Streaks of basal icequakes, subglacial geomorphology, and the evolving basal boundary of a major Antarctic ice stream

Icequakes that originate at or near the basal sliding surface of an ice stream may provide insight into temporal and spatial variability of ice stream basal properties and basal traction. We relate spatial patterns of basal seismicity with inferred subglacial geomorphology of the Whillans Ice Plain (WIP), in West Antarctica, and we suggest that changing seismicity patterns show evolving basal geomorphology and changing ice stream basal boundary conditions. In addition to being known for accomplishing most of its sliding by stick-slip, the WIP is also slowing down over time, making it a natural laboratory to study the processes that affect basal resistance to ice sliding. We record basal icequakes beneath the WIP using a 2.5 km wide seismic network. Basal icequakes are detected and located using a beamforming method, revealing elongate streaks of seismicity during several weeks of recording in two consecutive austral summers. Streak spacing and elongation are consistent with mega-scale glacial lineations (MSGL), and bed topography from one ice-penetrating radar line shows a low amplitude ( 2m) gently undulating bed, indicating MSGL in the area. One WIP icequake streak appears to occur in a shallow trough next to a MSGL, suggesting that MSGL formation is, at least in part, erosional. Previous work has suggested that basal icequakes beneath the Rutford Ice Stream happen where stiff, brittle, low-porosity till subcrops the ice base and causes stick-slip sliding, while aseismic areas have soft, deformable till or lubricated basal sliding. We propose that the WIP basal icequakes happen where erosion has removed shallow, deformable aseismic till and exposed deeper, stiffer, more consolidated sediment that can cause stick-slip basal sliding. Changing basal icequake patterns over the two years analyzed show the MSGLs beneath the seismic network are evolving on annual timescales, suggesting, importantly, that the evolution of subglacial geomorphology may change basal conditions and affect area-averaged basal traction values at annual to decadal timescales.