A joint statement by the U.S. Geological Survey and Oklahoma Geological Survey says that 183 earthquakes of magnitude 3.0 or greater occurred in Oklahoma from October 2013 through April 14, 2014. The long-term average from 1978 to 2008 showed only two magnitude 3.0 or larger earthquakes per year. The increased number of small and moderate shocks has led them to predict a higher likelihood of future, damaging earthquakes for central and north-central Oklahoma.
Oklahoma’s heightened earthquake activity since 2009 includes 20 magnitude 4.0 to 4.8 quakes, plus one of the two largest recorded earthquakes in Oklahoma’s history – a magnitude 5.6 earthquake that occurred near Prague on Nov. 5, 2011, which damaged a number of homes and the historic Benedictine Hall at St. Gregory's University in Shawnee.
As a result of the increased seismicity, the Oklahoma Geological Survey has increased the number of monitoring stations and now operates a seismograph network of 15 permanent stations and 17 temporary stations. Both agencies are actively involved in research to determine the cause of the increased earthquake rate and to quantify the increased hazard in central Oklahoma.
The joint statement says that the increase in earthquakes may be linked to wastewater disposal by injection into deep geologic formations - that's traditional drilling, not hydraulic fracturing, commonly called fracking. Water injection can increase underground pressures, lubricate faults and cause earthquakes – a process known as injection-induced seismicity.
The recent earthquake rate changes are not due to typical, random fluctuations in natural seismicity rates.
Information about earthquake preparedness can be found at the following websites: http://www.ready.gov/earthquakes and http://www.shakeout.org/centralus/.