Mystery: Alabama earthquake swarm stuns residents and geologists alike

June 2015 – ALABAMA – Early one morning last November, Jim Sterling was frightened when the ground began shaking outside his 156-year-old antebellum home in Alabama. He grabbed his gun and ran outdoors, where he found horses galloping, cows mooing and dogs barking. It was an earthquake. “I heard a boom and felt the shaking,” Sterling said. “It really upset me.” Since that day, more than a dozen weak earthquakes have shaken western Alabama’s Greene County. Geologists are now working to find out what has caused this swarm over the last seven months, in an area of the South that’s used to large tornadoes but not light tremors. “It is interesting that recently there has been more activity there than in the last four decades,” said Sandy Ebersole, an earthquake expert with the Geological Survey of Alabama.

Records from the U.S. Geological Survey show the first of 14 earthquakes occurred on Nov. 20, when a magnitude 3.8 earthquake was recorded about 10 miles northwest of the community of Eutaw. The second occurred in mid-December, followed by another in January and three within a few hours of each other on Feb. 19. The tremors have continued ever since, with the most recent occurring June 6, when a magnitude 3.0 quake rattled the area. All the tremors have been weaker than the initial jolt in November, and Ebersole said some have been too slight for residents to detect.

Located about 35 miles from Tuscaloosa, the whole of Greene County has only about 8,700 residents, and the area where the quakes are occurring is sparsely populated. Farmlands and forests are dotted by hunting preserves and old homes left over from Alabama’s past as a cotton-producing, slave-holding state. Experts have installed a seismic monitor in a field to enable them to get better information about the quakes, none of which has caused major damage. Ebersole said researchers are trying to rule out potential causes such as blasting for quarries and sonic booms. They’ve even held meetings with rattled area residents.

The earthquakes could be linked to underground cracks, or faults, found in the area in recent years at varying depths, Ebersole said. But just what has been causing the ground to shake is unclear. One potential source that regulators are discounting is hydraulic fracturing or “fracking,” a process for extracting underground oil or natural gas that has been blamed for earthquake swarms elsewhere, including Oklahoma. Wastewater is sometimes injected underground, a method the government has blamed for quakes. While Greene County is on the edge of Alabama’s primary region for oil and gas production, state geologist Nick Tew said no such production or disposal work is going on in the area where the earthquakes are occurring.  –Weather