Customers who stop by Mike Kahn's insurance agency in Oklahoma City are increasingly looking to buy a policy that was unheard of a decade ago: earthquake insurance.

Kahn, who opened the Lynnae Insurance Group in 2002, said he sold earthquake coverage to two homeowners during the first decade he was in business. During the past six months, he sold more than 125 policies.

"We used to get to that part of the policy, and I'd tell customers, 'You don't need that. This is Oklahoma,'" Kahn said, referring to the days when earthquake coverage was an add-on to a homeowner policy. "We used to laugh about it."

But much has changed in Oklahoma, which leads the continental United States in earthquakes so far this year. From 1978 to 2008, Oklahoma experienced an average of one earthquake a year of magnitude 3 and higher, according to the U.S. Geological Survey. As of last week, the state experienced 258 earthquakes in that range, almost twice as many as California.

A growing body of research has tied the spike to wastewater injection, a process in which water from oil and natural gas extraction, including fracking, is pumped into underground wells for disposal. Research has also tied wastewater injection to quakes in Arkansas, Colorado, Ohio, and Texas.

But none of those states have seen as many earthquakes as Oklahoma. Because the oil and gas industry is a major employer in the state, the possibility that drillers might be responsible for the earthquake surge has put industry on the defensive and residents on edge, while sending state and local governments scrambling to respond. (Related: "Scientists Warn of Quake Risk From Fracking Operations.")

"The anger is palpable," said John Wood, a member of the city council in Guthrie, Oklahoma, a small city near the epicenter of a 4.3 magnitude quake that struck in mid-July, one of seven earthquakes that hit in the state in a two-day period. "Our bread and butter is oil and gas, so we have been very slow to question the industry on quakes. But it's becoming a daily occurrence."

View Images EMILY M. ENG, NG STAFF. SOURCE: OKLAHOMA GEOLOGICAL SURVEY

Although the majority of the quakes have been harmless, they range from temblors that are barely felt to a 5.7 magnitude quake in 2011 that damaged homes. People can feel vibrations of a magnitude 3 temblor. A magnitude 4 quake feels like a heavy truck striking a building. At magnitudes 5 to 6, according to the USGS, dishes break, heavy furniture moves, chimneys fall, and a poorly built home can sustain serious damage.

Energy Industry Link?

The surge in Oklahoma earthquakes dates to 2008, and a recent study published in the journal Science bolstered the theory that they have been spurred by injecting wastewater produced during oil and gas development, including fracking, deep into underground wells.

Katie Keranen, a Cornell University seismologist and lead author of the study, also published a 2013 study in the journal Geology that tied wastewater injection to the largest earthquake in Oklahoma history, a magnitude 5.7 earthquake that in 2011 struck the town of Prague, in central Oklahoma. (Related: "Scientists Say Oil Industry Likely Caused Largest Oklahoma Earthquake.")

Despite the growing evidence, Chad Warmington, president of the Mid-Continent Oil and Gas Association of Oklahoma, said it's too early to know if injection wells are responsible or if the rise in earthquakes is part of a natural cycle.

"There is going to be studies that say lots of things," Warmington said of Keranen's latest findings, adding that the oil and gas industry is working with regulators and scientists to share data and make sure they are not putting citizens at risk. "We are concerned about it because we live here, but we don't want to have a knee-jerk reaction and have a bunch of regulation put on us that is not effective in minimizing the risk of seismic activity."

The Oklahoma Corporation Commission, the state's oil and gas regulatory agency, also says a direct link between wastewater injection and the rise in earthquakes in the state could not be established.

Ernest Majer, a geophysicist and induced seismicity expert at the Lawrence Berkeley National Laboratory in California, said injection wells rarely cause earthquakes. Majer said the oil and gas industry is as interested as scientists are to learn what conditions cause induced earthquakes and how to prevent them, with an eye toward avoiding increased regulation.

"The great majority are waking up to the question: Do they want to be regulated, or do they want to have best practices to follow?" he said.

Concern for Bridges, Property

As scientists, regulators, and the oil and gas industry search for the causes of the earthquake spike, Oklahoma's state and local governments have begun adjusting to the new seismic reality.

The state began dispatching crews to inspect bridges after magnitude 3 and higher earthquakes in 2010. The department recently changed the inspection level to magnitude 4 and over, after consulting with California officials about earthquake response policies, according to Terri Angier, a spokesperson for the Oklahoma Department of Transportation.

Although Oklahoma is consistently ranked at or near the bottom of lists ranking the country's worst bridges, Angier said inspectors have found only minor bridge damage after earthquakes. But she is concerned about the future and the potential for larger seismic events: "If there is a crisis for us, it's that we're not sure where it's headed."

That concern is echoed in Oklahoma City. Kristy Yager, a spokesperson for the city, said officials are updating the city's disaster plan, which focuses heavily on the tornadoes that traditionally plague the state, to prepare for earthquakes.

So far, residents have been concerned primarily about the potential for property damage, Yager said, adding that Oklahomans are worried about the prospect that the oil and gas industry might be responsible for the earthquakes.

"We hope it's not a byproduct of oil and gas because so many of our jobs depend on it," Yager said. "It's a difficult position to be in."

Uncertainty in Forecasting

Despite the uptick in Oklahoma seismicity, a recently updated USGS earthquake hazard map does not reflect an increased threat for dangerous quakes in the state. That's because the map, which is updated every six years, does not take into account earthquakes suspected of being induced by wastewater injection or other human activity.

Although Oklahoma is leading the lower 48 states in earthquakes so far in 2014, it is not included among 16 states the USGS says are at the highest risk for hazardous earthquakes.

Austin Holland, a seismologist at the Oklahoma Geological Survey, thinks the state's earthquakes should be included in the USGS hazard map, which is used to set standards for seismic safety in building codes.

"By removing them," he said, "we are underestimating the potential of serious seismic hazard in Oklahoma."

But scientists at USGS say it's difficult to predict future earthquake hazard based on recent temblors that may have been induced by the oil and gas industry.

William Ellsworth, a research geophysicist at the USGS Earthquake Science Center, said his organization uses knowledge of faults and data from earthquakes that occurred over thousands of years to develop hazard maps. The organization set aside earthquakes suspected of being induced by the oil and gas industry because it is unclear why they are occurring and how long they may continue.

If earthquakes are caused by a specific injection well, for example, and the oil and gas industry stops using that well, the seismic activity would likely stop, Ellsworth said, thereby ending the hazard.

Nonetheless, Ellsworth said the USGS is developing a hazard model that takes induced earthquakes into account. "Everyone here thinks quakes, regardless of origin, need to be accounted for in our hazard model," he said.

So far, the largest impact of the Oklahoma quakes might be on the insurance industry. Kelly Collins, a spokesperson for the Oklahoma Insurance Department, said the number of homeowners in the state with earthquake coverage has risen from around 3 percent in 2011 to 15 percent in 2014, according to an informal survey of the top five insurance companies.

In Oklahoma City, insurance agent Mike Kahn recently bought an earthquake policy after quakes knocked a piece of facade from his home and twice crumbled a neighbor's chimney. The policies have a 10 percent deductible, so they are meant for a quake far larger than what Oklahoma has seen in recent years.

And that prospect, Kahn said, is the fear.

"It's scary when they happen in the middle of the night," he said. "It's a weird feeling to feel your house shaking. Your heart is racing, you are running down the hall to check on your kids, then you run back and check on your wife. They make your heart skip."