The number of earthquakes in Oklahoma declined in 2012 following a two-year spike, said Austin Holland, a seismologist at the Oklahoma Geological Survey in Norman.
Holland moved to Oklahoma to study the sudden swarm of earthquakes that started in late 2009.
Before the spike began, the state averaged about three reported earthquakes felt each year dating back to when records were first kept. An earthquake of magnitude 2.5 to 3.0 is the smallest generally felt by people.
Thirty-eight earthquakes were felt in 2009.
Then 104 earthquakes were reported in 2010 and 97 in 2011, Holland said. The number dropped to 64 in 2012.
The area of Oklahoma with the most earthquakes continues to be centered in Jones in eastern Oklahoma County.
About 35 miles east in Lincoln County, the big one hit Nov. 6, 2011.
The 5.7 magnitude quake — the largest recorded in state history — was centered near Prague along the Wilzetta fault line, Holland said.
That record earthquake reportedly was felt as far away as Illinois and Tennessee.
Earthquakes aren't the deadliest natural disasters in Oklahoma, but they can cause the most damage suddenly over the largest area, said Joey Wakefield, Lincoln County's emergency management director.
Assessing the damage from the 5.7 magnitude quake took much longer than determining tornado damage, Wakefield, said.
There were 460 reports of damage to buildings — broken bricks, cracks in walls, roof damage, chimneys separated from houses — and two homes destroyed, he said.
Holland said scientists at the Oklahoma Geological Survey continue to study Oklahoma earthquakes. Many contributing factors are possible, such as weather or drought specifically, he said.
“We are working on studying many aspects of seismicity,” Holland said.
Hydraulic fracturing, or fracking, can cause small earthquakes for short periods of time, but it does not explain the increase in Oklahoma in recent years, Holland said.
Fracking, a well stimulation technique that is used with high pressure water to crack rock, may have contributed to about 10 percent of earthquakes from 2010 through 2012 and about 10 quakes were 3.0 magnitude or greater, Holland said.
He said fracking did not contribute to the big one Nov. 6, 2011.
Another concern is fluid injection from saltwater disposal wells. Scientists are studying links to earthquakes in Texas and saltwater disposal wells.
Bill Ellsworth, a seismologist at the U.S. Geological Survey's Menlo Park (Calif.) Science Center, said Arkansas also has had a significant increase in earthquakes in recent years. Links to wastewater disposal and earthquakes is being studied there, he said.
Prague City Manager Jim Greff said he continues to notice more small earthquakes, but nothing like the big one that caused all the damage.
“There are lots of little ones all the time,” Greff said. “Be aware, earthquakes can happen at any time.”
Wakefield said most of the damage has been repaired in the Lincoln County area.
“You're not seeing as many blue tarps on roofs and houses these days,” he said.