Oct. 7, 2003 Question: When is it reasonably safe for researchers to work near a volcano?
Answer: When the numbers in the probability model say it’s OK.
Volcanoes can erupt unexpectedly, with disastrous results for those who study them up close. Although it’s been a decade since the Galeras Volcano in Colombia erupted unexpectedly on January 14, 1993 killing 11 people, including six volcano research professionals, volcanologists have not found a foolproof way to forecast when a volcano will erupt. However, forecasting eruptions has taken a big step forward with new prediction research carried out by University of South Florida geologists, who published their findings in a recent issue of Geophysical Research Letters (Vol. 30, No. 13, 1701).
USF volcano researchers, who have studied volcanoes in Montserrat in the Lesser Antilles island arc, in Mexico, Nicaragua, Armenia, Japan, and elsewhere, have come up with a way to determine when it’s safe to work around a volcano by developing a probabilistic model that can be applied to a volcano’s “repose interval.”
“Volcanic explosions are caused by a build up of gas pore pressure to a certain threshold in the upper conduit,” says long time volcano researcher, Charles Connor, chair of USF’s Department of Geology. “However, gas can escape and pressure can be reduced, especially if the magma becomes permeable or fracture networks develop. These competing processes can increase or decrease the chances of a volcanic eruption.”
Can researchers plug these competing processes into a mathematical model that will predict which course will be taken?
“Yes. We can analyze what we call the volcano’s ‘repose interval,’ the time elapsed between each volcanic explosion, using probability models,” explains Connor. “One such model is the ‘materials failure model’ that assumes that the probability of eruptions increases exponentially as the time since the last eruption increases.”
However, volcanoes can have shorter or longer repose intervals than predicted by a materials failure model because additional factors, such as gas loss, need to be taken into account. When these additional processes are accounted for, the forecast improves.
To try out their model, Connor and his fellow researchers studied a major eruption episode (September-October 1997) of the Soufriere Hills Volcano on the island on Montserrat. Each explosion started with a ten second intense phase of peak discharge, reports Connor, followed by a low intensity phase of venting gas and ash lasting some ten minutes each. Repose intervals lasted between just over two hours to up to almost 34 hours.
Using the Montserrat volcano’s physical attributes as a case study, the probability model suggested that an eruption after a 40-hour respose interval was a low probability.
“That probability, while low, was not low enough to conclude that the eruptions were over,” explains Connor. “We opted on the conservative side and, using another equation, found that an 85 hour might be better suited to consider the eruption sequence over.”
The researchers concluded that the model might have wide application as an eruption-forecasting tool that would not only benefit researchers and possibly prevent disasters such as the one in 1993, but also benefit those who live in the shadow of a volcano’s wrath.
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