In some places, notably Ohio and Oklahoma, the injection of used fracking fluid in deep disposal wells appears to have produced a significant uptick in earthquake activity. The earthquakes are mostly much too small to be felt at the surface, but a magnitude 5.6 quake in Oklahoma was large enough to cause some damage in 2011.
This has made lots of news because of its scale, but it’s not our first experience with injection-triggered earthquakes. It’s a concern for geothermal power designs that inject water to depths where it can turn to turbine-driving steam, for example. And in the future, it could be a concern for efforts to store carbon dioxide in underground reservoirs.
Earthquakes occur where two blocks of rock suddenly slip past each other along a fault, releasing energy that causes the shaking that bothers us up at the surface. The blocks are generally stuck in place by friction, but the strain of being pushed (or pulled) in different directions slowly builds. Eventually, that strain overcomes the friction keeping it in place and the rocks slip some distance along a portion of the fault, relieving strain.
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