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Lookup NU author(s): Professor Matt King
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Stick-slip motion was discovered in the mouth of Whillans Ice Stream from analysis of GPS observations processed to 5-minute point positions (Bindschadler and others, in press). Daily motion of the ice plain occurred almost exclusively during two sudden and rapid motion events of from 20 to 40 cm each, that were closely associated with the end of the ocean high tide and the middle of the falling tide. Initiation of the events appeared to propagate across the ice plain at a mean speed of 88 m/sec, suggesting a shear wave moving through the water-saturated sub-glacial till. Here we report on subsequent analysis of this surprising phenomenon. GPS data are processed to give differential motions between occupied sites allowing 30-second temporal resolution. On the lower reach of the ice plain, a nearly stationary station within the network shows ice on the mobile ice plain rapidly accelerates in a single 30-second interval to a speed corresponding to a perfectly lubricated bed. The deceleration takes only slightly longer. Even with the improved temporal resolution, no vertical motion associated with the horizontal slip events is seen. Between one pair of motion events, ice-plain stations are observed to move 5 cm orthogonal to the downstream direction, in concert with a few-cm increase in vertical position, approximately midway between the high -tide and falling-tide events with most of the displacement recovered before the falling-tide event. From our brief observational record, it is not known how often this situation occurs. It may represent an elastic character of the till when it is stiff. Farther upstream, but still on the lightly-grounded ice plain, only relative motions between moving stations could be produced at 30-second intervals because no near-stationary stations were available. In a typical example involving two moving stations on roughly the same flowline, most, but not all, of the increase of the distance between the stations is confined to the motion events. The onset appears slightly less sudden than downstream, but this could be the result of the event not initiating at exactly the same instant at the two receivers (delays of a few minutes were observed in the earlier analysis). The deceleration is more gradual than farther downstream with most of the deceleration occurring within the first half-hour. Some of this longer term lengthening of the distance between stations is recovered in the hourslong interval before the next motion event. Again, this may represent some elastic character of the till. A near-plastic spring-block model of the till is presented that can reproduce some of the observed characteristics. The increased detail of the differentially processed data also reveal smaller changes (less than 5 cm.) in the horizontal component of the baseline connecting station pairs that correlate with small changes (less than 10 cm.) in the vertical component of the baseline. These smaller scale signatures may express the subglacial transport of water, as has been observed on mountain glaciers. However, until GPS data with longer time spans are collected on Whillans Ice Stream it will be difficult to determine the correct level of GPS data uncertainty, especially for these smaller motions.
Author(s): Bindschadler R, King M, Vornberger P
Publication type: Conference Proceedings (inc. Abstract)
Publication status: Published
Conference Name: Geophysical Research Abstracts: EGS-AGU-EUG Joint Assembly
Year of Conference: 2003
Pages: 02985
ISSN: 1029-7006
Publisher: Copernicus GmbH
Library holdings: Search Newcastle University Library for this item
ISBN: 16077962
