Basal seismicity of the Northeast Greenland Ice Stream

Abstract

Seismic studies of glaciers yield insights into spatio-temporal processes within and beneath glaciers on scales relevant to flow and deformation of the ice. These methods enable direct monitoring of the bed in ways that complement other geophysical techniques, such as geodetic or ground penetrating radar observations. In this work, we report on the analysis of passive seismic data collected from the interior of the North East Greenland Ice Stream, the Greenland ice sheet's largest outlet glacier. We record thousands of basal earthquakes, many of which repeat with nearly identical waveforms. We also record many long-duration glacial tremor episodes that migrate across the seismic network with slow velocities (e.g. ~4–12 m s−1). Analysis of the basal earthquakes indicates a transition between times of individual event activity and times of tremor activity. We suggest that both processes are produced by shear slip at localized asperities along the bed. The transition between discrete and quasi-continuous slipping modes may be driven by pore-water pressure transients or heterogeneous strain accumulation in the ice due to strength contrasts of the underlying till.

Stick‐Slip Tremor Beneath an Alpine Glacier

“…No such surging behavior was observed for the tremor presented in this study as ice velocities measured at several coordinates on Gornergletscher's surface (Garcia et al, 2019) did not reveal an anomaly such as a widespread or unusually large flow acceleration ( Figure S11). Our tremor seems more similar to recent observations at a Greenland ice stream, where stable or discrete stick-slip sliding transitions into stick-slip tremor (McBrearty et al, 2020). The authors of the Greenland study explain such transitions in terms of strain imbalances or pore water pressure changes at the ice stream bed.…”

Not all Icequakes are Created Equal: Basal Icequakes Suggest Diverse Bed Deformation Mechanisms at Rutford Ice Stream, West Antarctica

“…Instead, we favor a concept of asperities within the subglacial till, which are randomly built by the glacial movement and subsequently destroyed through a sequence of stick‐slip events. Such asperities may be envisaged as sites of increased friction that develop during continuous ice stream movement as sediment is transported and dilates and reorganizes (McBrearty et al., 2020; Thornsteinsson & Raymond, 2000; Van Der Meer et al., 2003). If glacial till is sheared, its pore volume is increased (Boulton & Hindmarsh, 1987).…”

Application of Constitutive Friction Laws to Glacier Seismicity

“…Unstable glacial slip may be akin to movements of a seismogenic fault, where periods of little or no motion are punctuated by periods of sudden acceleration and deceleration (Aster & Winberry, 2017). Basal‐slip seismicity has been observed for many glacier morphologies and bed conditions, from Antarctic and Greenland ice streams (e.g., Blankenship et al, 1987; Bindschadler et al, 2003; Winberry et al, 2009, McBrearty et al, 2020) to alpine glaciers (e.g., Allstadt & Malone, 2014; Thelen et al, 2013) and from hard beds (more rigid than ice) to deformable beds (less rigid than ice), indicating that glaciers spanning the full spectrum of basal slip conditions can slip unstably under certain conditions. Even glaciers that appear to have stable surface motion may have much of their slip accommodated through the combination of many small‐scale unstable slip events (McBrearty et al, 2020).…”