There are also questions about which chemicals are used in the fracking process. Companies might use different materials in the fluids that help break up rock and extract petroleum. And without fully understand why fracking causes earthquakes when it does, we also can't figure out why it doesn't always cause them. Correlation, of course, isn't causation. Why does fracking seem to be linked to an uptick in earthquakes in some places but not in others?

"We don't know exactly why," Williams said. (Some of the other states where seismic activity is on the rise: Arkansas, Texas, and to a lesser extent, Ohio and Colorado.) "There are a lot of questions yet to be answered. But a key point: There are thousands of wastewater wells across Oklahoma and we're seeing this concentration [of earthquakes] in central Oklahoma."