Why is Oklahoma experiencing a spike in earthquakes?

Over the past six years, earthquakes in Oklahoma have skyrocketed – from less than a handful of 3.0 earthquakes before 2009 to well over 900 last year. The likely culprit: salty wastewater that bubbles up during oil and gas drilling. The rash of quakes has led to tough questions for the energy industry that provides one in five jobs in the state and comprises nearly a third of Oklahoma’s economy.