National Geographic reports that "...much has changed in Oklahoma, which leads the continental United States in earthquakes so far this year. From 1978 to 2008, Oklahoma experienced an average of one earthquake a year of magnitude 3 and higher, according to the U.S. Geological Survey. As of last week, the state experienced 258 earthquakes in that range, almost twice as many as California."
"A growing body of research has tied the spike to wastewater injection, a process in which water from oil and natural gas extraction, including fracking, is pumped into underground wells for disposal. Research has also tied wastewater injection to quakes in Arkansas, Colorado, Ohio, and Texas."
"But none of those states have seen as many earthquakes as Oklahoma. Because the oil and gas industry is a major employer in the state, the possibility that drillers might be responsible for the earthquake surge has put industry on the defensive and residents on edge, while sending state and local governments scrambling to respond."