From The New Yorker:
Until 2008, Oklahoma experienced an average of one to two earthquakes of 3.0 magnitude or greater each year ... In 2009, there were twenty. The next year, there were forty-two. In 2014, there were five hundred and eighty-five, nearly triple the rate of California. Including smaller earthquakes in the count, there were more than five thousand. This year, there has been an average of two earthquakes a day of magnitude 3.0 or greater.
So, what's happening? You can read the article here.