Shale-rich Oklahoma experienced more than a dozen seismic events in the five days ending Friday, data from the U.S. Geological Survey show.

A magnitude-2.7 tremor was reported shortly after midnight local time Friday near the north-central town of Enid. The largest of the 16 seismic events since Monday was a magnitude-3.6 quake reported Wednesday in north-central Alva. Several towns in and around the region recorded more than one quake in the last week.

On Wednesday, after the USGS recorded 9 recent events in the Blanchard area, the Oklahoma Corporation Commission's Oil and Gas Division said it was investigating all of the oil and gas activity in the area.

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.

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. Last week, oil services company Baker Hughes recorded 59 rigs in service in the state. Rig numbers serve as a loose barometer to gauge the health of the energy sector and, for Oklahoma, the number of rigs in service is 45 percent lower year-on-year.

USGS data show 888 tremors hit Oklahoma last year, a 51 percent increase from the previous year. The state recorded 34 seismic events in 2012.

Oklahoma Gov. Mary Fallin in January approved $1.38 million in costs to back earthquake research in the state