09 Nov 2023
 | 09 Nov 2023
Status: this preprint is open for discussion.

Firn Seismic Anisotropy in the North East Greenland Ice Stream from Ambient Noise Surface Waves

Emma Pearce, Dimitri Zigone, Coen Hofstede, Andreas Fichtner, Joachim Rimpot, Sune Olander Rasmussen, Johannes Freitag, and Olaf Eisen

Abstract. We analyse ambient noise seismic data from 23 three-component seismic nodes to study firn velocity structure and seismic anisotropy near the EastGRIP camp along the Northeast Greenland Ice Stream (NEGIS). Using 9-component correlation tensors, we derive dispersion curves of Rayleigh and Love wave group velocities from 3 Hz to 40 Hz. These velocity distributions exhibit anisotropy along and across the flow. To assess these variations, we invert dispersion curves for shear wave velocities (Vsh and Vsv) in the top 150 m of NEGIS using a Markov Chain Monte Carlo approach. The reconstructed1-D shear velocity model reveals radial anisotropy in the firn, with Vsh 12 %–15 % greater than Vsv, peaking at the critical density (550 kg m–3). We combine density data from firn cores drilled in 2016 and 2018 to create a new density parameterisation for NEGIS, serving as a reference for our results. We link seismic anisotropy in the NEGIS to effective and intrinsic causes. Seasonal densification, wind crusts, and melt layers induce effective anisotropy, leading to faster Vsh waves. Changes in firn recrystalisation cause intrinsic anisotropy, altering the Vsv to Vsh ratio. We observe a shallower firn-ice transition across flow (≈ 50 m) compared to along flow (≈ 60 m), suggesting increased firn compaction due to the predominant wind direction and increased deformation towards the shear margin. We demonstrate that short-duration (nine-day minimum), passive, seismic deployments, and noise-based analysis can determine seismic anisotropy in firn, and reveal 2-D firn structure and variability.

Emma Pearce et al.

Status: open (until 21 Dec 2023)

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  • CC1: 'Comment on egusphere-2023-2192', Hanbing Ai, 03 Dec 2023 reply
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Emma Pearce et al.

Emma Pearce et al.


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Short summary
Our seismic study near EastGRIP camp in Greenland reveals varying firn properties by direction, crucial for studying ice stream stability, structure, surface mass balance and past climate conditions. We used dispersion curve analysis of Love and Rayleigh waves to show firn is non-uniform along and across flow of an ice stream due to wind patterns, seasonal variability and the proximity to the edge of the ice stream. This approach better informs firn structure advancing ice stream understanding.