Ice Quakes

Most earthquakes last for only seconds, and because they involve the release of a lot of energy over a such short period of time, they can be easily detected by standard seismic methods. If the same amount of energy is released over a longer period (minutes, hours, or days), then the earthquake will not be detectable at the frequencies that are monitored at most seismic stations. For example, slip events on the deeper parts of subduction zones can last for several days and do not have typical seismic signatures. However, as shown but Rogers and Dragert (2003) for the Cascadia Subduction Zone they do produce long period vibrations (or seismic tremor).

Göran Ekström and colleagues from Harvard and Boston Universities have carried out a detailed analysis of long-period seismic data from about 100 seismometers distributed around the world (Ekström et al. 2003). Using a computer-intensive procedure to optimize the alignment of signals, they have been able to narrow down the source

Top: A series of long-period seismic records

Middle: time adjusted records, based on distance

Bottom: resulting cumulative seismic record

(from Ekström et al., 2003)

locations of over 7000 seismic events. Of these, all but 521 are interpreted to be long-period vibrations associated with known earthquakes, and of the 521 all but 71 are located in seismically active areas (eg. at plate boundaries). 46 of the 71 unexplained events are located in glaciated areas - most of them in Greenland (42), with a few in Antarctica (3) and Alaska (1).

The Alaska event (M 5.0, Dall Glacier area, September 4, 1999, 30 to 60 sec. duration) occurred in an area with a comprehensive seismic network. Close inspection shows that it did produce a very weak signal on the broad-band seismometer (see figure below), and a strong signal on the long-period instrument at the same location. A model of the sliding mechanism of the Alaska event shows that it is parallel to the interpreted direction of movement the Dall Glacier. Similar models for some of the Greenland events, most of which are situated close the edge of the ice sheet, also show mechanisms that are consistent with presumed ice-movement directions.

Ekström et al. conclude that these 46 long-period seismic events are related to the stick and slip nature of glacial ice movement. They note that most of the Greenland events occurred from April to December, and speculate that this is because surface melting during the summer months leads to increased water pressure at the ice-rock interface, and thus to period slip.

Ekström G, Nettles M and Abers A, 2003, Glacial earthquakes, Science, V. 302, p. 622-624 (October 2003)

Rogers G and Dragert H, 2003, Episodic tremor and slip on the Cascadia subduction zone: the chatter of silent slip, Science, V. 300, p. 1942-43.