Thirst for groundwater caused fatal earthquake

On 11 May 2011, nine people were killed and dozens injured by a magnitude 5.1 earthquake near Lorca in southern Spain. Now it seems that the earthquake was triggered by human activity. What's more, it may have been shallower, and thus more destructive, than if it had happened following a slow, natural build-up of stress.

There could be a silver lining to the tragedy, though. It may provide seismologists with a rare insight into how an earthquake can be triggered, and so give fresh hope to people who try to forecast such events. It may even be possible to weaken the strength and scale of such quakes in future.

We know that small earthquakes have been triggered by human activities like fracking, but until now there had been little compelling evidence of human involvement in larger, fatal quakes.

The Lorca quake was unusually shallow, a point noted by Pablo Gonz?lez at the University of Western Ontario in Canada and his colleagues. They decided to investigate whether it might have been triggered by the removal of vast quantities of groundwater for irrigation. This has caused the Lorca water table to fall sharply. Since 1960, its level has dropped by 250 metres, and the land has subsided by around 15 centimetres per year.

Radar measurements

Using radar to measure ground deformation caused by the quake, Gonz?lez and colleagues modelled how much the land slipped and identified the point where the quake originated. They found it involved about 20 centimetres of slip and occurred 3 kilometres below the Earth's surface ? a third as deep as would be expected for quakes of that magnitude.

The team then modelled the effect of this disappearance of groundwater on stress patterns in the crust. The stresses coincided to such a degree with the rupture pattern seen in the quake that extracting groundwater must have triggered the event, the researchers concluded.

Gonz?lez adds that the quake would have happened eventually, even if the water had been left in the ground. "This portion of the fault was tectonically loaded," he says Gonz?lez. As the load continued to build, it would eventually rupture.

Even so, the quake might not have happened the way it did if the groundwater had been left untapped, says Jean-Philippe Avouac from California Institute of Technology in Pasadena, who was not involved in the study. He thinks the evidence suggests the event was shallower than it would have been without human intervention, and therefore more destructive.

Quicker release

Avouac agrees that stress in the earth's crust has to be released one way or another. But he says human activity ? like water extraction ? can cause the stress to be released quickly, rather than dissipating slowly over time.

"It's not just that you're advancing an earthquake that would have happened anyway. It's that you're creating more or larger earthquakes," he says.

A key part of predicting and possibly even controlling earthquakes is understanding what triggers them, Avouac adds. In most cases, pinning that down precisely is very hard, but the possibility that human activity is implicated in some events provides new opportunities for investigation.

A distant dream, Avouac says, is to tame natural faults by breaking them up and so decrease the magnitude of individual quakes and the size of the area they affect. It may also be possible to purposely trigger quakes to release tectonic pressures in a controlled way.

According to Gonz?lez, a more realistic approach is simply to avoid triggering seismic activity.

Journal reference: Nature Geoscience, DOI: 10.1038/ngeo1610

If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.

Have your say

Only subscribers may leave comments on this article. Please log in.

Only personal subscribers may leave comments on this article

Subscribe now to comment.

All comments should respect the New Scientist House Rules. If you think a particular comment breaks these rules then please use the "Report" link in that comment to report it to us.

If you are having a technical problem posting a comment, please contact technical support.

Source: http://feeds.newscientist.com/c/749/f/10897/s/24b77359/l/0L0Snewscientist0N0Carticle0Cdn2240A30Ethirst0Efor0Egroundwater0Ecaused0Efatal0Eearthquake0Bhtml0Dcmpid0FRSS0QNSNS0Q20A120EGLOBAL0Qonline0Enews/story01.htm