Collaborative Research: Seismic Investigation of the Sub-ice Environment and Crustal Composition of Antarctica

Part I: Nontechnical The geology beneath the Antarctic Ice Sheet sets up a key condition to its moving glacial systems: a weak, porous, and possibly water bearing sedimentary layer provides a slippery bed, whereas hard basement rock at the contact may reduce the ice flow. Knowing the geology, however, has been extremely difficult since the continent is remote and the ice cover is thick. Using seismic stations deployed on top of the ice sheet provides an alternative tool to see the continent through the ice, as seismic signal they record contains sensitive information about the base of the ice-sheet and the geology of the crust below. For example, a thick and porous sedimentary layer will cause the seismic waves to travel more slowly. In this project, the investigators will 1) utilize the seismic data collected in the past 18 years at more than 200 sites in Antarctica, 2) perform a suite of modern seismic data analysis tools to produce a variety of seismic signals, and 3) translate them into sets of maps about the physical properties (such as the speed of the seismic wave) and chemical composition (such as silica content) of the underlying crust. These seismic speed maps, with accuracy quantified by uncertainty measurements, provide a first-order assessment to the distribution of the sub-ice sedimentary layer. Additional information such as other crustal properties and inferred chemical composition beneath these sites will further shed light on the geological evolution of the continent buried by the thick ice sheet. Notably, determining the silica content of the crust will help produce better estimates of the thermal profiles of the sub-ice crust. Part II: Technical Description The underlying crust of Antarctica Ice Sheet holds strong importance from perspectives of both cryosphere and lithosphere: at local and regional scale, the shallow physical properties at the base of the Antarctic Ice Sheet sets an important boundary condition for the dynamics of the fast moving ice sheet; at a greater scale, the architecture (thickness and Poissons ratio) of the crust that varies across the continent retains the signature of the past geological evolution and its chemical composition contributes significantly to the geothermal structure. However, these properties have not been systematically studied due to both the remoteness of the continent and its thick ice-coverage. In this proposed study, the investigators will analyze 18 years of seismic records from more than 200 permanent and portable seismic stations, to constrain the sub-ice seismic structure, crustal architecture, and draw implications on the chemical composition of the bulk crust using multiple seismic techniques. Their analysis will include: 1) systematic measurement of the broad-band Rayleigh wave horizontal-vertical ratios (H/V) from both ambient noise and earthquakes; 2) analysis of high frequency receiver function waveforms and 2-layer h-k stacking to provide information on Poissons ratio and the depth of sub-ice discontinuities; 3) measurement of Love wave dispersion and inversion for radial anisotropy; 4) measurement of Rayleigh wave azimuthal anisotropy from both ambient noise and earthquakes; 5) Joint interpretation of Rayleigh wave dispersion, H/V ratios, and receiver function waveforms for detailed sub-ice isotropic structure; 6) a Monte Carlo transversely isotropic (TTI) modeling of all seismic anisotropic measurements. These analyses will be combined and synthesized to produce a set of seismic deliverables including the determination of a map of thickness of the sub-ice sediments with hierarchical-resolution, more accurate crustal thickness and Poissons ratio determination, and quantification of crustal composition. The proposed work will meet two major scientific objectives. First, it will provide detailed information about the bottom contact of the Antarctic ice-sheet and sediment thickness at localities of seismic stations, improving the boundary conditions for the ice-sheet modeling and determining the locations of major sedimentary basins within Antarctica. Second, constraints on the thickness and composition of the Antarctic crust will improve the understanding of the geology history of the continent. Notably, the crustal architecture and composition will further facilitate future geological investigation to various tectonic processes that have shaped Antarctica, such as rifting, lithospheric removal, orogeny, and mountain building. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.