AFTER 8 years of monitoring small earthquakes in the Dead Sea rift valley, Israeli seismologists report that the quakes were more likely to occur after a rise in emissions of the radioactive gas radon. Although the signal didn鈥檛 occur before every quake, the researchers say the link is statistically significant 鈥� a controversial claim, as it is generally accepted that individual quakes are impossible to predict.
Radon emissions have been touted as a possible quake precursor since the discovery that levels of the gas in water rose sharply before a major earthquake in Tashkent, Uzbekistan, in 1966. Advocates believe that as underground strain nears the threshold for triggering an earthquake, it releases radon that has accumulated in the rock from radioactive decay. Yet the few reported examples could not be reproduced, and no one had collected systematic data.
To fill that gap, the Geological Survey of Israel monitored radon flux in gravel along a fault zone by the Dead Sea for 8 years. They measured 110 radon peaks in that time, but saw no strong correlation with earthquakes in the surrounding area. But within the rift valley itself, 40 quakes occurred within three days of a radon peak 鈥� well above the 22 that would have been expected by chance (Geology, vol 31, p 505). 鈥淲e don鈥檛 yet have a prediction scheme,鈥� says lead researcher Gideon Steinitz, but he believes a link does exist. He compares it to smoking and cancer: we know smoking causes cancer, but we can鈥檛 predict which smokers will contract the disease.
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Other seismologists are sceptical, citing other reports of links that have failed to stand up. The fault zone should be studied over much longer intervals 鈥� more than 50 years 鈥� before drawing any firm conclusions, says Yehuda Ben Zion of the University of Southern California. And Evelyn Roeloffs of the US Geological Survey in Vancouver, Washington, points out that the radon emissions varied with the seasons, which could have affected the statistics. She thinks the next step should be to measure the crustal strain directly.
Steinitz鈥檚 group is now setting up more measurement stations, and leading a European programme to study links between radon emissions and volcanic eruptions. He is looking for partners in California too, where radon anomalies have also been reported.