Assessing ice anisotropy using basal icequakes at Sermeq Kujalleq in Kangia, Greenland

Glacial ice is formed as snow is compressed under its own weight, forming ice crystals with initially random orientations i.e. isotropic ice. Over time, under sustained accumulation and overburden stress, the ice crystals transition from a random arrangement to a more aligned structure, forming anisotropic ice. With continued stress, the ice starts flowing, further modifying the anisotropy. Unlike isotropic ice, which responds equally to stress in all directions, anisotropic ice can deform up to 10 times faster due to its aligned crystal structure. Widely used glacier flow laws, such as Glens flow law, assume the ice to be isotropic. Anisotropy significantly impacts flow dynamics and should therefore be included in ice sheet and glacier models. While enhancement factors are sometimes used to mimic anisotropy, they often do not accurately represent these effects.

In order to correctly represent anisotropy in ice flow, in-situ measurements of ice fabric are needed. However, obtaining such measurements is challenging, particularly in dynamic regions such as ice streams and outlet glaciers. Due to the evolving stress patterns they are subjected to over time, ice streams and outlet glaciers develop distinct anisotropic characteristics. This anisotropic signal contrasts with areas dominated by vertical compression, such as accumulation zones, where anisotropic measurements are typically conducted through ice cores. By applying the concept of seismic anisotropy, specifically shear wave splitting (SWS), we can effectively determine the ice fabric in these fast-flowing areas. This approach provides insights into ice anisotropy of ice streams and glaciers that is difficult to achieve with other methods.

Here, we present ice fabric measurements at Sermeq Kujalleq in Kangia (Jakobshavn Isbræ), Greenland's fastest flowing outlet glacier, with flow velocities reaching 30–40 m/d. By utilizing shear wave splitting observed using basal icequakes, measured directly within the main ice stream, we are able to make a first estimate of the ice anisotropy in such a fast-flowing ice stream.