Examples of seismic signals from glaciers

Low frequency seismic sources in Greenland

Remote detection of major glacial movements is now possible using remote seismic monitoring techniques. The signals are probably due to significant basal slip events on large glaciers. They are very low frequency and easily missed by detection algorithms designed to find normal earthquakes. The figure below shows the locations of seismic sources in Greenland. They correspond to large glaciers flowing into the ocean. However, many such large glaciers exist that do not show this kind of seismicity - there is still much to learn!

Fig 01. Locations of low frequency seismic sources in Greenland (Ekstrom, 2006)

Detection of low frequency glaciogenic seismic events in the southern hemisphere is even more difficult. There are much fewer permanent recording stations in Antarctica and data from temporary deployments is essential in identifying events and locating the source of the seismicity. Data from the TAMSEIS temporary deployment, designed primarily to investigate seismic structure across the Transantarctic Mountains, showed unidentified events. Further analysis strongly suggested that these were due to basal slippage of the Whillans Ice Stream (B) between Siple Dome and the Transantarctic Mountains (shown by a red star in the diagram below).

Fig 02. The TAMSEIS deployment and location of the low frequency seismic signals (red star) in the vicinity of the Whillans Ice Stream

The low frequency signals are detected by filtering the seismic records between 50-100 seconds and stacking the records. An unexpected arrival of energy (later found to be a glacial signal) can be seen in the plot below, along with incoming energy from earthquakes in the Kurile Islands and Indonesia.

Fig. 03. Unidentified low frequency energy recorded on the stations of the TAMSEIS deployment.The energy was later found to have a source at the Whillans Ice Stream (B) in West Antarctica.

Fig 04. Glaciogenic seismic waves from a basal slip event in the Whillans Ice Stream (B). Two slip events occurred, separated by about 5 minutes. The energy is very low frequency, and hard to detect by conventional means.

The energy also radiates horizontally from the glacier, so does not couple well to the Earth beneath. This means that Antarctic seismic stations are very important in detecting motion in this way, and hence monitoring the Whillans, and possibly other important ice-streams that drain the great Antarctic ice sheets.