By Deborah Zabarenko
WASHINGTON (Reuters) - Dozens of small earthquakes occurred in central Texas over a two-year period, and 23 of them were close to injection wells where waste water from energy extraction was pumped deep underground for disposal, a new study reported on Monday.
The study used temporary seismographs to detect earthquakes of magnitude 1.5 or higher in a geologic area called the Barnett Shale, a swath of land the size of England that includes Dallas and Fort Worth.
Earthquakes with magnitudes of 1 to 3 would be felt by few people, and only under particularly favorable conditions, according to the U.S. Geological Survey.
Study author Cliff Frohlich of the University of Texas at Austin located 68 earthquakes in this area, more than eight times as many as the U.S. National Earthquake Center found over the same period from November 2009 to September 2011.
Of those, 23 were located within about two miles of high-volume injection wells that pumped more than 150,000 barrels per month of water underground, Frohlich wrote in the Proceedings of the National Academy of Sciences.
His study did not examine any possible link between earthquakes and hydraulic fracturing - commonly called fracking - where water, sand and chemicals are pumped underground at high pressure to break up rock formations that contain oil and natural gas.
"Drilling never causes earthquakes," Frohlich said in a telephone interview. "Fracking almost never causes earthquakes ... While there are probably millions of hydrofracking jobs, only a few have caused earthquakes and they've all been little tiny earthquakes."
Many high-volume injection wells in the Barnett Shale were not near earthquakes, and the study does not specify what made the difference.
Frohlich theorized that injection of water only triggers an earthquake if a nearby fault is "experiencing tectonic stress" - that is, the fluid underground might relieve the stress by getting the fault unstuck, causing a mild earthquake. To test this theory would require data about subsurface faults that is not available now.
This research sparked interest in Britain, where shale gas extraction came under fire last year after tremors were measured in Blackpool, where fracking was taking place. The government temporarily halted fracking at the site.
British science and engineering bodies reported on June 29, 2012 that shale gas fracking is unlikely to cause big earthquakes or contaminate drinking water.
Cuadrilla Resources, a UK shale gas company, said on July 12 that it will improve monitoring of earth tremors at its drilling sites.
(Reporting by Deborah Zabarenko, Environment Correspondent; Editing by Marilyn W. Thompson and Eric Walsh)