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    Người gửi: Khuất Trung Thành (trang riêng)
    Ngày gửi: 20h:24' 14-12-2008
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    Earthquakes can `spark eruptions`
    
    
    The researchers analysed records from southern Chile
    
    Very large earthquakes can trigger an increase in activity at nearby volcanoes according to a new study.
    The controversial findings come from an analysis of records in southern Chile.
    It showed that up to four times as many volcanic eruptions occurred during the year following very large earthquakes than did so in other years.
    The work, by a team at the University of Oxford, appears in the journal Earth and Planetary Science Letters.
    The researchers say volcanoes lying up to at least 500km away from an earthquake`s epicentre were affected.
    Previously, scientists had identified only a few cases where volcanic eruptions followed very large earthquakes.
    It has been very difficult to show statistically that these earthquakes were the cause of an increase in eruptions, rather than coincidences. And the idea is not widely accepted among scientists.
    Shaken or stirred
    The researchers 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 analysing historical records, Sebastian Watt along with Oxford colleagues David Pyle and Tamsin Mather, discovered that volcanic activity increased for about a year after each of the largest earthquakes in southern Chile - those greater than 8.0 magnitude - over the past 150 years.
    
    The Osorno volcano was one of those that figured in the analysis
    
    Mr Watt said it was unexpected to find eruptions occurring so far from the earthquake rupture zone and also the length of time over which increased volcanic activity was seen.
    "This suggests that seismic waves, radiating from the earthquake rupture, may trigger an eruption by stirring or shaking the molten rock beneath volcanoes," he said.
    "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 the magma to move towards the surface, an eruption may not occur until some months after the earthquake."
    `Primed for eruption`
    The volcanoes most likely to be affected lay within about 500km of the earthquake epicentre, and included both dormant and active volcanoes.
    The powerful Chilean earthquakes in 1906 and 1960 - the latter, a 9.5 magnitude event, was the largest earthquake ever recorded - were each followed by activity at six or seven volcanoes. The researchers say this represents a significant increase on the average eruption rate of about one per year.
    Mr Watt commented: "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.
    Previous studies have suggested that where a volcano is primed, the specific timing of an eruption could be triggered by seismic activity. But one expert told BBC News that the idea was not "widely held" among the volcanology community.
    
    
    
     
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