The Oklahoma Geological Survey said it's "very likely" that a majority of recent earthquakes, particularly in central and north-central Oklahoma, experienced in recent years were triggered by the injection of produced water in disposal wells.
Oklahoma experienced a seismicity rate in 2013 that was 70 times greater than the background seismicity rate observed before 2008, OGS said in a statement issued Apr. 21, which noted the accelerating seismicity rate continued to increase during 2014.
OGS acknowledged the accelerated rate could stem from natural variations in earthquake rates.
"The seismicity rate is now about 600 times greater than the background seismicity rate and is very unlikely the result of a natural process," OGS said, adding that naturally occurring regional earthquake swarms have happened.
Background seismicity involves naturally occurring seismicity vs. induced seismicity caused by human activity.
Oklahoma earthquakes averaged 2/week during 2013 that measured a magnitude of 3+ compared with a historic average of 1.5/year of M3+. Currently, OGS reports an average of 2.5/day measuring M3+.
OGS is increasing its staff and adding seismic equipment to improve seismic monitoring throughout the state. The agency also is compiling a database of known fault locations within Oklahoma using scientific literature and information from the Oklahoma Independent Petroleum Association. In addition, OGS works with the US Geological Survey and other researchers.
"As communicated in the joint USGS-OGS statement dated May 2, 2014, the earthquake hazard in Oklahoma has increased," the state agency said.
"The primary suspected source of triggered seismicity is not from hydraulic fracturing," OGS said, noting a distinction between fracturing and the injection of water associated with oil and gas production..
"While there are large amounts of wastewater generated from hydraulic fracturing, this volume represents a small percentage of the total volume of wastewater injected in disposal wells in Oklahoma," OGS said.
Most Oklahoma earthquakes occurred within crystalline basement, deeper than most oil and gas operations, OGS said. Arbuckle formations overlie the crystalline basement.
"Reactivation of deeper basement faults from water injection [and] disposal at shallower depths is often observed in cases of triggered seismicity," OGS said.
Pressure from water injection and disposal can be transmitted several miles from an injection site, OGS said, adding the observed seismicity of greatest concentration tends to follow oil and gas plays characterized by large amounts of produced water.
Seismicity rates are observed to increase after a time delay that can be up to 1 year or more, the agency said.