Asked by: Steve Peters, by email
When an earthquake strikes, different vibrations travel through the ground at different speeds. The Primary (P-wave) vibrations travel about twice as fast as the Secondary (S-wave) vibrations that do most of the actual shaking. P-waves are generally too subtle to be felt by humans, although seismographs will pick them up. But some animals may be able to detect P-waves before the S-waves arrive. This would give them less than two minutes’ notice for any quake near enough to affect them.
Stories of snakes leaving their burrows, dogs barking excessively or birds flying in unusual patterns, days or weeks before an earthquake actually takes place are more contentious.
But there may be subtle changes prior to an earthquake that animals are able to detect. A 2011 study at The Open University found that the stresses that build up along earthquake fault lines cause pockets of positive charge to move through the rocks to the surface and will trigger chemical changes in the groundwater. This could have been the reason that toads suddenly left their breeding pond a few days before the earthquake that hit L’Aquila, Italy in 2009. Their pond was 74km away from the earthquake’s epicentre.
The positive charge could even affect the electromagnetic fields that bats and birds use for navigation, but we don’t have any direct evidence for this yet.