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

Abstract

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...

Distributed Acoustic Sensing (DAS) for Natural Microseismicity Studies: A Case Study From Antarctica

“…All P-phase polarities are correctly fitted by the geophones, as are the majority of S-phase polarities, although there is more uncertainty for the S-wave matches due to some of the shear wave splitting not being entirely compensated for. Unfortunately, geophone coverage of the focal sphere for the network configuration is not as well configured for source mechanism analysis as previous studies (Hudson et al, 2020;Kufner et al, 2021;Smith et al, 2015), leaving the most likely DC source poorly constrained compared to previous observations. While the slip vector is approximately the direction of ice flow, and therefore in agreement with the DAS source inversion, the uncertainty, denoted by the dashed lines, indicates ~270° azimuthal uncertainty.…”

“…This effectively acts as a moving average measurement of strain over 10 m of fiber, in 1 m increments, with the maximum resolvable frequency signals for ice therefore being ~200 Hz and ~90 Hz, for P-and S-waves with velocities of 3841 m s -1 and 1970 m s -1 , respectively. These frequencies are greater than typical observed icequake corner frequencies at Rutford Ice Stream (Hudson et al, 2020;Kufner et al, 2021;Smith et al, 2015). The seismic data were acquired in January 2020, during the austral summer.…”

Distributed Acoustic Sensing (DAS) for natural microseismicity studies: A case study from Antarctica

“…This effectively acts as a moving average measurement of strain over 10 m of fiber, in 1 m increments, with the maximum resolvable frequency signals for ice therefore being ∼200 and ∼90 Hz, for P-and S-waves with velocities of 3,841 and 1,970 m s −1 , respectively. These frequencies are greater than typical observed icequake corner frequencies at Rutford Ice Stream (Hudson et al, 2020;Kufner et al, 2021;Smith et al, 2015).…”

“…The source inversion method is a full-waveform Bayesian source mechanism inversion, randomly sampling the model space millions of times in order to obtain an estimate of the posterior probability distribution. We constrain the source model to be a Double-Couple (DC) model, which is appropriate for the predominantly stick-slip seismicity observed at Rutford Ice Stream (Hudson et al, 2020;Kufner et al, 2021;Smith et al, 2015). The DAS source inversion workflow is as follows:…”

“…A spatial detection bias is also apparent in the data, with icequakes from clusters beyond the SW end of the linear DAS configuration detected, but icequakes at closer offsets but perpendicular to the cable not detected. This sensitivity is likely a result of the single component nature of the DAS making it insensitive to certain S-phase polarizations, for the basal icequake radiation pattern orientations observed at Rutford Ice Stream (Hudson et al, 2020;Kufner et al, 2021;Smith et al, 2015).…”