Further evidence has emerged that injection wells associated with fracking cause minor earthquakes.
New research from The University of Texas at Austin indicates that most earthquakes in the Barnett Shale region of north Texas occur within a few miles of one or more injection wells used to dispose of wastes associated with petroleum production such as hydraulic fracturing fluids.
None of the quakes identified in the two-year study was strong enough to pose a danger to the public.
"You can't prove that any one earthquake was caused by an injection well," says senior research scientist Cliff Frohlich. "But it's obvious that wells are enhancing the probability that earthquakes will occur."
Frohlich analyzed seismic data collected between November 2009 and September 2011 by the EarthScope USArray Program, detecting earthquakes down to magnitude 1.5 - far too weak for people to feel at the surface.
He found that the most reliably located earthquakes — those accurate to within about 0.9 miles — occurred in eight groups, all within two miles of at least one injection well. Previously, the National Earthquake Information Center had only identified two earthquake groups in the area strongly associated with specific injection wells.
The finding suggests injection-triggered earthquakes are common. It supports recent data from the US Geological Survey that earthquakes of magnitude 3 or above are also triggered by wastewater injection.
"I didn't find any higher risks from disposal of hydraulic fracturing fluids than was thought before," says Frohlich. "My study found more small quakes, nearly all less than magnitude 3.0, but just more of the smaller ones than were previously known. The risk is all from big quakes, which don't seem to occur here."