A document by Ian R.J. Lee. Click on the document to view its contents.

The nucleation and triggering of basal microseisms, or icequakes, at the bottom of glaciers as the ice flows over it can grant us valuable insights about deformation processes that occur at the bed. The collaborative efforts of Penn State University and the British Antarctic Survey (BAS) during the 2018/2019 austral summer enabled the deployment of several seismic arrays over 3 months in the Rutford Ice Stream in West Antarctica for monitoring natural source seismicity. Using the earthquake detection and location software QuakeMigrate, we generated unique high-resolution icequake catalogs, particularly at Rutford’s grounding line. Our data showed an unprecedented number of detected events which we used to resolve key topographical features and characteristics at the bed like sticky spots, and how they related to the continuous ice loading-slipping process at the bed. To properly quantify relations between events, we performed rigorous testing via manual event inspection at each array to determine a trigger threshold that aims to balance event coverage with artefact minimization. To handle the massive amounts of incoming seismic data and subsequent located icequakes, we also created a systematic data processing pipeline, and used machine learning clustering algorithms to resolve inter- & intra-clusters spatial and temporal relations. We present our pre-processing methods on handling similarly large datasets and present findings from our seismic data in combination with other data sources, like GPR and tidal gauge data, that improves our understanding of ice flow dynamics in the region.

Basal properties beneath ice streams and glaciers are known to be a control for ice flow dynamics, hence knowledge of them is crucial for predicting sea level due to changes in glacial dynamics. Basal properties, processes and topography also drive the formation of subglacial landforms. Bed properties beneath Rutford Ice Stream (West Antarctica) have previously been described using seismic acoustic impedance measurements at a sparse spatial coverage. Here, we derive bed properties in a 15 x 17 km grid of surface radar data with coverage and sampling much higher than previous seismic studies. Bed reflection amplitudes in surface radar data were calibrated using sediment porosities (ranging from 0.4 – 0.5) derived from seismic acoustic impedance. We find the bed properties are spatially variable, consisting of low porosity material in some areas and soft sediment in other areas. Comparison of seismic and surface radar data imply the low porosity material to be a consolidated sediment or sedimentary rock. Mega-scale glacial lineations (MSGLs) are ubiquitous on the bed and consist of soft, high porosity, probably deforming sediment, consistent with previous interpretations of MSGLs. We find some MSGLs have high reflectivity on their crest, interpreted as water bodies overlying high porosity sediment, whereas the trough around and the upstream end of some landforms consist of low porosity material. Integrating these different observations, we place constraints on possible explanations for the occurrence of water on the crest of landforms.

Microseismicity, induced by the sliding of a glacier over its bed, can be used to characterize frictional properties of the ice-bed interface, which are a key parameter controlling ice stream flow. We use naturally occurring seismicity to monitor spatiotemporally varying bed properties at Rutford Ice Stream, West Antarctica. We locate 230000 micro-earthquakes with local magnitudes from –2.0 to –0.3 using 90 days of recordings from a 35-station seismic network located ~40 km upstream of the grounding line. Events exclusively occur near the ice-bed interface and indicate predominantly flow-parallel stick-slip. They mostly lie within a region of interpreted stiff till and along the likely stiffer part of mega-scale glacial landforms. Within these regions, micro-earthquakes occur in spatially (<100 m radius) and temporally (mostly 1-5 days activity) restricted event-clusters (up to 4000 events), which exhibit an increase, followed by a decrease, in event magnitude with time. This may indicate event triggering once activity is initiated. Although ocean tides modulate the surface ice flow velocity, we observe little periodic variation in overall event frequency over time and conclude that water content, bed topography and stiffness are the major factors controlling microseismicity. Based on variable rupture mechanisms and spatiotemporal characteristics, we suggest the event-clusters relate to three end-member types of bed deformation: (1) continuous creation and seismogenic destruction of small-scale bed-roughness, (2) ploughed clasts and (3) flow-oblique deformation during landform-formation or along bedrock outcrops. This indicates that multiple processes, simultaneously active during glacial sliding, can accommodate stick-slip behaviour and that the bed continuously reorganizes.

