WASHINGTON, June 6 (UPI) -- Four minor earthquakes were reported in and around the shale reserve basins in Oklahoma in the last 24 hours, the U.S. Geological Survey found.
The USGS recorded three minor tremors, including two near the metropolitan area of Oklahoma City, inside state borders and one less than 100 miles across the border in Kansas. The largest of the tremors was the 3.5-magnitude quake recorded about 20 miles east from Oklahoma City, the state's capital.
A March report from the USGS found central U.S. states have experienced a dramatic increase in seismic activity over the past six years. Between 2009 and 2015, there have been an average 318 quakes in the region, against an average of 24 in the 35 years ending in 2008. The average rate last year was 1,010.
Regionally, the report said the disposal of oil and gas-related wastewater is the "primary reason" for the increase in seismic activity in the central United States. Many states, including Oklahoma, have active management plans for wastewater injection as a way to curb tremors.
Oklahoma is the No. 6 oil producer and No. 3 natural gas producer in the country.
Oklahoma Gov. Mary Fallin in January approved $1.38 million in costs to back earthquake research in the state. State energy regulators, meanwhile, have called on energy companies to reduce the amount of wastewater disposed in underground wells in an effort to curb seismic activity.
Last month, the Oklahoma Insurance Department called for state hearings after fielding complaints that just four insurance companies claim more than half of the state market for insurance coverage for earthquakes.
With tremors becoming more frequent, the department said insurers have started raising their rates, cutting lower deductible plans or halting the issuance of new policies altogether.
No major damage was reported from the recent tremors.