IOWA STATE UNIVERSITY
Grant: $299,419 - National Science Foundation - Jul. 27, 2009
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Award Description: Intellectual Merit The temporal and spatial variability of glacier sliding is responsible for the most dynamic behavior of ice masses. Microseismicity measured on glacier surfaces contains information about basal movement, including slip velocity and displacement, slip areas, and basal shear stresses. However, extracting this information from seismicity is highly uncertain due to a lack of direct measurements at glacier beds that can be used to guide seismological inversions. Without such guidance, the seismological methods used extensively to characterize slip along crustal faults will continue to be underutilized for revealing characteristics of slip beneath modern ice masses. We propose to locally induce rapid glacier slip, measure it directly at the bed, and study the seismic expression of that slip. This work can be accomplished only at the Svartisen Ice Cap in Norway, where tunnels in subglacial rock provide unusual access to the bed of a thick sliding glacier. Previous work there has indicated that rock friction locally dominates resistance to basal slip, making parts of the bed especially prone to stick-slip movement and resultant microseismicity. This prior work has also demonstrated important experimental capabilities: basal motion and water pressure can be measured continuously at multiple locations, shear tractions on the bed can be measured locally, seismicity can be measured subglacially, and basal water pressure can be increased during pump tests that perturb 10-50 m2 of the bed. Two sets of measurements will be made and repeated the following year. In April prior to significant melting on the glacier surface, pump tests will be conducted to bring the basal water pressure above the ice-overburden pressure, inducing local slip. Resultant seismicity will be measured at multiple locations, both at the glacier surface and in tunnels within rock beneath the glacier. Hypocenter location, slip kinematics, and basal shear stresses inferred from surface seismicity will be compared with direct subglacial measurements and with seismic data gathered subglacially to test and calibrate methods of seismological inversion. During May and June, natural seismicity will be monitored and interpreted, as fluctuating water input to the bed causes variations in water pressure and storage. Results will help optimize the use of microseismicity for studying basal movement remotely over large areas of glacier beds and will be relevant to the study of all sliding ice masses, regardless of their bed type, size, or location. Broader Impacts Adequately parameterizing basal motion of wet-based portions of ice sheets is arguably the single largest impediment to developing predictive ice-sheet models. By improving techniques for inferring glacier-slip characteristics from microseismicity, this study could benefit such models and thereby contribute to climate and sea-level research. Also, by characterizing fault slip through both direct measurements and inversions of seismic data gathered above and below the fault surface, our project could positively influence the broader field of crustal seismology. 69% of the project’s direct costs will provide salaries, tuition, benefits, and research support for students and recent Ph.D. recipients who are beginning their careers. In addition, as has been shown through media coverage of our past research beneath the Svartisen Ice Cap, this project can serve as a platform for outreach that emphasizes to nonscientists both the role of glaciers in Earth’s past and modern environment and the role of NSF in supporting basic research.
Project Description: Intellectual Merit The temporal and spatial variability of glacier sliding is responsible for the most dynamic behavior of ice masses. Microseismicity measured on glacier surfaces contains information about basal movement, including slip velocity and displacement, slip areas, and basal shear stresses. However, extracting this information from seismicity is highly uncertain due to a lack of direct measurements at glacier beds that can be used to guide seismological inversions. Without such guidance, the seismological methods used extensively to characterize slip along crustal faults will continue to be underutilized for revealing characteristics of slip beneath modern ice masses. We propose to locally induce rapid glacier slip, measure it directly at the bed, and study the seismic expression of that slip. This work can be accomplished only at the Svartisen Ice Cap in Norway, where tunnels in subglacial rock provide unusual access to the bed of a thick sliding glacier. Previous work there has indicated that rock friction locally dominates resistance to basal slip, making parts of the bed especially prone to stick-slip movement and resultant microseismicity. This prior work has also demonstrated important experimental capabilities: basal motion and water pressure can be measured continuously at multiple locations, shear tractions on the bed can be measured locally, seismicity can be measured subglacially, and basal water pressure can be increased during pump tests that perturb 10-50 m2 of the bed. Two sets of measurements will be made and repeated the following year. In April prior to significant melting on the glacier surface, pump tests will be conducted to bring the basal water pressure above the ice-overburden pressure, inducing local slip. Resultant seismicity will be measured at multiple locations, both at the glacier surface and in tunnels within rock beneath the glacier. Hypocenter location, slip kinematics, and basal shear stresses inferre
Jobs Summary: Physical Scientists (19-2000): Work Period - 08/01/2009 to 09/30/2009 Hours Worked - 84 FTEs - 0.26 (Total jobs reported: 0)
Project Status: Less Than 50% Completed
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