Icequake rupture suggests Antarctic ice stream beds may be stronger, yet more dynamic than previously assumed

Ice slip controls the flow of fast-flowing glaciers that contribute most to sea-level rise. The ice-bed interface shear stiffness partially regulates the shear strength, which dictates how fast ice can slip. Here we present the first measurements of slip-generated icequake rupture velocities at Rutford Ice Stream, Antarctica, providing the first direct remotely-measured estimates of in-situ bed stiffness. We find that the bed is stiffer than previously measured from active seismic measurements. Assuming the bed fails elastically, we find that the bed is also stronger, with the stiffness-strength relationship validated using laboratory shear failure experiments. Bed strength estimates are important because they can be parameterised in ice dynamics models. Furthermore, we find rupture direction is controlled by a fine balance of downstream tidal stresses and topographic hydrological gradients. If our results hold for other fast-flowing ice streams, then the strength of the bed may currently be underestimated in models, resulting in overestimates of ice loss and hence sea-level rise. However, if bed conditions are also finely balanced, then as grounding lines retreat, bed conditions may become unstable and cause surging behavior and accelerated ice loss. This highlights the need for more observations to interrogate the dynamism of ice stream beds.