Frequent, rumbling tremors beneath the earth may be a common occurrence in California, but Oklahoma has now surpassed the state in the number of earthquakes felt this year-a trend that is surprising geophysicists and raising concern.
"It is actually very surprising to us as well," Pasadena, California-based U.S. Geological Survey Geophysicist Elizabeth Cochran said.
Between 1975 and 2008, only an average of two earthquakes with a magnitude of 3.0 or greater would occur each year in Oklahoma.
Since 2009, the number of quakes has climbed exponentially, and there are no signs of slowing.
"There has been a dramatic increase in that time," Cochran said. "We're seeing a very steep increase with 183 quakes occurring in a six-month period."
Between October 2013 and April 2014, 183 earthquakes occurred with a magnitude of 3.0 or higher, and continue to occur with an average depth of 3 miles.
Between 1975 and 2008, a total of 40 earthquakes with a magnitude of 3.0 or greater occurred, according to a USGS study.
Cochran said the rate will continue to climb in the future based on the current data.
Changes in pressure near faults in Oklahoma that surpass the fault's critical pressure threshold are likely the cause of the seismic activity surge.
The cause for this increase does not appear to be natural and has likely been induced by fluid injection, part of the crude oil and gas industry's disposal of wastewater through the creation of wastewater wells.
The injection of wastewater fluids deep into the ground can lubricate existing, dormant faults and change the stress and pressure of the fault, leading to increased seismic activity, Cochran said.
"These quakes tend to be shallower," she said. "At this point, we do not think this is a natural variation."
According to a recent research study Cochran cited, not only has the rate of occurrence increased, but the rate in which earthquakes have the potential to ignite other quakes.
"There is a fundamental change in the seismicity and statistically they differ from natural earthquake sequences," she said.
While there are currently a number of studies being conducted on the increased activity in Oklahoma, earthquakes caused by injection tend to occur more frequently in areas than they once did, have a shallower depth than tectonic quakes that occur at the basement rock, along with epicenters in proximity to wastewater wells, she said.
"These are the key things we look at," Cochran said. "[Fluids] may run into a fault and decrease the strength of that fault, essentially allowing it to slip.
According to Cochran, there are thousands of wastewater injection sites across the state, and many more across the country, but understanding the physics behind why some areas have seen dramatic increases and others have not depends on understanding a variety of variables.
Cochran said the rate in which injection occurs, the volume of fluid injected and the existing state of the geology may all play a role.
The largest event in the state to occur in the past century was a 5.6-magnitude quake that struck Oklahoma in November 2011.
"That was the largest event linked to injection," Cochran said.
With the growing number of quakes in the region, the hazards associated with an even larger event occurring near a major metropolitan area have geophysicists concerned.
"That is our big concern," Cochran said. "There is an increased probability of a larger event occurring."
With the increased frequency of quakes in the region, the potential for a larger, damaging earthquake is more and more likely, she added.
"We're just lucky that it [5.6 quake] did not occur in a major metropolitan area," she said, adding it struck about 45 miles from Oklahoma City.
Earthquakes cannot be predicted in advance and can occur without warning.
"The best we can do is make a long term forecast," she said.