There aren’t supposed to be earthquakes swarms in Oklahoma, and they were happening all the time. In 2016, there were 632 magnitude 3+ or higher temblors. In 2015, there were 903, 579 in 2014, and 109 in 2013. To say this got the attention of scientists is putting it mildly.
http://bigthink.com/robby-berman/earthquakes-have-to-do-with-one-very-specific-part-of-fracking