| Summary: | A major unknown in the estimation of the effects of climate warming is the rate of sea level rise. This is believed to be significantly impacted by the transfer of mass from ice sheets and glaciers to the ocean, both as melt water runoff and as ice bergs. 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. The PIs 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. At this facility, important experimental capabilities exist. 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.
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