Elucidating the Stick-Slip Nature of the Whillans Ice Plain

Elucidating the Stick-Slip Nature of the Whillans Ice Plain Station location map depicting continuous GPS network and broadband station names for the 2008 field season. Subglacial lake geometry is shown as IceSAT tracks, adapted from Fricker et al. [2007]. The green shaded circles 95% confidence level error ellipses encircling slip-start locations, shown as green squares. Yellow squares indicate slip-start locations with only three station observations.
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The Whillans Ice Plain (WIP) is an approximately 200 km x 100 km x 600 m portion of the West Antarctic Ice Sheet that slips up to 0.5 m over a duration of 30 minutes, twice daily. Such bi-daily, tidally modulated stick-slip speed-ups provide insight into glacier dynamics and may be a unique analogue to tectonic earthquake/slow slip rupture. We deployed a network of continuously-operating GPS receivers in 2007 and operated on-ice broadband seismometers (partially provided by PASSCAL) during the austral summer of 2008 on WIP. Previous work during the 2004 field season suggested that these speed- ups initiate as failure of an asperity on or near “Ice Raft A” that triggers rupture across the entire WIP [Wiens et al., 2008]. Our results for the 2008 field sea- son locate the slip initiation farther to the south of this feature, closer to the grounding line and the southernmost extent of the Ross Ice Shelf. A strong correlation between the amplitude of seismic waves generated at the rupture front and the total slip achieved over the duration of the slip event (~ 30 min) suggests slip-predictable behavior [Walter et al., submitted] or the ability to forecast the eventual slip based on the first minute of seismic radiation. Arrival time information compiled from stations QSPA and VNDA (continuous seismic data archived and provided by the IRIS DMC) show that successive slip events propagate with different rupture speeds (100-300 m/s) that strongly correlate (R-squared = 0.73, p-value = 0.0012) with the recurrence interval. In addition, the amount of slip achieved during each event appears to be correlated with the rupture speed. Our work suggests that the far-field transmission of seismic waves from glacier action is not dependent upon bulk ice movement, but rapid basal stress changes [Walter et al., submitted]. The availability of on-ice broadband seismometers and data at stations QSPA and VNDA yield important information regarding mechanics and dynamics of ice stream beds at the scale of 10s to 100s of km. Subglacial processes are notoriously difficult to constrain on these large scales, which are relevant to the understanding of regional and continental ice motion.
</p><p>References
</p><p>Fricker, H. A., T. Scambos, R. Bindschadler (2007), An active subglacial water system in West Antarctica mapped from space, Science, 315(5818): 1544-1548.
</p><p>Walter, J. I., Brodsky, E. Tulaczyk, S., Schwartz, S., and R. Pettersson, submitted, Slip- predictability and dynamically fluctuating rupture speeds on a glacier-fault, Whillans Ice Plain, West Antarctica, J. Geophys. Res. Earth Surface.
</p><p>Wiens, D. A., S. Anandakrishnan, J. P. Winberry, and M. A. King (2008), Simultaneous teleseismic and geodetic observations of the stick-slip motion of an Antarctic ice stream, Nature, 453: 770774.
</p><p>Acknowledgements: This work was funded primarily by NSF Antarctic Sciences Division Grant number 0636970. Support for JW is provided by a NASA Earth and Space Science Fellowship. Some broadband seismic equipment was provided by PASSCAL. The IRIS DMC manages and archives data from stations QSPA and VNDA.</p>

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