Oklahoma’s threat for a potentially damaging human-caused earthquake in 2017 is similar to 2016 and remains on par with natural shaking hazards in California, according to the latest U.S. Geological Survey study.
In Oklahoma and southern Kansas, about 3 million people live in an area of heightened risk of damaging shaking from an induced earthquake. Oklahoma’s greatest probability is a 10 to 12 percent chance in a central portion of the state, which includes the Pawnee and Cushing areas.
The USGS on Wednesday published its research in Seismological Research Letters. The overall level of induced seismic risk across the central and eastern U.S. declined, but Oklahoma remains similar to a year ago.
“That’s partly because on the one hand you have an overall decrease in frequency of earthquakes counter-balanced by the fact that in (2016) we had those three large earthquakes,” said Jeremy Boak, director of the Oklahoma Geological Survey. “And they skew the calculated distribution of earthquakes and frequencies.”
There were 623 quakes of magnitude-3.0 or greater in 2016 in Oklahoma, a 31 percent reduction from 2015’s record of 903. The 90-day moving average of quakes of at least 3.0 is at about 0.7 per day, according to Boak. That rate is substantially down from a peak of 3.0 a day in January 2016.
The 2017 USGS forecast pegs about 3.5 million people in the central and eastern U.S. with “significant potential” for a damaging quake induced by humans. In 2016 that number was double at about 7 million people.
The largest factor involved is the removal of an induced seismic threat in the Dallas-Fort Worth area because of a “dramatic decline” in quakes in North Texas, according to Mark Peterson, chief of the USGS hazard mapping project.
”The hazard in Oklahoma and south-central Kansas still remains hundreds of times higher than before the man-made activity began,” Peterson said. “So there is still work to be done in reducing the hazards in those areas.”
The study’s threshold for a damaging quake is considered to be minor, such as cracking of plaster and weak masonry.
“If you’re close to a (magnitude-) 4, you can have some damage,” Peterson said.
He noted that chances of experiencing a damaging earthquake could rise or fall with industry practices, “which are difficult to anticipate.”
Last week, the Oklahoma Corporation Commission unveiled a directive addressing that key issue. The latest regulations put a cap on how high and fast disposal volumes linked to man-made quakes can climb.
Tim Baker, director of the OCC’s Oil and Gas Division, said the state is “on a good track” given the correlated reductions in saltwater disposal and earthquake frequency.
Baker said operators in the quake prone part of Oklahoma are cumulatively injecting about 1.5 million barrels of saltwater into deep disposal wells per day.
Without the latest directive, Baker said the volume rate potentially could have climbed to 4.5 million a day.
During peak activity, there were about 3 million barrels per day being injected, he said.
The new cap limits overall disposal to about 2.5 million barrels, Baker said.
Notably, the latest directive includes 71 deep disposal wells in the northeast and southeast corners of the OCC’s 15,000-square-mile area of quake-related regulations. Those wells previously weren’t under volume restrictions because of a lack of seismicity, Baker said.
“What we don’t want to see is the volumes shift over to these wells that have no restrictions,” Baker said. “Is that likely? Probably not. But we’re wanting to reduce that risk of that scenario to occur.”
Katie Brown, a spokeswoman for Energy In Depth, said the USGS forecast is a clear sign that collaborative efforts among industry, scientists and regulators are working.
”In Oklahoma, the number of earthquakes last year dropped by 31 percent, and there were zero felt earthquakes in North Texas in 2016,” Brown said. “This study is in line with what numerous experts have said: the risk of induced seismicity is small, rare, and manageable.”
Energy In Depth is a research and education program of the Independent Petroleum Association of America.