AGU2024

The initiation of sliding on a fault or frictional interface is thought to be preceded by localized slow slip that may expand in the form of a front and herald an impending rupture. We study precursory slip associated with regular stick-slip events on the Whillans Ice Plain in West Antarctica and link our observations to laboratory experiments and frictional strength relevant to earthquakes. The glacier basal interface is a natural fault that slips about ½ m in roughly 30 minutes once or twice daily, which we term mainshocks. We use data from 20 GNSS stations placed 800 m above the basal interface on a 60 km segment of the ice plain surface for 2 months during the 2010-2011 austral summer. 70 slip events in this time window were found to contain precursory slip lasting 5 minutes to over an hour in duration prior to the mainshock. We focus on the shorter precursors, and analysis shows the event moves across the station coverage as a front with a velocity of 17-130 m/s. The front velocity correlates with the interevent displacement near the central highly locked region, which is likely related to the local basal shear stress. The locked area basal shear stress is estimated from its elastic resistance to upstream and downstream ice displacement. A 14 kPa increase in shear stress is found to cause approximately a 5x increase in the precursor front velocity. These findings are a striking resemblance to recent slow slip fronts in laboratory experiments that showed a 10 kPa increase in shear stress caused a 10x increase in the slow slip front velocity. The ability to monitor the ice plain in such detail showcases parallels of premonitory motion on a natural fault and the laboratory scale.