A recent study provides strong evidence that a November 2011 5.7 earthquake (and related aftershocks) in Oklahoma resulted from fluid injection as part of "unconventional resource" extraction activities. The quake was the largest wastewater-induced earthquake ever recorded
At the site in question, fluid was injected into effectively sealed subsurface compartments. Using aftershocks to illuminate faults that ruptured during a series of earthquakes in Nov. 2011 in Oklahoma, researchers were able to demonstrate that the tip of the initial rupture plane (aka fault) was within 200 yards of active injection wells and within 1 kilometer (0.62 miles) of the surface. 30% of the aftershocks occurred within this sedimentary section. (See http://en.wikipedia.org/wiki/Sedimentary_rock.)
The researchers concluded that, after 18 years of injection, the net fluid volume lowered the effective stress on faults about the subsurface sealed compartments. The researchers also note that decades-long lags between commencement of injection and the onset of induced earthquakes is possible, and thus modifies traditional geological science common criteria for fluid-induced events.
The researchers also note that the progressive rupture of three fault planes indicates that stress changes from the initial rupture triggered the successive earthquakes. Such progressive ruptures fits into commonly accepted geological principals of earthquake aftershocks. See http://en.wikipedia.org/wiki/Aftershock.
Although this earthquake was associated with "traditional" drilling activities, it indicates what may also happen with injection activities undertaken as part of hydraulic fracturing methods.
The article can be found at: http://geology.gsapubs.org/content/early/2013/03/26/G34045.1.abstract.
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