Pore-pressure and deformational response of subglacial till to seismicity

Glaciers and ice sheets decouple from and slide over their beds. The Siple Coast ice streams draining the interior of West Antarctica are commonly thought to slide on a layer of deforming subglacial till that is water saturated and behaves in a critical state. Recent observations from Whillans Ice Stream in Antarctica reveal stick--slip behavior that is tidally modulated and coeval with basal seismicity. These observations invite the possibility that seismic waves induce a deformational response of subglacial till and permit sliding. Geometrically, ice ~1000 m thick overlies meters to tens of meters of fine-grained till, and seismicity is generated at or near the ice--bed interface. Here, we develop and explore a simple theoretical model for pore-pressure-induced failure of a weak, Coulomb till that transmits pressure pulses much more rapidly than water volumes. In our model, stress driving ice motion is supported by shear along the bed and resisting stresses. Failure is governed by effective pressure and physical characteristics of till. We split resisting stress into a background wall stress supporting the ice stream laterally and a longitudinal stress operating along flow. Exploration of parameter values for driving stress, resisting stresses, and till properties yields a range of possible stick--slip scenarios. We conclude by relating our work to longer-term slip rates of glaciers and ice sheets.