(TIME, February 18, 2014)
An unusually high number of tremors have shaken the state lately, leading some to point their fingers at the emerging hydraulic fracturing industry, though the real culprit might be a type of wastewater storage system.
No strangers to nature’s fury, Oklahomans grow up accustomed scorching heat, blizzards, wrecking-ball thunderstorms and tornadoes. What they don’t see a lot of are earthquakes, which have been rattling the Sooner State with rare frequency of late — at least 115 earthquakes of varying intensities in the first two months of 2014.
The question on everyone’s mind is: why? The area has been seismically active since time immemorial but the latest swarm of earthquakes is unheard of. According to earthquake monitors EQ Charts, between 1990 and 2008 there were between 0 and 11 earthquakes of magnitude 2.0 or greater in Oklahoma every year. In 2009 there were 49. In 2010 there were 180. In 2013 there were 291, and so far in 2014 there have been 59-plus and counting. More than a dozen notable earthquakes have shaken north-central Oklahoma in the past three days.
Spent drilling water injected back into the ground for storage at high pressure, some scientists believe, may be forcing fault lines under pressure to shift. Katie Keranen, a geophysics professor at Cornell, says “the evidence is strong” that the earthquakes are caused by fracking and wastewater disposal, both of which have become more frequent amid today’s boom in oil and gas drilling.
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Significant earthquakes are increasingly occurring within the continental interior of the United States, including five of moment magnitude (Mw) ≥ 5.0 in 2011 alone. Concurrently, the volume of fluid injected into the subsurface related to the production of unconventional resources continues to rise.
Waste water injection well.
Here we identify the largest earthquake potentially related to injection, an Mw 5.7 earthquake in November 2011 in Oklahoma. The earthquake was felt in at least 17 states and caused damage in the epicentral region. It occurred in a sequence, with 2 earthquakes of Mw 5.0 and a prolific sequence of aftershocks. We use the aftershocks to illuminate the faults that ruptured in the sequence, and show that the tip of the initial rupture plane is within ∼200 m of active injection wells and within ∼1 km of the surface; 30% of early aftershocks occur within the sedimentary section.
Subsurface data indicate that fluid was injected into effectively sealed compartments, and we interpret that a net fluid volume increase after 18 yr of injection lowered effective stress on reservoir-bounding faults. Significantly, this case indicates that decades-long lags between the commencement of fluid injection and the onset of induced earthquakes are possible, and modifies our common criteria for fluid-induced events. The progressive rupture of three fault planes in this sequence suggests that stress changes from the initial rupture triggered the successive earthquakes, including one larger than the first.
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