High Frequency Seismic Waves Recorded By the Greenland Ice Sheet Monitoring Network (GLISN) During the Drainage of a Supraglacial Lake

Abstract:

Supraglacial lake drainage is a major source of subglacial water under the Greenland Ice Sheet, impacting the ice dynamics at different temporal and spatial scales. Previous studies have shown that fast drainage of a supraglacial lake can produce seismic waves that are detected by local seismometers; however, little work has been done on the regional detection of such waves. Here we present the results of a study focusing on seismic data and their potential linkage to the drainage of a supraglacial lake (Lake Ponting) in the Paakitsoq region of the West Greenland ice sheet. The formation and subsequent drainage of this lake were documented in the summer of 2011 by a multidisciplinary team of researchers (Tedesco, et al., 2013). The present study used all available high frequency data from the Greenland Ice Sheet Monitoring Network (GLISN) for one-half hour before and one hour after the onset of sudden drainage on June 19, 2011. Arrival times were picked manually, using a minimum amplitude of twice the noise level. The data were then plotted on a time-distance graph, using the location of Lake Ponting as the origin of the distance axis. Four linear trends are seen on the graph. Least-squares fitting of these linear trends show that two of the lines have apparent velocities of around 1,000 m/s. These two lines are approximately parallel, do not emanate from the time of lake drainage, and may be unrelated to it. The other two lines, however, do intersect the time axis at approximately the onset of the drainage. Based on this, we believe that these latter lines result from seismic waves associated with the drainage. The velocities derived from the curve fits for these lines are 378 m/s and 292 m/s. These velocities are too slow for the waves to be traveling through either the rock or the solid ice. Our current hypothesis is that they are traveling in a low-velocity channel of till underneath the ice. This would be consistent with the low attenuation required for the propagation of high frequency energy over regional distances. Further analysis, and the implications of these waves for the study of glacial processes, will be presented.