Amberlee Darold has never felt an earthquake, but she has some feelings about the rate at which they have increased in Oklahoma.
“Certainly it’s exciting - also overwhelming, perplexing and frustrating,” she said.
A research seismologist with the Oklahoma Geological Survey, Darold has been studying the increase in seismicity that has attracted the attention of scientists around the globe.
She will present the results of a recent study at the upcoming Mid-Continent Section meeting in a session called “Seismicity Rates in Oklahoma: A Look at the Seismicity Increase of 2014.”
A Record-Setting Year
“Thus far (as of Aug. 10, 2015), we, the OGS, have located 634 earthquakes of a magnitude 3.0 or greater. In 2014 we located 585 earthquakes of a magnitude 3.0 or greater,” she said.
The state’s seismicity rate for 2014 was greater than any previous year, including those that already had seen a significant increase.
“Overall the seismicity rate in 2013 was 70 times greater than the background seismicity rate observed in Oklahoma prior to 2008. While unlikely, this rate could have been potentially explained by natural variations in earthquake rates from naturally occurring swarms. The current seismicity rate is now about 600 times greater than the background seismicity rate mentioned above,” Darold said.
“We at the Oklahoma Geological Survey believe the rates and trends in seismicity are very unlikely to represent a naturally occurring process,” she said.
“Most likely, waste water injection wells are the cause for the increase,” she said.
In 40 of the state’s 77 counties, 5,417 earthquakes were reported during the year.
Of those, 967 were reported as felt to the OGS or USGS and 585 were of a local magnitude of 3.0 or greater, the report states.
In 2013, 284 earthquakes were reported as felt. In 2012, the number felt was 75. The number felt was around 100 in each of the two previous years, an apparent increase from earlier years, according to the OGS.
Speaking on a cellphone from the field, where she was checking on some of the agency’s network of about 40 seismometers, she said: “Nowhere else in the world sees the concentration that Oklahoma is seeing. There’s no comparison,” Darold said.
Potential Causes
The increased activity correlates with the areas of two major oil plays, the Mississippi Lime play and the Hunton dewatering play, she said.
Darold said that about 70 percent of the wastewater is being injected into the Arbuckle, which lies directly above the basement rock.
The increased pore pressure decreases the effective stress in the basement rock and, she said, “Basically, you’re pushing that fault to failure.”
The Oklahoma Corporation Commission (OCC) has been working with disposal operators in areas of most interest, and more than 50 disposal wells found to touch the basement have plugged back their depths to the Arbuckle. Others have cut their volumes, according to various news reports.
Darold said it is too early to tell if mitigation efforts are working.
“Statistically speaking, there is no clear sign that seismicity is decreasing,” she said.
She would not speculate on how long it would take to know if mitigation efforts are effective.
While most of the temblors have been relatively weak, a 5.6 earthquake was recorded in 2011. The last time anything close to that magnitude occurred in Oklahoma was a 5.2 quake in 1952.
“Both caused damage, but luckily no lives were lost,” Darold said.
“As seismologists, we can’t predict earthquakes. We need more knowledge, but we assume we could see another 5.5,” she said.
“We at the OGS are not a regulatory agency; we are a state agency for research and public service. Examining the seismicity rates will give us, the OCC and operators insight to the effects of mitigation efforts currently under way,” she said.
Increased seismicity has been noted in other regions associated with petroleum or geothermal activities, but none to the extent experienced in Oklahoma, she said.
Knowledge gleaned from the Sooner state experience could prove useful elsewhere.
“Geology does not stop at the state line,” she said.
The increased seismicity - and the attention it is getting - is broadening researchers’ knowledge base.
“Honestly, we’re learning a lot about the faults from the seismicity we are seeing,” Darold said.
“It’s not exactly an ‘upside,’ but it is showing us a lot about the faults in Oklahoma - and it’s a great research playground for people all over the world.”