HR: 09:15h
AN: C21A-06    [PDF]
TI: Glacial earthquakes
AU: * Ekstr\"om, G
EM: ekstrom@eps.harvard.edu
AF: Dept of Earth and Planetary Sciences, Harvard University, 20 Oxford St, Cambridge, MA 02138
AU: Nettles, M
EM: nettles@eps.harvard.edu
AF: Dept of Earth and Planetary Sciences, Harvard University, 20 Oxford St, Cambridge, MA 02138
AU: Abers, G A
EM: abers@bu.edu
AF: Dept of Earth Sciences, Boston University, 685 Commonwealth Av, Boston, MA 02215
AB: We have identified a new class of moderate earthquakes (seismic magnitude around 5) that occur beneath glaciers. The previously unknown glacial earthquakes generate long-period (20--60~sec) seismic surface waves that are well recorded on globally distributed seismic stations, but which have previously gone undetected because they do not generate the high-frequency seismic waves on which traditional earthquake detection and location methodologies are based. A glacial earthquake on September 4, 1999, beneath the Dall glacier (Denali range, Alaska) was well recorded by the BEAAR temporary broad-band seismic network. Inverse modeling of the regional long-period seismograms shows that the seismic waves are inconsistent with tectonic faulting beneath the glacier, but instead consistent with sudden and massive glacial sliding. Using the centroid-single-force formalism developed by Kawakatsu (1989) for analysis of landslide-generated seismic waves, we obtain an estimate of the product of sliding mass and sliding distance. The value of this parameter for the Dall glacier event is $1.3 \times 10^{14}$~kg-m, consistent with the displacement of 10~km$^{3}$ of ice by 13 meters, though we cannot constrain the mass and distance independently. The duration of sliding was approximately 40 seconds. By analysis of data from the Global Seismographic Network for 1999--2001, we have detected and located forty-two glacial earthquakes of similar magnitude beneath the Greenland ice sheet. Preliminary analysis of the five best-recorded earthquakes indicates that these, too, are consistent with glacial sliding. We speculate that the dynamics of these earthquakes are controlled by stick-slip motion of the glacier along its basal surface, and that the phenomenon involves the displacement of a large mass over a relatively short distance. Microearthquakes occur in association with glaciers, and some have been linked to sliding motion at the base, and in particular to motion on so-called sticky spots. Alternatively, rapid variations in pore pressure at the glacier base or the non-linear weakening of a deforming till may generate conditions for rapid lowering of the effective friction, growth of a slipping patch, and sudden large-scale motion of the glacier.
DE: 1827 Glaciology (1863)
SC: Cryosphere [C]
MN: 2003 Fall Meeting