Glacier Seismicity and High Resolution Motion Records: Relation to Glacier Erosion

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This is a multi-disciplinary study of basal processes on a large mountain glacier. Glaciers are among the most effective agents of erosion. There are indications that glacier basal motion is directly (maybe linearly) related to erosion rates. Yet, basal motion is difficult to assess, in models as well as from observations. A recent deployment of seismometers near the Bering Glacier, Alaska, is offering an unprecedented opportunity to monitor basal processes with modern broadband seismology. Data from these seismometers is being used to look for events of glacial origin, and in particular basal sources. These data are supplemented by five glacier stations of continuously recording GPS and an in-ice seismometer. The GPS are recording an ice velocity timeseries in 3-d that will be correlated with the seismic records. The GPS are laid out in an array just below the equilibrium line on the main trunk of the Bering Glacier where it drains from the Bagley Ice Valley. The in-ice seismometer is co-located with the GPS station at the center of the GPS array. This sensor is frozen into the surface of the glacier, in a borehole several meters deep, made using a steam drill. With the GPS laid out in an array, the experiment is designed to detect whether discrete slip patches, whole bed sliding events, or increasing ice-bed stresses due to the growth of basal cavities lead to glacial seismic events. Each of these mechanisms may contribute to different mechanisms of erosion at the base. Moreover, the combined seismic and ice velocity data may allow observation of glacial quarrying. Broadband seismometers also allow some determination of the role of fluids at the bed, possibly manifest as harmonic tremor. The expected outcome will be a powerful new data record that may be used to derive improved models of basal motion.The seismic data come from the summer 2005 deployment of the STEEP (St. Elias Erosion/tectonics Project, funded by Continental Dynamics) seismic array, which has surrounded the Bering Glacier with an IRIS/PASSCAL seismic network. The basic focus of STEEP is to explore the links between erosion and tectonic processes, and the seismic network was built to study tectonics in the St. Elias. By analyzing glacier seismicity within this network, the glacier seismic study is leveraging new results from the seismic data. Scientifically, the broader impacts of the proposed work are through its direct link to the large multi-disciplinary STEEP activity. The synergies between these projects are large, because an understanding of glacier basal processes is essential. The seismic data establishes a high quality data set that is unprecedented in glaciology and that is enhanced by the GPS data. This project is funded by Geophysics and the EAR Geomorphology and Land-Use Dynamics Program.