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SECULAR CHANGES IN GLACIER MOTION
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Slip-start locations, shown as green and yellow squares, were determined from broadband seismic stations deployed during the 2008 field season. Continuous GPS confirmed that the seismic events corresponded to times of accelerated motion of the ice stream. (From Fricker et al., 2007. An active subglacial water system in West Antarctica mapped from space. Science, doi:10.1126/science.1136897. Reprinted with permission from AAAS.)
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An increase in ice flow over the past decade is suggested on the basis of secular changes in long-period seismic sources associated with glacier motion. The relationship to ice flow is only now being calibrated by direct observation, but surface waves from slip events during a GPS deployment on the Whillans Ice Stream show that the seismic origin time corresponds to slip nucleation on the bed. A region of the bed acts like an “asperity” in traditional fault models. Seismic waves are also generated tens of minutes later when the slip terminates at the ice stream edge and the grounding line. Seismic amplitudes are modest, often equivalent to MS < 4, so some parameters—including the total amount of slip—cannot be determined without improving permanent regional monitoring networks. Nevertheless, because seismic radiation from ice movement is proportional to the rate of slip acceleration, long-period seismic waves are thus useful for detecting and studying sudden ice movements.</p>