Farmers and other residents pumping groundwater from Earth’s crust probably triggered an earthquake that killed nine people last year in southeastern Spain, scientists have found.
Sucking up water for decades would have unloaded stresses within the ground and hastened a quake that was likely to happen anyway, says Pablo González, a geologist at the University of Western Ontario in London, Canada.
“Even without the groundwater extraction, the earthquake was overdue,” he says. But human activities provided “a kind of triggering or controlling.”
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González and his colleagues report the discovery online October 21 in Nature Geoscience.
Scientists know that people can change the rate of earthquakes by piling up water within the crust, such as behind a dam. Some researchers argue — though it is not entirely accepted — that filling a nearby reservoir may have set off the magnitude 7.9 quake that killed some 80,000 people in Sichuan, China, in 2008. Injecting water into the crust, such as during hydraulic fracturing or other drilling, can also trigger quakes; geothermal drilling operations in Basel, Switzerland, set off a small earthquake there in 2006.
Overall, these triggered tremors are usually quite small, on the order of magnitude 3. The quake that struck Lorca, Spain, on May 11, 2011, was magnitude 5.1. But that isn’t uncommon for that region of Spain, the most seismically active part of the country, González says.
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Lorca is also a place where lots of water gets pumped from underground for farming and drinking. As a result, the land surface is subsiding at about 10 centimeters each year, the highest rate known in Europe.
When the quake hit, González and his colleagues decided to see whether there was any connection. They used satellite radar data to determine how the ground shifted before and after the quake, and then developed a mechanical model to explain what kind of ground stresses would cause that movement.
The quake struck at surprisingly shallow depths, just 2 to 4 kilometers down, the scientists found. The ground’s movement almost precisely matched the areas where stress had changed the most within Earth’s crust because of groundwater pumping, the team calculated. Both findings suggest that draining the aquifers pushed the crust over some threshold and accelerated the quake, González says.
Stress changes caused by groundwater extraction are tiny compared with the usual stress brought about by tectonic forces. That’s why it would have been hard to predict the Lorca quake based just on groundwater use, González says. And the findings probably don’t apply to other quake-prone areas where groundwater is also being pumped. “If the fault is not located right on the border of the aquifers where you have the changes in water level, it would probably not have much effect,” he says.
The new study is the first to show how changing stresses within the crust by removing water — as opposed to adding it, through drilling or dams — can trigger earthquakes. That finding underscores how little scientists still know about the consequences of messing with the crust, writes Caltech geologist Jean-Philippe Avouac in a commentary accompanying the study.
“We know how to start earthquakes,” Avouac writes, “ but we are still far from being able to keep them under control.”