More and bigger drilling-linked earthquakes rattle Oklahoma

Several earthquakes shook Oklahoma on Monday as the state experiences a sharp increase in the frequency of tremors linked to wastewater disposal from gas and oil drilling, including from fracking, state and federal officials said.

Three of Monday's quakes measured above a magnitude 4.0, with a 4.5 earthquake centered just north of Crescent, roughly 45 miles (72 km) north of Oklahoma City, the U.S. Geological Survey (USGS) said.

There were no reports of damage.

The rate of earthquakes in Oklahoma has increased by about 50 percent since 2013, greatly increasing the chance for a damaging quake, according to the USGS.

Noticeable quakes - above magnitude 3.0 - now hit the state at a rate of two per day or more, compared with two or so per year prior to 2009. During the past seven days, Oklahoma has experienced about 40 earthquakes, according to the USGS.

Scientists say the seismic activity is triggered by the injection of wastewater from booming oil and gas drilling operations into deep geological formations.

The state's oil and gas regulator released a directive this month expanding "Areas of Interest", parts of the state that have been worst-hit by the quakes, and adding restrictions for 211 disposal wells.

In March, the regulator - the Oklahoma Corporation Commission - also directed 347 wells to reduce their injection depths to above the Arbuckle formation. High-volume injections into the Arbuckle, the state's deepest formation, have the highest potential for seismic activity, according to the USGS.

Twenty-one of Oklahoma's 77 counties are under the order, and oil and gas drilling operators have until Aug. 14 to comply with reducing injection depth.