Bering Glacier Seismic Study

This is a multi-disciplinary project to study 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 related to erosion rates. Yet, basal motion is difficult to assess, in models as well as from observations. A recent deployment of broadband seismometers near the Bering Glacier, Alaska, offers an unprecedented opportunity to monitor basal processes with modern broadband seismology. These data have been supplemented by our deployment of five glacier stations of continuously recording GPS and in-ice seismometers. The purpose of the GPS deployment is to derive a 3-D ice velocity timeseries that will be correlated with the seismology records. The in-ice seismometers will help control depth and locations of glacial events.

With the GPS and in-ice seismometers laid out in an array, it may be possible 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 changes 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 will 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.

Nearby broadband seismic data come from the recent deployment of the STEEP (St. Elias Erosion/tectonics Project) seismic network, which 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, our Bering Glacier study leverages significant new and unanticipated results from the seismic data. Algorithms are being developed by Shad O'Neel to automatically detect and locate glacial seismic sources. These sources will be characterized according to origin and relationship to motion events. Ice dynamical models will be used to interpret the measured surface velocities and help find basal velocities; these velocities will be compared with seismic moment vectors.

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 erosion is essential to the STEEP activity. On the other hand, the seismic data collected as part of STEEP establishes a high quality data set that is unprecedented in glaciology and that will be greatly enhanced through the use of continuous GPS to track ice motion. This work will form the core of a PhD thesis (Laura LeBlanc). The GPS data will be made public through the UNAVCO data archive, and the seismic data will be made public through the PASSCAL data archive.

In April of 2007, we established the 5 glacier stations at the coordinates listed below. These sites all have a seismometer (3 axis L22 recorded by a Q330), and are arraigned in a cross pattern array with 2 km spacing, and all but the center site are collocated with a continuous GPS. The first two sites listed also have a Guralp 6TD instrument alongside the L22.

One additional site is located up glacier, near the "throat" between the Bagley Ice Valley and the Bering Glacier. This site is a continuous GPS station without a seismometer, and is located at 60° 32'40.3"-142° 26'36.4". All the glacier GPS sites are referenced to a continuous GPS basestation located near the Bering Glacier Base Camp at 60° 7'13.5" -143° 17'2.8".

In July of 2006 we had two GPS receivers on the Bering Glacier near the sites listed above, and measured ice velocities greater than 3.2 m/day. With this amount of motion, we hope for lots of icequake activity!

This project is supported by the NSF Geophysics program (EAR-0607872). GPS equipment is provided by UNAVCO. Seismic equipment is provided by PASSCAL. Logistical support for the fieldwork is provided by NSF Arctic Logistics (http://www.vecopolar.com/).