9 december 2008

Large earthquakes trigger a surge in volcanic eruptions

Osorno volcano in Chile
The Osorno volcano in Chile erupted in 1837 following a very large earthquake

New evidence showing that very large earthquakes can trigger an increase in activity at nearby volcanoes has been uncovered by Oxford University scientists.

An analysis of records in southern Chile has shown that up to four times as many volcanic eruptions occur during the year following very large earthquakes than in other years.

This ‘volcanic surge’ can affect volcanoes up to at least 500 km away from an earthquake’s epicentre. A report of the work will be published in the journal Earth and Planetary Science Letters. 

Previously, scientists had identified a few cases where volcanic eruptions follow very large earthquakes – but up until now it had been difficult to show statistically that such earthquakes may be the cause of an increase in eruptions, rather than the events just being a coincidence.

‘The most unexpected part of this discovery was the considerable distance from the earthquake rupture where these eruptions took place, and the length of time for which we saw increased volcanic activity,’ said Sebastian Watt, a Dphil student in Oxford’s Department of Earth Sciences, who conducted the analysis.

This work shows that the risk of volcanic eruption increases dramatically following large earthquakes in parts of the world affected by these phenomena. Hopefully, our findings could help governments and aid agencies in these regions to manage volcanic hazards.

Sebastian Watt, a Dphil student in Oxford’s Department of Earth Sciences, who conducted the analysis.

‘This suggests that seismic waves, radiating from the earthquake rupture, may trigger an eruption by stirring or shaking the molten rock beneath volcanoes. The disturbances that result from this lead to eruption but, because of the time it takes for pressure to build up inside a volcano and for magma to move towards the surface, an eruption may not occur until some months after the earthquake,’ Sebastian added.

Sebastian examined the volcanic eruption and earthquake records of southern Chile – where, in 1835, Charles Darwin first speculated on the link between earthquakes and eruptions. By careful analysis of historical records, he discovered that volcanic activity increased for about a year after each of the very largest earthquakes in southern Chile during the past 150 years.

The volcanoes most likely to be affected lay within about 500 km of the earthquake epicentre, and included both dormant and active volcanoes.The great Chilean earthquakes in 1906 and 1960 (the largest earthquake ever recorded) were each followed by activity at six or seven volcanoes – a significant increase on the average eruption rate of about 1 per year.

Sebastian said: ‘This work is important because it shows that the risk of volcanic eruption increases dramatically following large earthquakes in parts of the world, such as Chile, affected by these phenomena. Hopefully, our findings could help governments and aid agencies in these regions to manage volcanic hazards by showing the need for increased awareness of volcanic activity after large earthquakes.’