WASHINGTON (Reuters) - The number of earthquakes in the central United States rose "spectacularly" near where oil and gas drillers disposed of wastewater underground, a process that may have caused geologic faults to slip, U.S. government geologists report.
The average number of earthquakes of magnitude 3 or greater in the U.S. midcontinent - an area that includes Arkansas, Colorado, Oklahoma, New Mexico and Texas - increased to six times the 20th century average last year, scientists at the U.S. Geological Survey said in an abstract of their research.
The abstract does not explicitly link rising earthquake activity to fracking - known formally as hydraulic fracturing - that involves pumping water and chemicals into underground rock formations to extract natural gas and oil.
But the wastewater generated by fracking and other extraction processes may play a role in causing geologic faults to slip, causing earthquakes, the report suggests.
"A remarkable increase in the rate of (magnitude 3) and greater earthquakes is currently in progress," the authors wrote in a brief work summary to be discussed Wednesday at a San Diego meeting of the Seismological Society of America.
"While the seismicity rate changes described here are almost certainly manmade, it remains to be determined how they are related to either changes in extraction methodologies or the rate of oil and gas production," the abstract said.
From 1970 through 2000, the rate of magnitude 3 or greater quakes was 21 plus or minus 7.6 each year, according to USGS figures. Between 2001 and 2008, that increased to 29 plus or minus 3.5.
But the next three years saw the numbers increase "much more spectacularly," said Arthur McGarr, of the geologic survey's Earthquake Science Center in California: 2009 had 50, 2010 had 87 and 2011 had 134 such events.
"We don't know why, but we doubt that it's a natural process, because in nature, the only time you see such a big increase is during an aftershock sequence (with a series of quakes) or in a volcanic setting where you often get swarms of earthquakes due to magmatic activity," McGarr said by telephone.
When swarms of quakes occurred in Colorado and Oklahoma last year, the U.S. Environmental Protection Agency asked the geologic survey to investigate possible links to energy extraction in the area.
Among other sites, they examined an August 2011 earthquake centered around Trinidad, Colorado, near the New Mexico border, that registered a magnitude of 5.3, said McGarr, a co-author of the abstract.
That quake "turned out to be really close to two of the highest injection volume waste water disposal wells in the field," McGarr said. "So that gives us quite a strong hint that these earthquakes are being triggered by these wastewater disposal facilities."
There were different responses on either side of the Colorado-New Mexico line, he said. New Mexico, where the policy was to inject all wastewater underground, experienced more earthquakes than Colorado, where some wastewater is disposed at the surface.
America's Natural Gas Alliance, which represents major energy companies involved in natural gas fracking, said it was difficult to conclude anything based on an unpublished abstract.
"We are committed to monitoring the issue and working with authorities where there are concerns, but it should be noted that currently there is no scientific data associating hydraulic fracturing with earthquakes that would cause damage," ANGA spokesman Dan Whitten said in an email.
The disposal of wastewater underground, called injection, has long been known to have the potential to cause earthquakes, the Interior Department said in a blog post here .
What is new is the ability to precisely locate earthquakes of magnitude 3 or greater (magnitude 3 is recognized as the threshhold for detection) and a signature shape of the waves on a seismogram indicating a shallow quake, McGarr said.
Human-induced quakes are typically quite shallow, he said.
Reporting By Deborah Zabarenko, Environment Correspondent; Editing by Marilyn W. Thompson and Philip Barbara