The rate of earthquakes in Oklahoma has increased by about 50 percent since October 2013, significantly increasing the chance for a damaging quake in the central part of the state, federal and state agencies said Monday.
The U.S. Geological Survey and Oklahoma Geological Survey reported that 183 earthquakes of magnitude 3.0 or greater occurred from October 2013 through April 14. This compares with an average of only two magnitude 3.0 or larger earthquakes per year from 1978 to 2008.
"As a result of the increased number of small and moderate shocks, the likelihood of future, damaging earthquakes has increased for central and north-central Oklahoma," a statement from the agencies said.
The agencies' statement indicates that a likely factor in the increase in earthquakes is wastewater injection.
"The water injection can increase underground pressures, lubricate faults and cause earthquakes — a process known as injection-induced seismicity. Much of this wastewater is a byproduct of oil and gas production and is routinely disposed of by injection into wells specifically designed and approved for this purpose," the report stated. "The recent earthquake rate changes are not due to typical, random fluctuations in natural seismicity rates."
Dr. Bill Leith, USGS senior science adviser for Earthquakes and Geologic Hazards, said he hopes residents, schools and businesses in central Oklahoma will consider the warning when looking at earthquake preparedness.
"Building owners and government officials should have a special concern for older, unreinforced brick structures, which are vulnerable to serious damage during sufficient shaking," Leith said.
The U.S. Geological Survey says Oklahoma is now the second most-active state seismically, behind California.
Brian Woodward, the vice president of regulatory affairs for the Oklahoma Independent Petroleum Association, said the state's top industry is working with researchers but that the recent increase in earthquake activity cannot necessarily be blamed on it.
"Granted, we've not seen this level of seismic activity in Oklahoma in the last 60 to 80 years and before that we don't have a record. It causes us all concern, but the rush to correlate this activity with our industry is something we don't believe is necessarily fair," he said.
A study published in March in the Journal of Geophysical Research suggested that the sharpest earthquake to strike Oklahoma, a magnitude 5.7 quake centered near Prague, may have been triggered in part by wastewater injection — which if true, would make the 2011 temblor the strongest ever linked to disposal practices within the oil and gas industry. The quake caused at least $4.5 million in damages, including knocking over four spires at a university 17 miles away and shaking a college football stadium that moments earlier had held more than 57,000 fans. Fourteen homes suffered significant damage and two people near the epicenter suffered minor injuries.
State Farm Insurance spokesman Jim Camoriano said 16 percent of the company's Oklahoma policyholders are covered for earthquakes.
As a result of the increased seismicity, the Oklahoma Geological Survey has increased the number of monitoring stations and now operates a seismograph network of 15 permanent stations and 17 temporary stations. The state and federal agencies are involved in research to determine the cause of the increased earthquake rate and to quantify the increased hazard in central Oklahoma.