Firn densification profiles are an important parameter for ice-sheet mass balance and palaeoclimate studies. One conventional method of investigating firn profiles is using seismic refraction surveys, but these are limited to point measurements. Distributed acoustic sensing (DAS) presents an opportunity for large-scale seismic measurements of firn with dense spatial sampling and easy deployment, especially when seismic noise is used. We study the feasibility of seismic noise interferometry on DAS data for characterizing the firn layer at the Rutford Ice Stream, West Antarctica. Dominant seismic energy appears to come from anthropogenic noise and shear-margin crevasses. The DAS cross-correlation interferometry yields a noisy Green’s function (Rayleigh waves). To overcome this, we present two strategies for cross-correlations: (1) hybrid instruments – correlating a geophone with DAS, and (2) selected stacking where the cross-correlation panels are picked in the tau-p domain. These approaches are validated with results derived from an active survey. Using the retrieved Rayleigh wave dispersion curve, we inverted for a high-resolution 1D S-wave velocity profile down to a depth of 100 m. The inversion spontaneously retrieves a “kink” (velocity gradient inflection) at ~12 m depth, resulting from a change of compaction mechanism. A triangular DAS array is used to investigate directional variation in velocity, which shows no evident variations thus suggesting a lack of deformation in the firn. Our results demonstrate the potential of using DAS and seismic noise interferometry to image the near-surface and present a new approach to derive S-velocity profiles from surface wave inversion in firn studies.

Icequakes, microseismic earthquakes at glaciers, offer critical insights into the dynamics of ice sheets. For the first time in the Antarctic, we explore the use of fibre optic cables as Distributed Acoustic Sensors (DAS) as a new approach for monitoring basal icequakes. Fibre was deployed on the ice surface at Rutford Ice Stream, in two different configurations. We compare the performance of DAS with a conventional geophone network for: microseismic detection and location; resolving source and noise spectra; source mechanism inversion; and measuring anisotropic shear-wave splitting parameters. The DAS arrays detect fewer events than the geophone array. However, DAS is superior to geophones for recording the microseism signal, suggesting the applicability of DAS for ambient noise interferometry. We also present the first full-waveform source mechanism inversions using DAS anywhere, successfully constraining the horizontal stick-slip nature of the icequakes. In addition, we develop an approach to use a 2D DAS array geometry as an effective multi-component sensor capable of accurately characterising shear-wave splitting due to anisotropy of the ice fabric. Although our observations originate from a glacial environment, the methodology and implications of this work are relevant for employing DAS in other microseismic environments.

The crystal orientation fabric of glacier ice severely impacts its strength and flow. Crystal fabric is therefore an important consideration when modelling ice flow. Here, we show that shear wave splitting (SWS) of glacial microseismicity can be used to invert for seismic anisotropy and ice fabric at Rutford Ice Stream (RIS). RIS is a fast-flowing Antarctic ice stream, a setting crucial for informing flow models. We present ~2000,000 SWS measurements from glacial microseismicity, registered at a 38-station seismic network located ~40 km upstream the grounding line. A representative subset of this data is inverted for ice fabric. Due to the character of SWS, which accumulates along the ray path, our method works best if additional information on the depth structure of the ice is available, which are radar measurements in our case. We find that the following three-layer model fits the data best: a broad vertical cone near the base of RIS (500 m thick), a thick vertical girdle, orientated perpendicular to flow, in the middle (1200 m thick) and a tilted cone fabric in the uppermost 400 m. Such a fabric causes a depth-dependent strength profile of the ice with the middle layer being ~3.5 times harder to deform along flow than across flow. At the same time, the middle layer is a factor ~16 softer to shear than to compression or extension along flow. If such a configuration is representative for fast-flowing ice streams, it would call for a more complex integration of viscosity in ice sheet models.

