Diurnal expansion and contraction of englacial fracture networks revealed by seismic shear wave splitting

Daily variations in seismic S-wave anisotropy reveal the dynamic response of englacial fractures to changing meltwater supply, according to shear wave splitting measurements from a seismometer network on the Rhonegletscher, Switzerland. Fractures contribute to bulk elastic anisotropy of many materials in the Earth. This includes glaciers and ice sheets, whose fracture state controls the routing of water to the base and thus large-scale ice flow. Here we use anisotropy-induced shear wave splitting to characterize ice structure and probe subsurface water drainage beneath a seismometer network on an Alpine glacier. Shear wave splitting observations reveal diurnal variations in S-wave anisotropy up to 3%. Our modelling shows that when elevated by surface melt, subglacial water pressures induce englacial hydrofractures whose volume amounts to 1-2 percent of the probed ice mass. While subglacial water pressures decrease, these fractures close and no fracture-induced anisotropy variations are observed in the absence of meltwater. Consequently, fracture networks, which are known to dominate englacial water drainage, are highly dynamic and change their volumes by 90-180 % over subdaily time scales.

Automated summary

Daily variations in seismic S-wave anisotropy reveal the dynamic response of englacial fractures to changing meltwater supply, which play a crucial role in controlling water routing to the base of ice sheets and influencing large-scale ice flow. Fractures, which dominate englacial water drainage, are shown to be highly dynamic and their volumes can change significantly over short time scales.