Seismic evidence for frictional heterogeneity and transient basal slip beneath a fast Greenland outlet glacier

Basal friction and stick–slip processes beneath fast-flowing glaciers play a key role in modulating ice dynamics, yet the physical conditions at the ice–bed interface remain poorly constrained. Here, we use seismic observations of basal icequakes recorded within the ice stream of a fast-flowing Greenland outlet glacier to investigate frictional heterogeneity and transient slip behavior at the glacier bed. Using a three-sensor seismic array, we detect nearly 25,000 short-duration seismic events over 5-week period that occur in spatially coherent clusters, indicating repeated failure on localized basal asperities.

We analyze S-wave spectra within these clusters using the Brune source model and interpret the results within a rate-and-state friction framework to estimate relative variations in basal frictional stress through space and time. Our analysis reveals pronounced heterogeneity in basal seismic slip and stress behavior, with one persistent, spatially extensive region exhibiting systematically higher inferred frictional stresses throughout the observation period. This suggests that basal friction is not spatially uniform but instead governed by a patchwork of asperities that repeatedly load and fail, including at least one long-lived, dominant “sticky-spot”.
In addition to this localized behavior, we observe kilometre-scale downstream and upstream migration of icequake activity. These migration patterns suggest the presence of transient, propagating slip fronts, analogous to faster slip behavior previously observed beneath the Whillans Ice Stream, Antarctica, as well as in some tectonic fault systems. The inferred slip fronts propagate faster than glacier flow speeds and show a weak correlation with the tidal signal at the glacier terminus, indicating that their evolution might be controlled by small external stress changes.

Together, these observations support a view of glacier basal motion as a highly dynamic and locally controlled process rather than a spatially averaged frictional regime. The additional evidence for seismic migration highlights an interplay between localized stress accumulation at persistent asperities and more distributed, evolving slip processes, both of which may influence the dynamics and stability of fast glacier flow.