SYDNEY: A magnitude 5.1 earthquake that killed nine people in Lorca, southeastern Spain, in May 2011, was likely the result of human activity, according to new research.
Occurring at a depth of just two to three kilometres, the Lorca earthquake was extremely shallow and unusually destructive for its magnitude. Based on this, researchers hypothesised that it may have been connected to an already observed local shift in the Earth’s surface, called subsidence, caused by groundwater extraction near the epicentre.
“Moreover, the pattern of previously detected subsidence was bounded by the Alhama de Murcia fault,” said Pablo Gonzales, a geologist at the University of Western Ontario in Canada, and lead author of the paper published in Nature Geoscience.
Groundwater extraction adds stress
The researchers first modelled ground deformation using data from satellite images to confirm that the Alhama de Murcia fault, which runs for 100km through Spain’s Murcia province, was the cause of the quake. They then developed a mechanical model to simulate the stresses on that fault caused by long-term groundwater extraction in the region since 1960, which has reduced the Alto Guadalentin water table – declared officially overexploited in 1987 – by at least 250 metres.
Gonzales said that both models showed similar results: that the “unloading process” caused by groundwater extraction “could add a critical amount of stress that would control the occurrence and fault-slip pattern of the earthquake”.
“Our results imply that anthropogenic activities could influence how and when earthquakes occur,” the researchers wrote in the paper.
What triggers earthquakes?
Gonzales said the research would aid in our understanding of the earthquake process, including the types of geological stresses – both natural and man-made – that trigger, intensify or even prevent earthquakes.
“However, we should continue research to further confirm this hypothesis using refined modelling approaches and better data in future earthquakes,” he said.
Michael Asten, a professor of geophysics at Monash University in Melbourne, Australia, who wasn’t involved with the study, said the association with groundwater extraction and the earthquake’s trigger was “counter-intuitive”.
“There used to be a conventional wisdom that injection of water might lubricate faults and trigger earthquakes, while extraction of water from a fault zone might reduce the likelihood of fault movement,” he said, adding that the study “illustrates the remarkable complexity” of the earthquake process.
Carbon sequestration and fracking
In a news and views article that accompanied the paper, geologist Jean-Phillippe Avouac from the California Institute of Technology in Pasadena said that, based on the results, “We should remain cautious of human-induced stress perturbations, in particular carbon dioxide sequestration projects that might affect very large volumes of crust.”
Carbon sequestration involves capturing and storing carbon dioxide, and can be a natural process – such as in forests – or a man-made process. Capturing and storing C02 in underground reservoirs has been proposed as one of the ways to reduce emissions and ward off dangerous climate change.
However, Asten said the potential risk of projects such as carbon sequestration and fracking – drilling and injecting millions of litres of fluid into the ground to extract natural gas – shouldn’t be overstated.
“Southern Spain is a zone of very high earthquake risk being at the junction of two continental plates,” he said. “As Avouac points out, the movement associated with the earthquake was very much greater than what might be attributed to change associated with removal of the groundwater [alone].”
Instead, Asten said the study should act as a “cautionary note” informing public policy to “reconsider the benefits of such projects in areas of known and high natural earthquake risk”.
Avouac added a further caution in the conclusion of his commentary: “We know how to start earthquakes, but we are still far from being able to keep them under control,” he said.