HR: 0800h
AN: C51B-0284    [Abstracts]
TI: Stress Re-Distribution in a Transverse Cross-Section of Black Rapids Glacier, Alaska
AU: * Amundson, J M
EM: amundson@gi.alaska.edu
AF: Geophysical Institute, 903 Koyukuk Drive, Fairbanks, AK 99775-7320 United States
AU: Truffer, M
EM: truffer@gi.alaska.edu
AF: Geophysical Institute, 903 Koyukuk Drive, Fairbanks, AK 99775-7320 United States
AU: Lüthi, M P
EM: luthi@gi.alaska.edu
AF: Geophysical Institute, 903 Koyukuk Drive, Fairbanks, AK 99775-7320 United States
AB: We implement a finite element model that incorporates a simple inverse approach to model the mean seasonal stress distribution in a transverse cross-section of Black Rapids Glacier, Alaska, during summer 2002 and winter 2002-03. The model iteratively searches for a basal velocity function that minimizes the percentage error between modeled and measured surface velocities and ``tilt angles''. Tilt angles are described by tilt readings from tiltmeters installed in the ice, and are modeled by extracting model strain rates and generating synthetic tilt curves. The synthetic tilt curves are then subtracted from the measured tilt angles to obtain tilt deviation, which can be treated as a proxy for short term variations in strain rate. Our model results show that elevated surface velocities over a variety of time scales are achieved by a decrease in basal shear stresses a few hundred meters north of the glacier centerline (orographic left), which is accommodated by an increase in basal shear stresses south of the centerline (orographic right). This picture of glacier flow is consistent with a previous seismic study on Black Rapids Glacier, in which a weak till layer (up to 7 m thick) was first discovered in our region of depressed shear stresses, and with the presence of several marginal lakes on the glacier's northern margin. We suggest that these marginal lakes drain primarily through the northern half of the glacier, causing the till there to weaken at moderate to high subglacial water pressure and to decouple from the ice at higher pressures. This results in a local reduction in basal shear stresses that must be accommodated elsewhere if the glacier is to remain at equilibrium.
DE: 0720 Glaciers
DE: 0776 Glaciology (1621, 1827, 1863)
DE: 0798 Modeling
SC: Cryosphere [C]
MN: Fall Meeting 2005