When Austin Holland was being considered for his job as the sole seismologist at the Oklahoma Geological Survey in 2009, his interviewer posed a wry question: "Are you going to be able to entertain yourself as a seismologist in Oklahoma?"
Back then, the state had a 30-year average of only two earthquakes of magnitude 3.0 or higher per year. As it turns out, though, boredom has been the least of Holland's concerns. Over the last five years, the state has had thousands of earthquakes — an unprecedented increase that has made it the second-most seismically active state in the continental United States, behind California.
The state had 109 temblors measuring 3.0 or greater in 2013 — more than 5,000% above normal. There have already been more than 200 earthquakes this year, Holland said.
Scientists have never observed such a dramatic swarm of earthquakes "in what's considered a stable continental interior," Holland said. "Whatever we're looking at, it's completely unprecedented."
Oklahoma has always had the potential for earthquakes; it has a complex underlying fault system. But until recently, the most powerful quake of the modern era was a 5.5-magnitude temblor in 1952 that left a 15-meter crack in the state Capitol.
Scientists say the more likely cause of the recent increase is underground injection wells drilled by the oil and gas industry. About 80% of the state is within nine miles of an injection well, according to the Oklahoma Geological Survey.
Oklahoma has seen a boom in oil and gas production, including the use of hydraulic fracturing, or fracking — the process of shooting water, sand and chemicals deep into the earth at high pressure to extract oil and natural gas. Scientists from the U.S. Geological Survey and several universities suggest there is a link between the quakes and disposal wells, where wastewater from fracking is forced into deep geological formations for storage.
Industry officials are not ready to definitively accept responsibility but support additional studies of a possible link. "It warrants more research, but a rush to judgment is going to provide no clear understanding of the causes," said Cody Bannister, a spokesman for the Oklahoma Independent Petroleum Assn.
All the shaking has worried residents and prompted one Oklahoma City newspaper to explain earthquake measures. (Magnitude 4 to 4.3: "Vibrations like heavy trucks passing ... glasses clink." Magnitude 5.5 to 6.1: "Difficult to stand ... furniture breaks.")
A magnitude 5.7 earthquake in 2011 damaged homes and buckled a highway here in Prague, about 45 miles east of Oklahoma City. Now, "As soon as people walk in, the first thing out of their mouths is 'earthquake insurance,'" said Jennica Haddox, an insurance policy saleswoman at the local Farm Bureau.
Nerves have been on edge. When the Oklahoma City area was rattled Monday morning by several new earthquakes, one of them as large as a magnitude 4.3, it startled on-air broadcasters. "Earthquake!" KWTV Channel 9 meteorologist Lacey Swope yelled, her eyes wide as the studio lights shook. "I've never felt one that big!"
The Oklahoma Geological Survey, trying to keep up with all the work, hired a second seismologist this year and has added new seismic stations across the state.
Several residents described their shock at feeling their first quake. One man thought a truck had hit his house. Another ran to check his washing machine, thinking it had gone off balance.
"The only thing I'd ever felt that was like an earthquake was the bombing in Oklahoma City in 1995," said Charles Justice, 47, who lives in a mobile home in the town of Guthrie, about 30 miles north of the capital. After an early-morning quake a few months ago, he said, his "first thought was that ... the blocks under the house had fallen."
Oklahoma has about 4,500 active disposal wells, said Matt Skinner, a spokesman for the Oklahoma Corporation Commission, which regulates drilling. With the increase in seismic activity, the commission has asked the state Legislature to approve rules that would increase monitoring and testing requirements for many of the wells.
In September, the commission ordered a wastewater well operator in southern Oklahoma to "drastically reduce his injection pressure and volume" after a swarm of earthquakes nearby. There are no rules requiring seismic studies before wells begin operations, Skinner said.
The focus on drilling has been sensitive in Oklahoma, where 1 in 6 jobs is tied to the industry.
In the town of Marshall, farmers and oil field workers filled the tables at lunchtime on a recent afternoon at Earnheart Oil & Propane, where three wells were visible from the parking lot.
Inside, Dillon Davis, 63, a farmer and oil field worker, recalled a recent swarm of earthquakes that seemed to come every half-hour, cracking a wall in his house and throwing a door off balance. Davis has lived in Marshall, about 50 miles north of Oklahoma City, since 1972. Until the last three years, he had never felt an earthquake.
"Some folks say it's the oil field, but it's just speculation," Davis said.
In a series of recent studies, the U.S. Geological Survey and scientists at the University of Oklahoma and Brown University linked the November 2011 Prague earthquake sequence to drilling operations.
The 5.7-magnitude quake was preceded by a 5.0 foreshock, which was linked to several nearby wastewater injection wells, said Danielle Sumy, the lead researcher of a study about the phenomenon published in the March edition of the Journal of Geophysical Research. The foreshock ruptured the historically inactive Wilzetta fault, also known as the Seminole uplift, triggering the larger quake, she said.
Wastewater fluid injection had been going on for about 18 years before seismicity was felt along the fault, Sumy said. The heavily drilled ground was like a sponge that could hold injected water — to a point.
"You reach a critical threshold where the system can no longer support that much water being injected into the subsurface," Sumy said. The wastewater increases the pressure in the pores, which is sometimes released through an earthquake, she said.
The earthquakes, including a 5.0 aftershock, were at a depth of about 1.8 miles, the same depth as nearby wastewater wells and "shallow for an earthquake that size," Sumy said.
Holland, with the state geological survey, says it's too soon to attribute the increase in earthquakes entirely to injection wells, because there have been so many over a large area in such a short time that researchers have been unable to carefully study all of them. At least one oil industry scientist has suggested that changing water levels in underground aquifers could also trigger quakes.
But the increase has been so unusual that the quakes "do not seem to be due to typical, random fluctuations in natural seismicity rates," the state and federal geological surveys said in a joint statement in May. Researchers have recently linked earthquakes in Arkansas, Texas and Colorado to fluid injection, the agencies said. In April, Ohio announced new restrictions on drilling near faults after a series of earthquakes that state geologists said could have been caused by fracking.
In Prague, locals are trying to adjust to a new normal that includes frequent rattling.
Joe and Mary Reneau live on a 440-acre hay farm less than a mile from the epicenter of the 5.7 quake in 2011.
That night, there was a loud bang, followed by shaking so hard that Mary couldn't stand. Their brick chimney fell through the roof and into their den. It took months to repair the house; since then, earthquakes have cracked their new patio, and they keep finding cracks in walls. A few weeks ago, a new injection well went up half a mile down the road.
"We're not anti-oil," said Joe, 78. "We're pro-oil. We just want them to do it right, and if they're making mistakes, correct them."