Speaker: Brad Lipvosky

Title: "Stick-slip motion of the Antarctic Ice Sheet"

Abstract:

The reactivation of a single dormant Antarctic ice stream would double the continent’s mass imbalance. Despite importance of understanding the likelihood of such an event, direct observation of the basal processes that lead to the activation and stagnation of streaming ice are minimal. As the only ice stream in the early stages of stagnation, the Whillans Ice Plain (WIP) occupies a central role in our understanding of these subglacial processes.  Complicating matters is the observation, from GPS records, that the WIP experiences most of its motion during episodes of rapid sliding.  Sliding events are tidally modulated and separated by ~12 hour periods of quiescence.  A typical slip event affects a ~10^4 sq km area, has sliding velocities as fast as 65 m/d, lasts for about half an hour, and is accompanied by m-scale repeating earthquakes, or tremor, at the ice sheet bed.  

We conduct numerical simulations of ice stream stick-slip cycles and accompanying seismic tremor. Our simulations include rate- and state-dependent frictional sliding, tidal forcing, inertia, and upstream loading in a cross-stream, thickness-averaged formulation. Our numerical implementation makes use of higher order accurate, summation-by-parts finite difference operators and weak enforcement of boundary conditions. 

Our principal finding is that ice stream sliding may respond to ocean tidal forcing with one of two end member behaviors.  In one limit, tidally modulated slip events have rupture velocities that approach the shear wave speed and slip events have a duration that scales with the ice stream width divided by the shear wave speed.  This first limit more closely resembles tectonic earthquakes. In the opposite limit, tidal modulation results in a gradually varying ice stream velocity.  This latter behavior is observed on several other active ice streams (Bindschadler, Rutford). 

We find that WIP slip events exist between these two end member behaviors: rupture velocities are far below the inertial limit yet sliding occurs only episodically.  Borrowing terminology from the subduction zone community, we refer to this behavior as slow slip.  The transition between these two types of behavior may occur by changing subglacial water pressure, suggesting that such a mechanism may be responsible for stagnation of the WIP.