Basal microseisms in Antarctica, or icequakes, are valuable data sources that we can use to determine features and processes at the bed to improve our understanding of ice flow dynamics in the region. In the 2018/19 austral summer, we collaborated with the British Antarctic Survey (BAS) to deploy several seismic arrays of short period instruments over ~2 months in Rutford Ice Stream in West Antarctica to monitor natural source seismicity. During this recording period, we detected several swarms of repeating icequakes (~40 s interevent time) at our grounding line array that originate from a common basal source, which we hypothesize to be stick-slip motion over sticky spots/asperities. Smaller scale repeating icequakes, both in terms of amplitude and interevent times, also exist among the original larger repeating icequakes and are also hypothesized to originate from multiple smaller sticky spots that had less consistent loading and slipping. We built an auto-picker to detect these repeating icequakes over our recording period and located them using the automatic earthquake location Python package QuakeMigrate, and here we present our results as well as what they tell us about the basal topography. Further investigation of the interevent offsets between repeating signals of varying amplitudes and their frequency characteristics via FFT will provide more insights into the basal features, which we will corroborate with GPR basal topography data. Relations of the repeating icequakes to aseismic slip and tides will also be investigated. The findings at our grounding line array, where the repeating icequakes were first detected, can later support similar searches at the inland arrays. Antarctic ice streams remain a major source of uncertainty in projections of sea level rise, and our work seeks to constrain this uncertainty by improving our understanding of ice stream dynamics through basal conditions.

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 new, 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 tens to hundreds of cubic meters. Our method and findings provide a framework for studying a critical process, key for the stability of ice shelves and ice sheets, and hence rates of future sea-level rise.

Improving our understanding of glacial sliding is crucial for constraining basal drag in ice dynamics models. We use icequakes, sudden releases of seismic energy as the ice slides over the bed, to provide geophysical observations that can be used to aid understanding of the physics of glacial sliding and constrain ice dynamics models. These icequakes are located at the bed of an alpine glacier in Switzerland and the Rutford Ice Stream, West Antarctica, two extremes of glacial settings and spatial scales. We investigate a number of possible icequake source mechanisms by performing full waveform inversions to constrain the fundamental physics and stress release during an icequake stick-slip event. Results show that double-couple mechanisms best describe the source for the events from both glacial settings and the icequakes originate at or very near the ice-bed interface. We also present an exploratory method for attempting to measure the till shear modulus, if indirect, reflected icequake radiation is observed. The results of this study increase our understanding of how icequakes are associated with basal drag while also providing the foundation for a method of remotely measuring bed shear strength.

We use polarimetric radar sounding to investigate variation in ice crystal orientation fabric within the near-surface (top 40-300 m) of Rutford Ice Stream, West Antarctica. To assess the influence of the fabric on ice flow, we use an analytical model to derive anisotropic enhancements of the flow law from the fabric measurements. In the shallowest ice (40-100 m) the azimuthal fabric orientation is consistent with flow-induced development and correlates with the surface strain field. Notably, toward the ice-stream margins, both the horizontal compression angle and fabric orientation tend toward 45 degrees relative to ice flow. This result is consistent with theoretical predictions of flow-induced fabric under simple shear, but to our knowledge has never been observed. The fabric orientation in deeper ice (100-300 m) is significantly misaligned with shallower ice in some locations, and therefore inconsistent with the local surface strain field. This result represents a new challenge for ice flow models which typically infer basal properties from the surface conditions assuming simplified vertical variation of ice flow. Our technique retrieves azimuthal variations in fabric but is insensitive to vertical variation, and we therefore constrain the fabric and rheology within two end-members: a vertical girdle or a horizontal pole. Our hypotheses are that fabric near the center of the ice-stream tends to a vertical girdle that enhances horizontal compression, and near the ice-stream margins tends to a horizontal pole that enhances lateral shear.