Long-period seismic sources associated with glacier motion have been recently discovered1,2, and an increase in ice flow over the past decade has been suggested on the basis of secular changes in such measurements3. Their significance, however, remains uncertain, as a relationship to ice flow has not been confirmed by direct observation. Here we combine long-period surface-wave observations with simultaneous Global Positioning System measurements of ice displacement to study the tidally modulated stick–slip motion of the Whillans Ice Stream in West Antarctica4,5. The seismic origin time corresponds to slip nucleation at a region of the bed of the Whillans Ice Stream that is likely stronger than in surrounding regions and, thus, acts like an ‘asperity’ in traditional fault models. In addition to the initial pulse, two seismic arrivals occurring 10–23 minutes later represent stopping phases as the slip terminates at the ice stream edge and the grounding line. Seismic amplitude and average rupture velocity are correlated with tidal amplitude for the different slip events during the spring-to-neap tidal cycle. Although the total seismic moment calculated from ice rigidity, slip displacement, and rupture area is equivalent to an earthquake of moment magnitude seven (Mw 7), seismic amplitudes are modest (Ms 3.6–4.2), owing to the source duration of 20–30 minutes. Seismic radiation from ice movement is proportional to the derivative of the moment rate function at periods of 25–100 seconds and very long-period radiation is not detected, owing to the source geometry. Long-period seismic waves are thus useful for detecting and studying sudden ice movements but are insensitive to the total amount of slip.
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GPS receivers for the TIDES project were supplied by the University NAVSTAR Consortium. Seismic data were obtained from the Data Management Center of the Incorporated Research Institutions for Seismology. This research was funded by the Office of Polar Programs, US National Science Foundation. M.A.K. was partially funded by a NERC (UK) research fellowship. We thank R. B. Alley, R. A. Bindschadler, H. Horgan, I. Joughin, L. Peters and D. E. Voigt for planning and carrying out the TIDES field deployment.
Author Contributions D.A.W. found the ice slip signals on the seismic records and carried out the seismic processing and modelling. D.A.W. also filtered the GPS time series and calculated the slip nucleation locations and times from the GPS records. S.A. and J.P.W. carried out the GPS fieldwork and calculated the displacement time series from the three-dimensional GPS data. M.A.K. processed the raw GPS data to obtain the three-dimensional displacement time series. All authors participated in the interpretation of the results and preparing the paper.
The file contains Supplementary Tables 1-2.
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Poroelastic effects destabilize mildly rate-strengthening friction to generate stable slow slip pulses
Journal of the Mechanics and Physics of Solids (2019)