From Michael Slezak:
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 González 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.