Mass transfer from the ice sheet interior to the oceans is dominated by ice stream flow (Rignot et al., 2011), which, in turn, is governed by deformation within the ice, and friction and deformation at the bed, that is, the contact between ice and underlying sediments or bedrock. Furthermore, tidally induced modulations influence the flow dynamics of some ice streams, likely by introducing pressure modulation at the bed (Anandakrishnan et al., 2003; Gudmundsson, 2006). Poorly constrained processes and conditions at ice stream beds, therefore, contribute to the uncertainty in sea-level rise projections. Better understanding of the dynamic response of ice streams to a warming climate and oceans therefore requires improved models of these basal processes and the spatial variation in properties. Here, we focus on the understanding of basal sliding and deformation characteristics through the analysis of naturally occurring micro-earthquakes at the ice-bed interface. These events are used to examine the nature of basal slip, tidal influences, and spatial and temporal variations. The beds of ice streams consist of bedrock and sediment, often known as till. Till stiffness is variable and depends on the dynamic conditions and material properties. Ice flow at the bed is then facilitated by a combination of slip over a hard bed and by slip and deformation within a soft bed. Fluids further modulate basal ice stream flow. Where bedrock is exposed or subglacial till has relatively low permeability and is of Abstract 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 230,000 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 lineations. Within these regions, micro-earthquakes occur in spatially (<100 m radius) and temporally (mostly 1-5 days activity) restricted event-clusters (up to 4,000 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...

Three holes were drilled to the bed of Rutford Ice Stream, through ice up to 2154 m thick, to investigate the basal processes and conditions associated with fast ice flow and the glacial history of the West Antarctic Ice Sheet. A narrative of the drilling, measuring and sampling activities, as well as some preliminary results and initial interpretations of subglacial conditions, is given. These were the deepest subglacial access holes ever drilled using the hot-water drilling method. Samples of bed and englacial sediments were recovered, and a number of instruments were installed in the ice column and the bed. The ice–bed interface was found to be unfrozen, with an existing, well-developed subglacial hydrological system at high pressure, within ~1% of the ice overburden. The bed itself comprises soft, water-saturated sediments, consistent with previous geophysical interpretations. Englacial sediment quantity varies significantly between two locations ~2 km apart, and possibly over even shorter (~20 m) distances. Difficulties and unusual observations encountered while connecting to the subglacial hydrological system in one hole possibly resulted from the presence of a large clast embedded in the bottom of the ice.

We compared elastic moduli in polar firn derived from diving wave refraction seismic velocity analysis, firn-core density measurements and microstructure modelling based on firn-core data. The seismic data were obtained with a small electrodynamic vibrator source near Kohnen Station, East Antarctica. The analysis of diving waves resulted in velocity–depth profiles for different wave types (P-, SH- and SV-waves). Dynamic elastic moduli of firn were derived by combining P- and S-wave velocities and densities obtained from firn-core measurements. The structural finite-element method (FEM) was used to calculate the components of the elastic tensor from firn microstructure derived from X-ray tomography of firn-core samples at depths of 10, 42, 71 and 99 m, providing static elastic moduli. Shear and bulk moduli range from 0.39 to 2.42 GPa and 0.68 to 2.42 GPa, respectively. The elastic moduli from seismic observations and the structural FEM agree within 8.5% for the deepest achieved values at a depth of 71 m, and are within the uncertainty range. Our observations demonstrate that the elastic moduli of the firn can be consistently obtained from two independent methods which are based on dynamic (seismic) and static (tomography and FEM) observations, respectively, for deeper layers in the firn below ~10 m depth.

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Mass transfer from the ice sheet interior to the oceans is dominated by ice stream flow (Rignot et al., 2011), which, in turn, is governed by deformation within the ice, and friction and deformation at the bed, that is, the contact between ice and underlying sediments or bedrock. Furthermore, tidally induced modulations influence the flow dynamics of some ice streams, likely by introducing pressure modulation at the bed (Anandakrishnan et al., 2003;Gudmundsson, 2006). Poorly constrained processes and conditions at ice stream beds, therefore, contribute to the uncertainty in sea-level rise projections. Better understanding of the dynamic response of ice streams to a warming climate and oceans therefore requires improved models of these basal processes and the spatial variation in properties. Here, we focus on the understanding of basal sliding and deformation characteristics through the analysis of naturally occurring micro-earthquakes at the ice-bed interface. These events are used to examine the nature of basal slip, tidal influences, and spatial and temporal variations.The beds of ice streams consist of bedrock and sediment, often known as till. Till stiffness is variable and depends on the dynamic conditions and material properties. Ice flow at the bed is then facilitated by a combination of slip over a hard bed and by slip and deformation within a soft bed. Fluids further modulate basal ice stream flow. Where bedrock is exposed or subglacial till has relatively low permeability and is of Abstract 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 230,000 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 lineations. Within these regions, micro-earthquakes occur in spatially (<100 m radius) and temporally (mostly 1-5 days activity) restricted event-clusters (up to 4,000 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) ...