Intermediate-depth icequakes at Greenlands fastest outlet glacier: evidence for englacial thrust faulting?

In traditional glacier flow laws and consequently glacier models, a widely used assumption is that the ice behaves as a non-Newtonian viscous fluid that slides either across hard bedrock or via deforming subglacial till. Elastic effects and brittle deformation within the ice are often neglected for simplicity, as even ubiquitous surface crevasses are difficult to capture in numerical schemes. While there is ample seismological evidence that stick-slip motion plays a significant role in basal sliding of both alpine and polar glaciers, similar evidence is lacking for brittle deformation within the ice mass itself. Instead, it is commonly assumed that the ice moves and deforms in a purely viscous or ductile manner, which may not be an accurate representation of reality. Here, we present observations of high-frequency (>50Hz) signals of intermediate-depth seismic sources occurring along the fast ice-stream of Sermeq Kujalleq in Kangia (Jakobshavn Isbr), Greenlands fastest flowing outlet glacier. The waveform characteristics of these events closely resemble the known characteristics of waveforms associated with basal stick-slip events, making them easily distinguishable from the more prevalent icequake signals generated by surface crevasse opening and propagation. However, differences in P and S wave arrival times as well as probabilistic source locations show that these events occur at ~170-400 m depth, whereas at those locations the glacier has a total depth of approximately 2000 m. Hence, these events cannot be caused by stick-slip motion at the base of the glacier, but must originate from englacial dislocations such as e.g., thrust faulting. Hundreds of these englacial icequakes are observed at several seismic arrays that were temporarily deployed in 2022 and 2023 along the fast ice-stream of Sermeq Kujalleq. Using waveform clustering and source mechanism analysis, we discuss the role of these events in ice dynamics and in particular englacial deformation.