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. –