Three minor seismic events were recorded near the shale reserve areas in Oklahoma in the past 24 hours, data for the U.S. Geological Survey show.

USGS data show the strongest of the three seismic events was a magnitude-2.9 tremor recorded shortly after midnight in southern Kansas, a few miles from the state border with Oklahoma. A magnitude-2.5 event was recorded in Fairview, Okla., early Sunday afternoon.

State researchers said wastewater injection, not from hydraulic fracturing operations but producing wells, poses the largest potential risk for seismic events in Oklahoma. A 15,000 square-mile "area of interest" was established to monitor recent seismic events and their association with the state oil and gas sector.

State regulators last week ordered the closure of two wells included in the area of concern in an effort to reduce the seismic activity associated with wastewater. The order followed a magnitude-4.0 tremor recorded Wednesday in the area.

In a letter to operators working in the area, the state Oil and Gas Conservation Division called for the installation of special flow meters and regular reports of volumes and pressures on wells.

A March report from the USGS found central U.S. states have experienced a dramatic increase in seismic activity over the past six years. 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.

Oklahoma is one of the largest oil producers in the United States, hosting some of the more lucrative shale basins in the country. Federal data show state oil production has been on the rise since around 2010. Output in May, the last full month for which federal data are available, was 4.7 percent higher than the previous month.