A pair of small earthquakes struck north-central Oklahoma about an hour apart early Thursday morning, according to the U.S. Geological Survey (USGS).

According to the USGS, the first temblor struck at 5:36 a.m. CDT and measured 3.6 in magnitude on the Richter scale. The second, measuring 2.6 magnitude, came 56 minutes later, at 6:32 a.m. CDT. The epicenter for both quakes was located 13 kilometers (eight miles) south-southeast of Medford, OK.

No injuries or damage was reported. The first earthquake is the largest, in terms of magnitude, recorded by the USGS in Oklahoma since a 4.1 magnitude temblor struck 10 kilometers (6.2 miles) east of Medford on March 29.

Although it was unclear what caused the two earthquakes, regulators with the Oklahoma Corporation Commission (OCC) and its Oil and Gas Conservation Division (OGCD) have been attempting to mitigate induced seismic activity across the state for the last two years. They have especially been targeting wastewater injection wells targeting the Arbuckle formation.

Scientists with the Oklahoma Geological Survey attribute many of the recent quakes to the disposal of extremely salty water, a byproduct of oil and gas production, via underground injection wells.