Researchers with the U.S. Geological Survey (USGS) said a "remarkable" increase in the number of earthquakes in the Midcontinent region is more than likely from injection wells that handle wastewater from oil and natural gas drilling.

In a statement released before publication of a research paper on the issue, scientists from the USGS earthquake research center in Menlo Park, CA, said reported a six-fold increase in the number of earthquakes of 3.0 magnitude or greater since 2001 from 20th century levels.

According to the USGS, between 1970 and 2000 there were, on average, of 21 seismic events of 3.0 magnitude or greater every year in the Midcontinent, plus or minus 7.6. That rate increased to 29 events (plus or minus 3.5) from 2001 to 2008, a "modest increase" the researchers said was caused by increased seismicity in a coalbed methane field of the Raton Basin along the Colorado-New Mexico border, just west of Trinidad, CO. Many more earthquakes occurred in 2009, which had 50 seismic events, followed by 2010 (87) and 2011 (134).

"The acceleration in activity that began in 2009 appears to involve a combination of source regions of oil and gas production," the researchers said. "Strong evidence" exists that seismic activity in the area around Guy, AR, was directly related to operations at nearby wastewater injection wells.

In central and southern Oklahoma, seismic events of 3.0 magnitude or greater averaged 1.2 events/year over the previous half century, but in 2009 they skyrocketed to more than 25/year, researchers noted.

"A naturally occurring rate change of this magnitude is unprecedented outside of volcanic settings or in the absence of a main shock, of which there was neither in the region," the researchers said. "While the seismicity rate changes described here are almost certainly man-made, it remains to be determined how they are related to either changes in extraction methodologies or the rate of oil and gas production."

USGS researchers plan to discuss the increase in earthquakes at the annual Seismological Society of America conference April 17-19 in San Diego.

The total number of earthquakes in the United States increased by about 1.6% annually during the 1990s and 7.8% annually since then. Only two deaths in the United States since 2000 are attributable to earthquakes, according to USGS.

In early March the Ohio Department of Natural Resources (ODNR) said a dozen small earthquakes in the northeastern part of the state over the last year may have been triggered by a wastewater disposal well in Youngstown (see NGI, March 12). A series of tough new regulations for injection wells has been announced.

"There is a credible connection between the wastewater injection activities near Youngstown and the recent earthquakes, including the magnitude 4.0 earthquake that occurred on New Year's Eve 2011," the USGS stated on its website. "This connection is based on the close proximity of the earthquakes to the injection well and depth of injection, and the observation that these events began soon after the start of the injection activities."

However, the USGS said to date, "there is no conclusive example linking injection operations to [the] triggering of major earthquakes, however we cannot eliminate this possibility. More research is needed to either confirm or refute this possibility."

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