Evidences of melt water control on crevasse propagation using dense array seismic observations

Crevasses are inherent features of glaciers and Ice Sheets. They exert a primary control on glacier dynamics, such as, for example, along shear margins through reducing the overall glacier ice viscosity, or at glacier and Ice Sheet fronts through controlling the onset of serac falls and of ice sheet instabilities (calving, ice shelf disintegration). However, our understanding of crevasse formation and propagation, and in particular the effect of melt water, remains limited due to lacking observations. Here we provide novel observational insights into englacial fracturing, the depth of crevasses and their depth propagation rates using dense seismic array monitoring on an Alpine glacier. We systematically detect and locate englacial seismic events through applying matched-field-processing on a particularly dense seismic array of 98 sensors deployed on the Glacier d’Argentière during 1-month in spring 2018. We observe rupture fronts along crevasses, which propagate from the glacier center to the glacier side at typical velocities of few hundreds of meters per day, i.e. at velocities that are much lower than those of seismic waves but much higher than those of glacier flow. We argue based on a dedicated spatial and temporal analysis that crevasse rupture propagation is set by the migration of water along the crevasse tip. We also observe that crevasses are associated with a wide range of depths, varying from the near surface to the glacier base, which at the present site is located about a hundred meters below the surface. This observation is particularly interesting, since it provides evidences that (i) crevasses are water filled and (ii) crevasses play a role in the supply of water to the bed. These findings are further supported by the observation that surface melt modulates the seismic activity of crevasses including those reaching the bed. Finally, by evaluating coherent structures in the crevasse population, we are able to infer their depth propagation rate, which we find is constant through the ice column, as expected if the surrounding ice stress field is counterbalanced by the water pressure in the crevasse. These observationally-derived findings provide useful grounds to test and improve theories of crevasse dynamics and their control in the overall transfer of water from the surface to the bed.