Highly variable friction and slip observed at Antarctic ice stream bed

The slip of glaciers over the underlying bed is the dominant mechanism governing the migration of ice from land into the oceans, with accelerating slip contributing to sea-level rise. Yet glacier slip remains poorly understood, and observational constraints are sparse. Here we use passive seismic observations to measure both frictional shear stress and slip at the bed of the Rutford Ice Stream in Antarctica using 100,000 repetitive stick-slip icequakes. We find that basal shear stresses and slip rates vary from 10$^4$ to 10$^7$ Pa and 0.2 to 1.5 m per day, respectively. Friction and slip vary temporally over the order of hours, and spatially over 10s of metres, due to corresponding variations in effective normal stress and ice–bed interface material. Our findings suggest that the bed is substantially more complex than currently assumed in ice stream models and that basal effective normal stresses may be significantly higher than previously thought. Our observations can provide constraints on the basal boundary conditions for ice-dynamics models. This is critical for constraining the primary contribution of ice mass loss in Antarctica and hence for reducing uncertainty in sea-level rise projections

Bathymetry and bed conditions of Lago Subglacial CECs, West Antarctica

Although over 600 Antarctic subglacial lakes have been identified using radar and satellite observations, the bathymetry and bed properties, which are key to understanding conditions within the lake, have been determined in very few localities. We present measurements of water column thickness and lakebed properties from Lago Subglacial CECs (SLC), located beneath 2653 m of ice at the Rutford-Institute-Minnesota divide in Antarctica. Seismic profiles indicate a maximum water column thickness of 301.3 ± 1.5 m, at the widest part of the lake, with an estimated lake volume of 2.5 ± 0.3 km3. Seismic imaging and measurements of the reflection strength at the ice base and lakebed indicate >15 m of high-porosity fine-grained sediment in the central section of the lakebed, consistent with a depositional sequence with an age of up to 0.5 Ma. These observations, along with previous radar measurements and modelling, indicate a low-energy sedimentary environment with a long water-residence time. As such, SLC is a suitable target for exploration via direct access to recover sediment records of ice sheet and climate history and investigate microbial life with long periods of isolation

Differentiating flow, melt, or fossil seismic anisotropy beneath Ethiopia

Ethiopia is a region where continental rifting gives way to oceanic spreading. Yet the role that pre-existing lithospheric structure, melt, mantle flow, or active upwellings may play in this process is debated. Measurements of seismic anisotropy are often used to attempt to understand the contribution that these mechanisms may play. In this study, we use new data in Afar, Ethiopia along with legacy data across Ethiopia, Djibouti, and Yemen to obtain estimates of mantle anisotropy using SKS-wave splitting. We show that two layers of anisotropy exist, and we directly invert for these. We show that fossil anisotropy with fast directions oriented northeast-southwest may be preserved in the lithosphere away from the rift. Beneath the Main Ethiopian Rift and parts of Afar, anisotropy due to shear segregated melt along sharp changes in lithospheric thickness dominates the shear-wave splitting signal in the mantle. Beneath Afar, away from regions with significant lithospheric topography, melt pockets associated with the crustal and uppermost mantle magma storage dominate the signal in localized regions. In general, little anisotropy is seen in the uppermost mantle beneath Afar suggesting melt retains no preferential alignment. These results show the important role melt plays in weakening the lithosphere and imply that as rifting evolves passive upwelling sustains extension. A dominant northeast-southwest anisotropic fast direction is observed in a deeper layer across all of Ethiopia. This suggests that a conduit like plume is lacking beneath Afar today, rather a broad flow from the southwest dominates flow in the upper mantle

Measuring seismic attenuation in polar firn: method and application to Korff Ice Rise, West Antarctica

We present seismic measurements of the firn column at Korff Ice Rise, West Antarctica, including measurements of compressional-wave velocity and attenuation. We describe a modified spectral-ratio method of measuring the seismic quality factor (Q) based on analysis of diving waves, which, combined with a stochastic method of error propagation, enables us to characterise the attenuative structure of firn in greater detail than has previously been possible. Q increases from 56 ± 23 in the uppermost 12 m to 570 ± 450 between 55 and 77 m depth. We corroborate our method with consistent measurements obtained via primary reflection, multiple, source ghost, and critically refracted waves. Using the primary reflection and its ghost, we find Q = 53 ± 20 in the uppermost 20 m of firn. From the critical refraction, we find Q = 640 ± 400 at 90 m depth. Our method aids the understanding of the seismic structure of firn and benefits characterisation of deeper glaciological targets, providing an alternative means of correcting seismic reflection amplitudes in cases where conventional methods of Q correction may be impossible

