OKLAHOMA CITY (AP) — In Oklahoma, now the country's earthquake capital, people are talking nervously about the big one as man-made quakes get stronger, more frequent and closer to major population centers.
Next door in Kansas, they're feeling on firmer ground, though no one is ready yet to declare victory.
A year ago, the states had a common problem — earthquakes caused by the disposal of wastewater from oil and gas exploration. They chose different solutions.
Kansas, following early scientific studies, decided to restrict how much and how fast the wastewater could be pumped back underground. Oklahoma instead initially concentrated on the depth of the wastewater injections.
Developments since then haven't been reassuring in Oklahoma, where a quake knocked out power in parts of an Oklahoma City suburb several weeks ago and where fears are growing that the worst is yet to come.
A governor's task force is studying the problem, but officials have so far avoided taking tougher measures.
The quakes, which have been mostly small to medium size, have caused limited damage. No one foresees anything like the massive damage and deaths in the famous quakes in California, seismologists say.
Still, "It's a trend that's unsettling," said Cornell University geophysicist Katie Keranen, referring to the increasing number of quakes.
In Oklahoma, earthquakes of magnitude 2.7 and stronger increased by about 10 percent between the last half of 2014 and the last half of 2015, according to a data analysis by the Associated Press.
In Kansas, earthquakes of that magnitude went down by 60 percent in the same period.
According to earthquake experts, the pattern fits recent peer-reviewed studies suggesting injecting high volumes of wastewater could aggravate natural faults.
In Oklahoma's six most earthquake-prone counties, the volume of wastewater disposal increased more than threefold from 2012 to 2014.
For decades, drilling companies have disposed of oilfield wastewater by pumping it back underground.
But in recent years, improved technology has allowed for injecting more wastewater faster so more oil and gas can be produced.
Around here, above the Arbuckle geologic formation of limestone, water under pressure can set off a fault if there's enough tension, according to interviews with 10 earthquake experts.
In March 2015, Kansas regulators ordered a dramatic reduction in injection volumes in the most vulnerable area.
That same month, Oklahoma regulators directed the operators of 347 wells to check the depth of their injections, then three months later issued a broader order to avoid the Arbuckle's "basement." But by the end of November, the state had asked for volume cutbacks in fewer than 90 of the about 1,000 wells in a key area.
One Oklahoma official said it was not proper to compare Oklahoma to Kansas, which has fewer wells and less wastewater. But oil and gas operators in Oklahoma acknowledge their resistance to cutting back on their injections of wastewater.