EDMOND, Okla. (AP) - Earthquakes that have shaken Oklahoma communities in recent months have damaged homes, alarmed residents and prompted lawmakers and regulators to investigate what's behind the temblors - and what can be done to stop them.
Hundreds of people are expected to turn out in Edmond, Oklahoma, on Thursday night for a town hall meeting on the issue.
Earthquakes used to be almost unheard of on the vast stretches of prairie that unfold across Texas, Kansas and Oklahoma, but they've become common in recent years.
Oklahoma recorded nearly 150 between January and the start of May. Most recently, the U.S. Geological Survey recorded a magnitude 3.6 earthquake southwest of Guthrie early Thursday morning.
Though most have been too weak to cause serious damage or endanger lives, they've raised suspicions that the shaking might be connected to the oil and gas drilling method known as hydraulic fracturing, especially the wells in which the industry disposes of its wastewater.
Now after years of being harangued by anxious residents, governments in all three states are confronting the issue, reviewing scientific data, holding public discussions and considering new regulations. The states are trying to reconcile the scientific data with the interests of their citizens and the oil and gas industry.
Oklahoma state Rep. Jason Murphey, a Guthrie Republican, said though the damage from quakes hasn't been serious, it's still a big problem for his constituents. He said residents have reported cracks in interior and exterior walls, doors that no longer close properly, trim that is separating and even foundation problems.
"Those types of reports are becoming commonplace," Murphey said.
Murphey said many of his constituents believe there's a relationship between the earthquakes and injection wells that are used to dispose of wastewater from oil and natural gas drilling operations.
Austin Holland, a research seismologist with the Oklahoma Geological Survey, said the agency is closely monitoring the area's seismic activity to determine whether the earthquakes are a natural phenomenon or are man-made.
"It's one thing to have suspicions. It's another thing to demonstrate that scientifically," Holland said. "We have a lot of faults in Oklahoma."
Seismologists already know that hydraulic fracturing - which involves blasting water, sand and chemicals deep into underground rock formations to free oil and gas - can cause microquakes that are rarely strong enough to register on monitoring equipment.
However, fracking also generates vast amounts of wastewater, far more than traditional drilling methods. The water is pumped into so-called injection wells, which send the waste thousands of feet underground. No one knows for certain exactly what happens to the liquids after that. Scientists wonder whether they could trigger quakes by increasing underground pressures and lubricating faults.
Another concern is whether injection well operators could be pumping either too much water into the ground or pumping it at exceedingly high pressures.
Matt Skinner, a spokesman for the Oklahoma Corporation Commission, which regulates the oil and gas industry in the state, said the agency is monitoring the activity of every injection well in a seismically active area.
"We're looking for anomalies," Skinner said. "This is not an abstract exercise in policymaking. The reason that we're all here is that it's frightening."
In Texas, residents from Azle, a town northwest of Fort Worth, who have endured hundreds of small quakes, went to the state Capitol earlier this year to demand action by the state's chief oil and gas regulator, known as the Railroad Commission. The commission hired the first state seismologist, and lawmakers formed the House Subcommittee on Seismic Activity.
After Kansas recorded 56 earthquakes between last October and April, the governor appointed a three-member task force to address the issue.
Associated Press writers Emily Schmall in Azle, Texas, and Kristi Eaton in Oklahoma City contributed to this report.