Abstract
AbstractFirn is the pervasive surface material across Antarctica, and its structures reflect its formation and history in response to environmental perturbations. In addition to the role of firn in thermally isolating underlying glacial ice, it defines near-surface elastic and density structure and strongly influences high-frequency (> 5 Hz) seismic phenomena observed near the surface. We investigate high-frequency seismic data collected with an array of seismographs deployed on the West Antarctic Ice Sheet (WAIS) near WAIS Divide camp in January 2019. Cross-correlations of anthropogenic noise originating from the approximately 5 km-distant camp were constructed using a 1 km-diameter circular array of 22 seismographs. We distinguish three Rayleigh (elastic surface) wave modes at frequencies up to 50 Hz that exhibit systematic spatially varying particle motion characteristics. The horizontal-to-vertical ratio for the second mode shows a spatial pattern of peak frequencies that matches particle motion transitions for both the fundamental and second Rayleigh modes. This pattern is further evident in the appearance of narrow band spectral peaks. We find that shallow lateral structural variations are consistent with these observations, and model spectral peaks as Rayleigh wave amplifications within similarly scaled shallow basin-like structures delineated by the strong velocity and density gradients typical of Antarctic firn.
Highlights
Firn is present across approximately 99% of glaciated Antarctica
We study the ambient seismic excitation of Antarctic firn due to wind and anthropogenic sources using a passive threecomponent nodal seismograph array deployed near West Antarctic Ice Sheet (WAIS) Divide camp, Antarctica
We studied high-frequency (> 5 Hz) ambient seismic wavefield phenomena in Antarctic firn recorded by a 1 km circular array of 22 nodal seismometers deployed in the vicinity of WAIS Divide camp in January 2019
Summary
Firn is present across approximately 99% of glaciated Antarctica (van den Broeke, 2008; Ligtenberg and others, 2011). Classic estimations of long period surface wave basin response based on energy flux conservation typically rely on comparisons with hard rock benchmarks (e.g. Tromp and Dahlen, 1992) to produce relative measures of amplification This approach has been shown to be valid for short periods (Bowden and Tsai, 2017) and well-defined basin edges (Brissaud and others, 2020), more complete predictions of surface wave amplifications, for 3D structures, require full wavefield numerical methods (Komatitsch and others, 2004). We propose a revised interpretation of ambient absolute spectral amplifications recently documented by Chaput and others (2018) characterized in a microbasin resonance framework Such modes, processed at isolated geographically widespread stations, can constrain regional impacts of near-surface atmospheric wind, melt and temperature forcing on environmentally vulnerable cryospheric media. Increased understanding of these seismological phenomena more generally advances prospects for single and multi-station methods for inferring structures and monitoring temporal changes in cryospheric media
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