Hydraulically-forced crevassing is thought to reduce the stability of
ice shelves and ice sheets, affecting structural integrity and providing
pathways for surface meltwater to the bed. It can cause ice shelves to
collapse and ice sheets to accelerate into the ocean. However, direct
observations of the hydraulically-forced crevassing process remain
elusive. Here we report a new, novel method and observations that use
icequakes to directly observe crevassing and determine the role of
hydrofracture. Crevasse icequake depths from seismic observations are
compared to a theoretically derived maximum-dry-crevasse-depth. We
observe icequakes below this depth, suggesting hydrofracture.
Furthermore, icequake source mechanisms provide insight into the
fracture process, with predominantly opening cracks observed, which have
opening volumes of tens to hundreds of cubic meters. Our method and
findings provide a framework for studying a critical process, key for
the stability of ice shelves and ice sheets, and hence rates of future
sea-level rise.