Stream B, West Antarctica, in order to determine the role of the margins in the force balance of an active ice stream. The profile extended from near the ice-stream center line, through a marginal shear zone and on to the slow-moving ice sheet. The velocity profile exhibits a high degree of shear deformation within a marginal zone, where intense, chaotic crevassing occurs. Detailed analysis of the profile, using analytical and numerical models of ice flow, leads to the following conclusions regarding the roles of the bed and the margins in ice-stream dynamics:(i)The overall resistive drag on the ice stream is partitioned nearly equally between the margins and the bed and, thus, both are important in the force balance of the ice stream.(ii)The ice within the chaotic zone must be about 10 times softer than the ice in the central part of the ice stream.(iii)The average basal shear stress is 0.06 × 105Pa. This implies that the entire bed cannot be blanketed by the weak, deformable till observed by Engelhardt and others (1990) near the center of the ice stream — there must be regions of increased basal drag.(iv)High strain rates and shear stresses in the marginal zones indicate that strain heating in the margins may be significant.While the exact quantitative values leading to these conclusions are somewhat model and location-dependent, the overall conclusions are robust. As such, they are likely to have importance for ice-stream dynamics in general.

scholarly journals In search of ice-stream sticky spots

1993 ◽  
Vol 39 (133) ◽  
pp. 447-454 ◽  
Author(s):  
Richard B. Alley
Keyword(s):  
Shear Stress ◽  
Large Scale ◽  
Water Pressure ◽  
Pressure Measurements ◽  
West Antarctica ◽  
Ice Surface ◽  
Ice Stream ◽  
Bed Roughness ◽  
Basal Shear

AbstractThe basal shear stress of an ice stream may be supported disproportionately on localized regions or “sticky spots”. The drag induced by large bedrock bumps sticking into the base of an ice stream is the most likely cause of sticky spots. Discontinuity of lubricating till can cause sticky spots, but they will collect lubricating water and therefore are unlikely to support a shear stress of more than a few tenths of a bar unless they contain abundant large bumps. Raised regions on the ice-air surface can also cause moderate increases in the shear stress supported on the bed beneath. Surveys of large-scale bed roughness would identify sticky spots caused by bedrock bumps, water-pressure measurements in regions of thin or zero till might reveal whether they were sticky spots, and strain grids across the margins of ice-surface highs would show whether the highs were causing sticky spots. Sticky spots probably are not dominant in controlling Ice Stream Β near the Upstream Β camp, West Antarctica.

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