Tidally modulated seismicity at a major outlet glacier in Greenland

Abstract:

The outlet glaciers of the Greenland Ice Sheet are the source of observable seismic phenomena over a broad range of frequencies. We have found the globally observable, long-period events known as glacial earthquakes to have a close link to calving-front behaviour, occurring during calving-front retreat at glaciers with grounded or near-grounded termini. While glaciers in other areas are known to be an abundant source of high-frequency seismicity, much less is known about the nature of high-frequency seismicity emanating from outlet glaciers in Greenland. We consider the high-frequency seismicity of Helheim Glacier, a major outlet glacier on the East Coast of Greenland, by analyzing data recorded on a temporary network of six 3-component seismometers sampling at 100 sps. This network was deployed at bedrock sites surrounding Helheim Glacier for approximately seven weeks during the summer of 2009. We observe the seismograms from this network to be rich in high-frequency signals, and we identify thousands of seismic events by analysis of the envelope functions of the seismograms. We find that the occurrence of these events is tidally modulated, and we examine the relationship between this tidal modulation and previous geodetic observations of tidal forcing at outlet glaciers in Greenland. We infer that the high-frequency signals we observe are the result of brittle processes occurring in a localized source region near the glacier calving front. Continuing analysis of such high-frequency seismicity will lead to a better understanding of the full-range of glaciogenic seismicity and to understanding of calving process and glacier dynamics, including the link between the calving process and external forcings.