Chemical changes in groundwater occurring as a result of an imminent earthquake could be the reason for the bizarre behaviour of animals during such calamities, suggests a new study.
The team of researchers led by Friedemann Freund from NASA and Rachel Grant from the UK's Open University began to investigate these chemical effects after seeing a colony of toads abandon its pond in L'Aquila, Italy in 2009, days before a quake.
The researchers suggest that animal behaviour could be incorporated into earthquake forecasting.
"When you think of all of the many things that are happening to these rocks, it would be weird if the animals weren't affected in some way," the BBC quoted Grant as saying.
In the study, the researchers describe a mechanism whereby stressed rocks in the Earth's crust release charged particles that react with the groundwater.
Animals that live in or near groundwater are highly sensitive to any changes in its chemistry, so they might sense this days before the rocks finally "slip" and cause a quake.
The L'Aquila toads are not the first example of strange animal behaviour before a major seismic event. There have been reports throughout history of reptiles, amphibians and fish behaving in unusual ways just before an earthquake struck.
"It was very dramatic. It went from 96 toads to almost zero over three days.
"After that, I was contacted by NASA," she said.
Scientists at the US space agency had been studying the chemical changes that occur when rocks are under extreme stress. They wondered if these changes were linked to the mass exodus of the toads.
Their laboratory-based tests have now revealed, not only that these changes could be connected, but that the Earth's crust could directly affect the chemistry of the pond that the toads were living and breeding in at the time.
NASA geophysicist Friedemann Freund showed that, when rocks were under very high levels of stress, for example by the "gargantuan tectonic forces" just before an earthquake, they release charged particles.
These charged particles can flow out into the surrounding rocks, and when they arrive at the Earth's surface they react with the air, converting air molecules into charged particles known as ions.
"Positive airborne ions are known in the medical community to cause headaches and nausea in humans and to increase the level of serotonin, a stress hormone, in the blood of animals," Freund said.
This chemical chain of events could affect the organic material dissolved in the pond water - turning harmless organic material into substances that are toxic to aquatic animals.
The study has been published in the International Journal of Environmental Research and Public Health.