Oklahoma, a state known more for tornadoes than seismic shifts, is becoming all too familiar with earthquakes.
In the week ending Saturday, 48 quakes larger than the magnitude of 2.5 had struck, says Paul Caruso, a geophysicist with the U.S. Geological Survey (USGS).
In the past month, there have been 157 quakes larger than magnitude 2.5.
"We've never seen anything like this in Oklahoma," Caruso says.
The numbers back him up.
"Since 2009, the earthquake activity in Oklahoma has been approximately 40 times higher than in the previous 30 years," says a February report from the Oklahoma Geological Survey (OGS), a group that first began monitoring earthquakes 40 years ago.
From 1972 to 2008, about two to six earthquakes a year in Oklahoma were recorded by the USGS National Earthquake Information Center.
In 2009, about 50 earthquakes were recorded, and the numbers have continued to climb.
In 2011, the state had magnitude-4.7 and -5.6 earthquakes in November. The magnitude-5.6 event was "the largest quake to hit Oklahoma in modern times," the USGS says. At least two people were injured, 14 homes were destroyed and many were damaged, it says.
All the recent shaking has rattled residents' nerves — and has led to speculation, and debates, about what's behind the quakes. Some people have taken to social media and other public forums, contending that hydraulic fracturing, also known as fracking, and the deep-injection wells used for disposal of fracking wastewater could be causing the uptick.
The February OGS report says it "has not ruled out that some earthquakes may have a relationship to oil and gas activities such as water disposal/injection, and examining these issues remains a major focus of ongoing research."
Yet the report adds that "the majority, but not all, of the recent earthquakes appear to be the result of natural stresses, since they are consistent with the regional Oklahoma natural stress field."
An October statement from the USGS, made in partnership with the OGS, says "a contributing factor to the increase in earthquake triggers may be from activities such as wastewater disposal — a phenomenon known as injection-induced seismicity."
But at this point, "there is nothing conclusive" regarding the cause of increased earthquakes, says Priyank Jaiswal, an assistant professor at Oklahoma State University's Boone Pickens School of Geology.
While studies on possible causes are being conducted, he advocates for more education for the public to understand the basics of oil and gas drilling, fracking and deep-injection wells.
Whether the quakes are natural or human-induced, people need to understand the potential causes — and that that there could be more quakes to come, he says.
"When our homes shake, we are concerned," he says.