Brittle creep deformation observed in an ice stream from borehole distributed acoustic sensing

Ice streams are major contributors to ice sheet mass loss and critical regulators of sea level change. Despite their importance, standard viscous flow simulations of ice stream deformation and evolution have limited predictive power, mostly because our understanding of the involved processes is limited. This leads, for instance, to widely varying predictions of sea level rise during the next decades.

Here we report on a Distributed Acoustic Sensing experiment conducted in the borehole of the East Greenland Ice Core Project (EastGRIP) on the Northeast Greenland Ice Stream (NEGIS). For the first time, our observations reveal a brittle deformation mode that is incompatible with viscous flow over length scales similar to the resolution of modern ice sheet models: englacial ice quake cascades that are not being recorded at the surface. A comparison with ice core analyses shows that ice quakes preferentially nucleate near volcanism-related impurities, such as thin layers of tephra or sulfate anomalies. These are likely to promote grain boundary cracking, and appear as a macroscopic form of crystal-scale wild plasticity. A conservative estimate indicates that seismic cascades are likely to produce strain rates that are comparable in amplitude to those measured geodetically, thereby bridging the well-documented gap between current ice sheet models and observations.