Changes in the chemical make-up of underground water may be a warning that an earthquake is on the way.
This is not a new idea, but it is a difficult one to prove. Seismologists have seen changes in levels of radon gas, concentrations of dissolved elements and the ratios of stable isotopes in water. But the studies typically did not have enough data to confidently link any shifts to subsequent earthquakes. Now new data from Iceland offers a detailed picture of how water changed ahead of two earthquakes of magnitude 5.6 and 5.5 in 2012 and 2013.
Alasdair Skelton of Stockholm University in Sweden and his colleagues analysed weekly water samples from a borehole in northern Iceland for 5 years. Four to six months before each of the earthquakes, they observed a change in the mixture of hydrogen isotopes, and a shift in the levels of sodium ions.
Because they monitored water chemistry for a long time, and spotted specific changes before two earthquakes, the team had enough data for a statistical analysis to show that the chemistry changes were most likely associated with the earthquakes and not the result of random fluctuations.
“The whole idea of chemical changes in groundwater before earthquakes is highly disputed,” says Skelton. “Our study makes a pretty convincing case that these really were related to the earthquake.”
So far, Skelton has not determined what caused the chemical changes. It could be that other sources of water had started mixing with the water the team were sampling, perhaps as a result of shaking or stress-induced cracking of the underground rocks. Because of these uncertainties, says Skelton, “we’re not ready to use the ‘prediction’ word yet”.
“The results shown give cautious optimism,” says Bruce Malamud of King’s College London. “But there is still a lot more evidence that needs to be gathered, in other study sites and for longer time spans, before we can consider large changes of groundwater chemistry to always be a precursor to medium-to-large seismic activity.”
Seismologist Ian Main of the University of Edinburgh, UK, agrees: “There is a long way to go before observations such as these could be turned into diagnostic operational tools for forecasting earthquake probabilities.” Most geochemical signals fluctuate anyway, so he says it is “virtually inevitable” that some signals will coincide with earthquakes.
“For this reason,” says Main, “the global community is now focused on prospective tests: making a public forecast in real time, and only testing in retrospect when a realistic large sample of events can be analysed fully for hits, misses and false alarms.” Only after experiments like these will we be able to say whether possible predictors really work.
Journal reference: Nature Geoscience, DOI: 10.1038/NGEO2250
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