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Quiet period could hint of impending earthquake
04 March 2008
From New Scientist Print Edition.
Kate Ravilious

It is not exactly an early warning system, but earthquakes may indeed hint at their intentions before they rumble. According to a large satellite survey, a few hours before a night-time earthquake there is a significant reduction in the intensity of very low frequency radio waves coming from beneath the ground in the region.

Frantisek Nemec of the University of Orléans in France and colleagues put together two-and-a-half years of electromagnetic wave readings taken by the French satellite DEMETER. They analysed the intensity of these emissions for more than 9000 earthquakes with magnitudes 4.8 or greater, and compared them with background levels.

The team observed a decrease of around 5 decibels in emission intensity up to 4 hours before shallow earthquakes, which occur less than 40 kilometres below the surface. They only found this decrease for earthquakes that happened during the night. (Geophysical Research Letters, DOI: 10.1029/2007GL032517).

It isn't clear what causes this effect, but Nemec and colleagues speculate that a build-up of tension or fresh cracks in the earth, might alter the way the waves travel. Nanec reckons that we may only detect the signal at night because higher levels of ionospheric radiation in the daytime atmosphere mask any subtle changes.

Ian Main, a seismologist at the University of Edinburgh, UK, is not convinced, and says the statistical analysis may be flawed. "The authors have tuned many parameters, such as the depth and magnitude range of the earthquakes and the time period - day or night," he says. And he can't understand why electromagnetic intensity would drop rather than rise before an earthquake. "The biggest disturbance is the earthquake itself and yet there is no signal at the time of the earthquake, which strikes me as odd," says Main.

Colin Price, a geophysicist at the University of Tel Aviv in Israel, wonders if the intensity change could explain why some animals appear to know a quake is imminent. "It shows that earthquakes are not controlled by an on/off switch. However, the signal for an individual earthquake is very small and would need very sensitive ground-level equipment to use it as a precursor," he says.

From issue 2645 of New Scientist magazine, 04 March 2008, page 11

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