Researchers with the U.S. Geological Survey (USGS) said a 5.1-magnitude earthquake that struck Oklahoma last February was likely caused by nearby oil and natural gas disposal wells, noting that injected volumes of wastewater had increased seven-fold over the last three years.
The earthquake struck 32 kilometers (19.9 miles) northwest of Fairview, OK, on Feb. 13. According to the USGS, when the Fairview quake struck it was the largest seismic event recorded in the central and eastern United States since a 5.7-magnitude temblor shook Prague, OK, on Nov. 6, 2011. The Fairview quake is now the third-largest, after a 5.8-magnitude earthquake near Pawnee, OK, in early September (see Shale Daily, Sept. 8; Sept. 6).
In a report released Monday, the USGS said the Fairview earthquake occurred southwest of a group of wastewater disposal wells more than 12 kilometers (7.5 miles) away, but with high disposal rates. The region surrounding Fairview has seen the seven-fold increase in disposal volumes.
"The fact that seismicity is rather limited near the high-rate wells while the Fairview sequence occurred at a relatively larger distance from these wells, shows us the critical role preexisting, though possibly unknown, fault structures play in inducing large events," USGS scientist William Yeck said.
"The rapid deployment of seismic stations by the USGS allowed us to precisely locate the aftershock sequence. High-quality data sets such as these are critical when trying to understand the shaking produced by these events and therefore are an important basis for earthquake hazard modeling."
Yeck was lead author of the USGS report, which was published earlier this month in Geophysical Research Letters. The agency said five recent earthquakes of 4.4-magnitude or larger could potentially be linked to wastewater injection. According to the USGS, almost all of those quakes occurred in the Precambrian basement on a partially mapped 14-kilometer (8.7-mile) fault.
"We suggest far-field pressurization from clustered, high-rate wells greater than 12 kilometers from this sequence induced these earthquakes," the USGS said in the report's abstract. "As compared to the Fairview sequence, seismicity is diffuse near high-rate wells, where pressure changes are expected to be largest. This points to the critical role that preexisting faults play in the occurrence of large induced earthquakes."
Seismologists last month said the Pawnee earthquake occurred along a known fault line, but a series of aftershocks along a different line indicated that a new fault had opened up (see Shale Daily, Sept. 13). At the time, researchers said it was too early to tell if the Pawnee quake was triggered by injection well activity or was a natural occurrence.
Earlier this month it was revealed that the U.S. Environmental Protection Agency has recommended that state regulators -- specifically, the Oklahoma Corporation Commission (OCC) and its Oil and Gas Conservation Division (OGCD) -- enact a moratorium on disposal wells in sensitive areas of the Arbuckle formation (see Shale Daily, Oct. 11).
Since the beginning of the year, OCC has ordered operators of about 700 wastewater injection wells targeting the Arbuckle formation to cease or curtail their operations.
OGCD in February unveiled its Western Regional Reduction Plan, which called for a nearly 500,000 b/d reduction in wastewater injection volumes (see Shale Daily, Feb. 17). It released its Central Oklahoma Volume Reduction Plan, which called for a 300,000 b/d reduction in injection volumes, one month later.