Clarification: After publication, the Oklahoma Geological Survey revised its count of magnitude 3 and above earthquakes to 905 in 2015. That new total represents an increase of 55 percent above the 2014 total of 585 quakes.
The number of earthquakes in Oklahoma rose 50 percent last year, easily surpassing the record number that hit the state in 2014.
Oklahoma Geological Survey (OGS) data show that the state was shaken by 881 earthquakes of magnitude 3 or greater, or an average of 2.4 per day. That's up from 585 in 2014.
U.S. Geological Survey data show that California had 128 such quakes in 2015.
Scientists and state officials say the increase in earthquakes in Oklahoma likely has been caused by wastewater disposal from oil and gas operations. Oil production methods that yield unusually large volumes of water have combined with favorably aligned faults under the state to cause the unprecedented shaking.
The data are subject to revision. USGS records appear to show at least one late-December quake not reflected in OGS records.
The number of quakes per month hit a high of 105 in June. The lowest number occurred in September and December, which both saw 55 quakes magnitude 3 or greater.
Among the 29 quakes of magnitude 4 or higher was a magnitude-4.3 quake on Dec. 28. It made front-page news in Oklahoma City because it was centered just north of the city in Edmond. It was followed by a magnitude-4.2 quake on New Year's Day and numerous smaller shocks.
Today, oil and gas regulators at the Oklahoma Corporation Commission (OCC) are expected to announce their response to those Edmond quakes. OCC officials say their response is complicated by the fact that there are few high-volume disposal wells in the area injecting waste into the deep Arbuckle formation, considered the most troublesome.
State officials in April 2015 acknowledged that the increased shaking was related to oil and gas operations, ending years of waffling extended at times by interference from the politically powerful oil and gas industry (EnergyWire, March 3, 2015).
In the summer, OCC officials started asking operators of wells in earthquake-prone areas to reduce injection volumes or cease operations. The agency's "directives" are voluntary but backed by the threat of formal, mandatory action.
That policy has been put to the test by SandRidge Energy Inc., which refused the state's request to shutter six disposal wells in the north-central part of the state, near the Kansas line (EnergyWire, Dec. 21, 2015). The financially struggling company has focused most of its operations in the area, in a play called the Mississippi Lime.
OCC officials say they plan to file a formal request soon to make their shutdown request mandatory. State Rep. Jason Murphey (R) of Guthrie, one of the most outspoken legislators on the quake issue, said yesterday that the pushback from SandRidge, along with a Tulsa company that challenged OCC's authority, makes it more likely that the Legislature might act on the issue.
The Mississippi Lime area, near the cities of Cherokee and Medford, was the hardest hit by the quakes. Grant County, home to Medford, had the most quakes at 229. Neighboring Alfalfa County, home to Cherokee, had the third-highest total, 119.
The quakes spilled over the state line into Kansas, which had 55 quakes last year. Officials there say they've had success in reducing quakes since enacting mandatory volume cuts in March 2015. There hasn't been a quake of magnitude 3 or greater in Kansas since Nov. 9.
A report released on New Year's Eve by Oklahoma Geological Survey hydrogeologist Kyle Murray shows that Alfalfa County in particular saw a huge increase in disposal of oil and gas wastewater through 2014, the last year for which data are available.
In 2009, companies injected about 17 million barrels of oil and gas wastewater in Alfalfa County (a barrel is 42 gallons). In 2014, companies injected 299 million gallons, a seventeenfold increase.
Statewide, wastewater disposal volumes increased from about 830 million barrels to 1.5 billion barrels during that time period. In the Arbuckle formation, where scientists and regulators have focused their attention, the increase was about 140 percent.
Oklahoma County, home to Oklahoma City and Edmond, had 22 quakes of magnitude 3 or larger. Logan County, just to the north, had 122 quakes, ranking it second in the state.
Magnitude 3 is the strength at which almost any earthquake will be felt and recorded by instruments in the United States.