The Quiet Side of Earthquakes
Earthquakes aren't known for their subtlety: but science is revealing a hidden side to these killers. We now know that rocks give off invisible warnings long before faulting occurs. They don't move the Earth, but these non-seismic signals could be groundbreaking in a different sense: they're our best hope of predicting the unpredictable.
Earthquakes infamously emit energy in the form of seismic waves. But with a more sophisticated view, a wide spectrum of emissions emerges, the majority of which are imperceptible to humans.
They're all linked to stress. Seismicity occurs when stress which has gradually built up in the subsurface is suddenly released. By the time that happens it's too late for a prediction. But, luckily for would-be earthquake forecasters, rocks emit other signals before they reach breaking point.
In the early 2000s, it was discovered that when rocks are subjected to stress – especially igneous or metamorphic types – atomic defects in their crystal structure generate electric charge. And it now seems that the same charge is generated in the ground before an earthquake.
If it reaches the surface, this charge can give rise to a whole suite of strange phenomena: thermal anomalies, bizarre cloud formations, even ionic disturbances hundreds of kilometres above the ground – all have been recorded before major earthquakes. But for sheer weirdness, there's one pre-seismic signal that tops them all: earthquake lights.
It sounds like science fiction, but luminous apparitions have been reported to occur before earthquakes since Roman times. Eyewitness descriptions vary wildly, from the auroras seen before the 2008 Sichuan earthquake, to what look like flames leaking out of the ground, or even bright "globes" suspended above it.
Until recently, the subject hovered on the fringes of scientific credibility, almost to seismology what UFO sightings are to aeronautics. But the discovery of the electric potential of earthquakes presented a rational explanation. Earthquake lights are now believed to be the visual manifestation of stress-induced charge deep within the Earth – probably the result of ionisation at the surface causing air molecules to luminesce.
We've traveled quite far from seismic waves, so I'll make the next example the last. It's a good pace to end, though, because it might just be the most fascinating of the lot.
When electric charge is generated before earthquakes, basic physics dictates that magnetic fields must also be created. This effect has been seen in action; changes in the Earth's magnetic field have been detected before numerous seismic events. Learning to read these magnetic omens is a target for future predictive efforts. And next year, a revolutionary new space programme will try to do just that – in an extraordinary way.
It's been well established that certain species of flying animals can sense magnetic fields. They probably use this ability to navigate. But there are hints that this sense might also alert them to coming earthquakes. Currently, the only evidence for this is anecdotal. That's where ICARUS comes in.
When it launches late next year, the International Cooperation for Animal Research Using Space initiative will continuously track hundreds of birds and bats, from an instrument on board the International Space Station. Its main goal is to monitor migration routes. But it will also be looking for warning signs.
More than 1300 large earthquakes (magnitude 5 or greater) occur each year. As such, there's a good chance that the animals tagged by ICARUS will, at some point, fly near to a seismic fault which is about to rupture. Both birds and bats can detect magnetic fields – and the way they react could be of vital importance. For the whole of human history, we've been powerless against these random killers. In the end, though, the key to earthquake forecasting might be over our heads.