Increased bump and grind beneath Greenland’s ice-sheets

 

In 2003 earth scientists from Harvard and Boston Universities reported that they had detected seismic signals from movement of glaciers in Alaska and Greenland (Ekström et al., 2003) (see Ice Quakes).  Since 2003 Ekström and co-workers have examined thousands of additional seismic records, and they are now reporting a strong seasonality to the frequency of glacial movement, and a clear trend of increasing frequency of glacial movement on Greenland (Ekström et al., 2003).

 

Glacial earthquakes represent the periodic sliding of ice over bedrock.  Ice sheets do not move continuously; instead they move by stick and slip motion.  The seismic signals examined by the authors are consistent with movements of approximately 10 km3 of ice over distances in the order of 10 m, over periods of 30 to 60 seconds.

 

 

 

  Seasonal variation in the incidence of glacial and non-glacial earthquakes.  The glacial earthquakes (blue) are from Greenland only, while the non glacial earthquakes (grey) are from all areas north of 45º.

 

Over the 12 years from 1993 to 2004 the cumulative frequency of detected glacial seismic signals from Greenland was between 3 and 8 per month for the period from January to May, and between 10 and 25 per month for the period from June to December.  The summer and autumn increase in frequency is attributed to an increase in surface melting, an increase in the supply of water to the base of the glacier and a consequent decrease in friction along the ice-bedrock interface.

 

 

Long-term variation in the incidence of glacial and non-glacial earthquakes.  The glacial earthquakes are from Greenland only, while the non glacial earthquakes are from all areas north of 45º.

 

Over the period from 1993 to 2001 there was an average of 10 glacial earthquakes per year (range: 6 -14).  From 2002 to 2005 that number increased to 15, 20, 24 and 32 per year respectively (the 2005 number is based only on the period from January to October).  The authors conclude that these increases are a response to large-scale processes affecting the entire Greenland ice sheet. It is evident that these processes are now delivering a greater volume of water to the base of the glacier, and it is possible that this increase in melting is due to climate warming, although, as yet, the data set covers too short a period to conclusively support this hypothesis.

 

  

 

References

 

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

 

Ekström G, Nettles M and Tsai V, 2006, Seasonality and increasing frequency of Greenland glacial earthquakes, Science, V. 311, p. 1756-1758 (March 2006).