While it appears likely that the observed seismicity rate changes in the middle part of the United States in recent years are manmade, it remains to be determined if they are related to either changes in production methodologies or to the rate of oil and gas production.
We also find that there is no evidence to suggest that hydraulic fracturing itself is the cause of the increased rate of earthquakes. The fact that the disposal (injection) of wastewater produced while extracting resources has the potential to cause earthquakes has long been known. One of the earliest documented case histories with a scientific consensus of wastewater inducing earthquakes, is at the Rocky Mountain Arsenal well, near Denver. There, a large volume of wastewater was injected from 1962-1966, inducing a series of earthquakes (below magnitude 5).
We also have
previously reported that the production of oil and gas (extraction) can potentially cause earthquakes when changes in the underground stresses created by the removal of large volumes of oil, gas or water are large enough.
And, of course, we know that the Earth’s crust is pervasively fractured at depth by faults. These faults can sustain high stresses without slipping because natural "tectonic" stress and the weight of the overlying rock pushes the opposing sides of the fault together, increasing the frictional resistance to fault slip. The injected wastewater in deep wells can counteract the frictional forces on faults, causing an earthquake.
Not all wastewater disposal wells induce earthquakes. Of approximately 150,000 Class II injection wells in the United States, including roughly 40,000 waste fluid disposal wells for oil and gas operations, only a tiny fraction of these disposal wells have induced earthquakes that are large enough to be of concern to the public. Information on wastewater disposal wells and the US Environmental Protection Agency's Underground Injection Control program is available online.
Earthquakes induced by fluid-injection activities are not always located close to the point of injection. In some cases, the induced earthquakes have been located as far as 6 miles from the injection well.