LORCA, Spain, Oct. 22 (UPI) -- An unusually shallow earthquake that hit a Spanish city in 2011, killing nine people and injuring dozens, was triggered by groundwater removal, a study found.
The earthquake in Lorca was a relatively moderate magnitude 5.1 temblor but was only 0.6 miles below Earth's surface, resulting in significant damage to structures.
Scientists have long known pumping water into the ground can cause small earthquakes, but researchers studying the May 5, 2011, Lorca temblor said it was the first time removing groundwater has been shown to set off a quake.
"The fact that the very tiny stress changes due to normal processes, such as the extraction of groundwater, could have an effect on very large systems such as faults, that's very surprising," said researcher Pablo Gonzalez at the University of Western Ontario in Canada.
The groundwater table south of Lorca has dropped as much as 820 feet in 50 years, researchers said.
"When the tragic event occurred, we asked ourselves if the earthquake might be related to the subsidence," Gonzalez told OurAmazingPlanet. "The earthquake was very shallow and, moreover, this pattern of subsidence was bounded by the fault."
Although triggered by groundwater extraction, the earthquake released stress built up over several centuries of regional deformation in southeastern Spain, which is near the plate boundary region between the Eurasia and Africa tectonic plates, researchers concluded.