Oklahoma ranked No. 2 among the lower 48 states for earthquakes in 2013 amid a seismic swarm that many scientists attribute to increased oil and gas waste disposal.
There were 99 earthquakes of magnitude 3.0 or greater in Oklahoma last year, according to an EnergyWire review of U.S. Geological Survey data. That puts it far behind California's 236.
But Oklahoma is less than half the size of California and had almost half as many earthquakes. To Katie Keranen, a seismologist who has studied the Oklahoma quakes extensively, the seismic activity is comparable.
"Oklahoma is in that same ballpark" for earthquakes, said Keranen, a professor at Cornell University.
So far this year, Oklahoma has actually had more onshore quakes of magnitude 3.0 or greater than California. California has had 23, according to USGS data, while Oklahoma has had 30.
Oklahoma is a big part of the nation's drilling boom, which has been driven by advances in hydraulic fracturing technology that creates millions of gallons of waste fluid. Most of that gets disposed of by deep injection underground. Scientists have known for decades that such deep underground injection can sometimes lubricate faults and unleash earthquakes.
The larger, potentially damaging earthquakes come from the disposal of that wastewater, seismologists say, rather than hydraulic fracturing itself.
The U.S. Geological Survey says there has been a "remarkable" increase in earthquakes in the middle of the United States, most likely linked to disposal of waste fluid from oil and gas production. Central Oklahoma, USGS says, is in the midst of an injection-related earthquake "swarm."
But Oklahoma officials brush off the idea that drilling activities are causing the big surge of earthquakes. The Oklahoma Geological Survey put out a four-page statement this week after a series of quakes rocked the area around Guthrie, north of Oklahoma City.
"The majority, but not all, of the recent earthquakes appear to be the result of natural stresses," the agency said in the statement.
State officials note that oil and gas companies have been injecting waste fluid for decades in Oklahoma, and there are thousands of disposal wells spread across the state. But the oil and gas regulators at the Oklahoma Corporation Commission last year did sharply limit injection at a well in southern Oklahoma when earthquakes broke out shortly after it opened. The operator then shut the well.
The commission is also looking at rules requiring operators to keep more information about their wells for regulators and researchers.
Many of Oklahoma's quakes can be grouped into four seismic outbreaks since early 2009 that Keranen and other researchers have linked to oil and gas activities, primarily deep underground injection of drilling wastewater.
There's the Jones swarm, named after a northeastern suburb bordering Oklahoma City where oil comes up, along with millions of gallons of water that then gets reinjected underground. Hundreds of small quakes have been recorded in the area since 2009.
Then there was the state's biggest earthquake in 2011, a magnitude-5.7 rupture east of Oklahoma City near Prague that produced dozens of aftershocks. A study led by Keranen linked it to two injection wells near the epicenter.
The ground shook through much of October near Enid, in northwestern Oklahoma, where some of the state's highest-volume injection wells are located. That followed the September shaking just outside Marietta, Okla., near the Texas border.
From 1975 to 2008, Oklahoma averaged one to three quakes of magnitude 3.0 or greater a year, according to USGS. The number then rose abruptly. In 2009, it had 20 such quakes.
In 2012, Oklahoma ranked second to California, as it did last year.
Seismologists usually group the lower 48 "contiguous" states together, separating out Alaska and Hawaii. If Alaska were counted, Oklahoma would have ranked third in 2012 and 2013.
EnergyWire's review looked at onshore earthquakes of magnitude 3.0 or greater because that is the strength at which most earthquakes can be felt and reliably recorded. There are more seismic monitors on the East and West coasts, so many smaller earthquakes are recorded in those areas.
Other states, such as Arkansas, Colorado and Ohio, have also seen an increase in earthquakes amid the surge in drilling activity. Kansas Gov. Sam Brownback (R) this week formed a task force to look into a series of earthquakes in his state north of Oklahoma (EnergyWire, Feb. 19).
"There is data that point to a possible correlation between fluid injection and seismic activity," Brownback said in announcing the task force.
Texas had 16 earthquakes of magnitude 3.0 or greater, ranking it eighth for 2013. In 2009, it had two quakes of that strength. There have been many more quakes than that around Azle, Texas, but most were below magnitude 3.0.
The shaking in the area northwest of Fort Worth has many people who live in the area complaining that the state regulators at the Texas Railroad Commission haven't looked aggressively enough into the disposal wells in the area.