These human-induced earthquakes are not related to hydraulic fracturing, but to very deep disposal wells that inject waste fluids—formation brine, excess fracking fluid and the like. Over 100,000 injection wells of this type are operating in the United States. Almost all of them make no problems: they do their thing far away from water supplies and are monitored closely. But a handful of wells, maybe a dozen, have gone on to produce unexpected earthquakes.