Breaking the Ice: Identifying Hydraulically Forced Crevassing

Hydraulically forced crevassing is thought to reduce the stability of ice shelves and ice sheets, affecting structural integrity and providing pathways for surface meltwater to the bed. It can cause ice shelves to collapse and ice sheets to accelerate into the ocean. However, direct observations of the hydraulically forced crevassing process remain elusive. Here we report a novel method and observations that use icequakes to directly observe crevassing and determine the role of hydrofracture. Crevasse icequake depths from seismic observations are compared to a theoretically derived maximum dry crevasse depth. We observe icequakes below this depth, suggesting hydrofracture. Furthermore, icequake source mechanisms provide insight into the fracture process, with predominantly opening cracks observed, which have opening volumes of hundredths of a cubic meter. Our method and findings provide a framework for studying a critical process that is key for the stability of ice shelves and ice sheets and, therefore, future sea level rise projections

Ice Fabric in an Antarctic ice stream interpreted from seismic anisotropy

Here we present new measurements of an anisotropic ice fabric in a fast moving (377 ma−1) ice stream in West Antarctica. We use ∼6000 measurements of shear wave splitting observed in microseismic signals from the bed of Rutford Ice Stream, to show that in contrast to large-scale ice flow models, which assume that ice is isotropic, the ice in Rutford Ice Stream is dominated by a previously unobserved type of partial girdle fabric. This fabric has a strong directional contrast in mechanical properties, shearing 9.1 times more easily along the ice flow direction than across flow. This observed fabric is likely to be widespread and representative of fabrics in other ice streams and large glaciers, suggesting it is essential to consider anisotropy in data-driven models to correctly predict ice loss and future flow in these regions. We show how passive microseismic monitoring can be effectively used to provide these data

Subglacial lakes and hydrology across the Ellsworth Subglacial Highlands, West Antarctica

Subglacial water plays an important role in ice sheet dynamics and stability. Subglacial lakes are often located at the onset of ice streams and have been hypothesised to enhance ice flow downstream by lubricating the ice–bed interface. The most recent subglacial-lake inventory of Antarctica mapped nearly 400 lakes, of which ∼ 14 % are found in West Antarctica. Despite the potential importance of subglacial water for ice dynamics, there is a lack of detailed subglacial-water characterisation in West Antarctica. Using radio-echo sounding data, we analyse the ice–bed interface to detect subglacial lakes. We report 33 previously uncharted subglacial lakes and present a systematic analysis of their physical properties. This represents a ∼ 40 % increase in subglacial lakes in West Antarctica. Additionally, a new digital elevation model of basal topography of the Ellsworth Subglacial Highlands was built and used to create a hydropotential model to simulate the subglacial hydrological network. This allows us to characterise basal hydrology, determine subglacial water catchments and assess their connectivity. We show that the simulated subglacial hydrological catchments of the Rutford Ice Stream, Pine Island Glacier and Thwaites Glacier do not correspond to their ice surface catchments

Mapping the Ice-Bed Interface Characteristics of Rutford Ice Stream, West Antarctica, Using Microseismicity

Flow dynamics of the ice streams which drain the Antarctic Ice Sheet are heavily in uenced by the processes happening at the bed. Natural seismic activity generated beneath an ice stream is associated with the motion of the ice over its bed and can be used to map the characteristics of the ice-bed interface and to understand these basal processes. Basal microseismicity was recorded over a 34-day period on Rutford Ice Stream, West Antarctica, using ten three-component geophones 40 km upstream of the grounding line. Around 3000 microseismic events were located in discrete spatial clusters near the ice-bed interface. The activity of each cluster varies with time, and the source mechanism for the events is interpreted as sub-horizontal, low-angle faulting, slipping in the ice ow direction. Cluster locations are interpreted as `sticky spots' of stiff basal sediment at the ice-bed interface, where ice movement is accommodated by stick-slip basal sliding. The `sticky spots' occur in areas where independent active-source seismic surveys show low porosity sediments at the bed. We show that the `sticky spots' probably accommodate only a small amount of the total basal motion. Our results suggest that most of the ice motion is accommodated by aseismic deformation of soft,dilatant basal sediment or by a well-lubricated, stiffer be

Radon transform-based detection of microseismicity on DAS networks: A case study from Antarctica

Seismic arrays deployed using DAS generally suffer from a poorer SNR than those using conventional seismometers or geophones, however their high spatial resolution provides opportunities to supress noise and enhance coherent signals. Using a localised Radon transform-based detection method, we exploit the spatial resolution of DAS to identify low amplitude arrivals. We develop this approach using data acquired at the Rutford Ice Stream, Antarctica, where naturally occurring microseismicity is a regular occurrence due to high flow rates of the glacier. During January 2020 both linear and triangular arrangements of fibreoptic cable were deployed to recorded icequakes originating from the base of the glacier, and these were complimented by a network of 3-component geophones. Using a 6hr subset of this dataset we show that the DAS network can achieve a higher detection rates than the geophone network when Radon detection methods are employed. The linear array achieves better detection rates than the triangular array due to is larger spatial coverage, however the one-dimensional nature of the array results in significant ambiguities in event locations