Bradley Lipovsky, Harvard University

The Whillans Ice Plain region of the West Antarctic Ice Sheet experiences twice-daily, tidally-modulated slow slip events. During each event, 0.5 m of slip occurs over a 150x150 km area. Sliding initiates at one of several recurring locations and expands outwards with a typical rupture velocity ~200 m/s. Slow slip events are accompanied by seismic tremor due to small (meter-scale) repeating earthquakes. Understanding the mechanical processes that gives rise to these curious phenomena has implications for both sea level rise hazard and for our knowledge of faulting processes more generally. We explore these observations using a simplified model that includes the effects of inertia, elasticity, tidal forcing, and rate- and state-dependent frictional sliding. We find that, at the ~km-scale, slow rupture velocities are explained by basal conditions that are near the steady sliding limit. Such conditions arise in our simulations due to the presence of high pore pressures such that the subglacial effective normal stress is ~10 kPa where the ice overburden pressure is 7.2 MPa. Many other Antarctic ice streams experience tidally modulated flow but not sticks-slip, and our model is able to capture this behavior as well. The transition between steady sliding and stick-slip motion --a central hallmark of classical rate-and-state theory-- thus explains a wide range of fast sliding behavior within the Antarctic Ice Sheet.