A second M=4.7 earthquake shock struck northern Oklahoma at 3:49 am local time, at a shallow depth of 5 km (3 mi). The shock was felt as far away as Oklahoma City and Tulsa in Oklahoma, and Wichita and Dodge City in Kansas. The first M=4.7 struck on November 19 about 40 km (25 mi) to the west. It is unlikely that the two quakes are related, except that both are associated with deep wastewater injection associated with oil exploration and production activities.
In the past decade, the rate of earthquakes in Oklahoma has increased by a factor of 30 over the previous 35 years, as reported by William Ellsworth of the USGS in Science (2013). Today, the M≥3 quake rate in Oklahoma exceeds that of California. A new study by Susan Hough and Morgan Page, both at the USGS, and published in Bulletin of the Seismological Society of America, finds that most of the Oklahoma quakes since the 1930’s were themselves associated with wastewater injection, and so the factor of 30 underestimates the increase over the natural quake rate. In addition to the 2011 M=5.7 Prague, Oklahoma earthquake, the largest quake in the area was the 1952 M=5.7 El Reno, Oklahoma earthquake.
There is an active fault in Oklahoma, and so the background rate of quakes—those not associated with oil field operations—cannot be zero. The Meers fault can be seen on Temblor maps. It produced large earthquakes about 1,000 and 2,000-3,000 years ago. Today’s M=4.7 quake was a ‘strike-slip’ event, the same kind of motion as occurs on the Meers fault. Further, injection of wastewater primarily lubricates natural faults, and so these faults must already be stressed by tectonic forces to be able to trigger an earthquake. In this sense, oil field operations are greatly accelerating a natural process rather than replacing it by a man-made one.