The ground shook in Oklahoma Monday as seven earthquakes struck the state. The frequency of seismic activity in the state has been so high that it now outranks California for earthquakes of magnitude-3.0 or greater.
Sandra Ladra experienced the strongest, a 5.6, that hit the town of Prague in 2011. Ladra told CBS News it was hard to believe what was happening at the time because she had never been in an earthquake like that before. A rock that fell from her fireplace injured her leg and shattered her sense of safety.
"If you don't feel safe in your home, what do you do?" Ladra said.
Since late 2009, the U.S. Geological Survey found Oklahoma has had 300 times more earthquakes than in previous decades. The latest study says 300 million-year-old fault lines have been reactivated and are capable of producing a magnitude-6.0 quake.
"The reason for the study is to identify regions most likely able to produce a magnitude 6.0 earthquake and how that may affect infrastructure in the buildings in the region," said Dan McNamara with the U.S.G.S.
McNamara's research did not focus on a cause, but a 2014 study posted online in "Science" connected thousands of small earthquakes to oil and gas production. The study linked the increase in seismic activity to the process of pumping highly pressurized waste-water into the ground -- a byproduct of fracking.
Kim Hatfield, with the Oklahoma Independent Petroleum Association, says the science to prove a definitive link simply isn't there.
"Coincidence is not correlation," said Hatfield. "This area has been seismically active over eons and the fact that this is unprecedented in our experience doesn't necessarily mean it hasn't happened before."
But it isn't slowing down. Scientists say the number of quakes so far this year is on track to set a new record.