Because both beliefs are founded in the same kind of sloppy, simplified guilt-by-association reasoning applied to a complex problem.
Fivethirtyeight.com has a decent, not perfect*, description of Oklahoma’s earthquake dilemma. Now, I’m a thousand miles away and only mildly educated with the particulars, but it doesn’t take a genius to see that the problem is centered in the north-central part of the state where companies like SandRidge Energy were chasing the Mississippian Lime. (Note: not the Mississippian Shale.)
What characterizes the Mississippian Lime?
A disappointingly low oil cut, very high decline rates, high well costs in a fairly low-populated area and, perhaps most of all, the issue of saltwater in the geology. In some locations, the ratio of hydrocarbons to saltwater coming out of the well was as high as 1:10. Dealing with all that saltwater can be a huge deterrent, especially when other plays are yielding oilier cuts without the saltwater.
Shales, for the most part, don’t make lots of water. The Mississippian Lime does. The profitability of oil wells revolves around how cheaply you can get rid of all that excess water. (To be clear, the 10:1 ratio means that for every 100 bbls of oil, you have to get rid of 1,000 bbls of salt water.)
When you have a lot of water to put away, you pump it down a salt water disposal (SWD) well. You can put even more if you pump it away with pressure. Presumably, the State of Oklahoma regulates all this, but historically it hasn’t been much of an issue.
It’s an issue now, because the zone they put it away in, the Arbuckle, sits right on top of “basement”, Precambrian granite. Ancient faults in the granite can become lubricated and slip when pressured water is injected into them. (As I have noted before, they would slip if pressured mother’s milk were injected into them in sufficient volumes.)
Thus the problem. Faults that have probably been asleep for hundreds of millions of years have been awakened. Whether or not the production wells were fracked is really pretty immaterial, and blaming fracking for the earthquake problem blissfully ignores thousands of fracked wells that are nowhere near earthquake zones.
Yes, the volume of water is a real problem. Fracking is only circumstantially involved. Sorry to disappoint.
* “Not perfect” because the author asserts:
There’s … evidence that older oil and gas wells — like the kind you’d find in a state like Oklahoma, which has a long history of drilling — produce more wastewater than newer wells, such as North Dakota’s.