Interfacial controls on glacier stick-slip rupture dynamics and implications for slow earthquakes

English Abstract:

Slow slip events are now well-established in natural faults occurring under a myriad of physical conditions, though the processes controlling slow rupture remain poorly understood. The Whillans Ice Plain provides a window into these processes through bi-daily stick-slip seismic events that displace an ice mass over 100 km long with a variety of rupture speeds observed at a single location (e.g. Bindschadler et al., 2003). During periods between fast slip events, the ice flows downhill at a steady rate of less than 0.001 m/min, then suddenly increases its speed by more than an order of magnitude at tidal periods, slipping up to ~0.5 m in ~30 min. Typically individual rupture fronts initiate at two distinct nucleation regions that vary with the tide (Winberry et al., 2009). Events occurring during high tide have an average rupture speed (when averaged across the Ice Plain) that is faster than low tide events. Though, counter-intuitively, the low tide events tend to have significantly higher initial rupture speeds. We attribute this behavior to local stress configurations that are spatially and temporally heterogeneous and utilize laboratory measurements of stick-slip sliding on plastic blocks to mimic the WIP behavior (Walter et al., 2015). Basal interfacial stresses are known to control rupture speed in numerous laboratory analog experiments. We hypothesize a similar control on the WIP behavior and show that laboratory experiments can explain most of the rupture speed behavior. In addition, we show evidence that nucleation of the events is sensitive to triggering from distant earthquakes. If we simplify our notion of slow slip to events that rupture under the same conditions as earthquakes but at significantly reduced rupture velocities, then the glacier events provide a path forward for understanding slow slip on natural faults. The central conclusion of our studies is that the observed systematic variations in rupture velocities are governed by the applied stresses and may provide insight into some slow slip source processes.