Oklahoma and south Kansas had seventeen percent of the world's earthquakes M2.5 or greater in the past 7 days as of noon Oct. 16, 2015. Deep well injection of oil and gas waste has turned a seismically quiet area into one of the world's most active seismic regions.
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On October 10 a M4.5 earthquake rocked the critical infrastructure of the oil distribution and storage complex of Cushing, Oklahoma, but no major problems have been reported. Because the hidden deep faults in Oklahoma and Kansas cut across both states, larger, more destructive earthquakes are possible. In locations where these long ancient faults align (un)favorably with regional tectonic forces, they have been pushed apart by the enormous amounts of pressurized fluid produced from the dewatering of oil bearing rock formations. It has taken years to build up enough fluid pressure over a large area of the deep faulted crystalline rock to trigger earthquakes, but now they happen daily in the oil country of central Oklahoma and south central Kansas.
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Faults associated with the billion year old rip through the heart of north America that crosses Kansas from Lake Superior to central Oklahoma, called the mid-continent rift, have been reactivated by high pressure injection of oil and gas waste fluids. In 2014 the U.S. Geological Survey warned of an increased risk of damaging earthquakes of M5.0 or greater in Oklahoma. To date, the largest earthquake probably triggered by deep waste injection is M5.6, in Prague Oklahoma. The fault structures are long enough to be potentially capable of even larger earthquakes, but it is not clear how much larger because of the lack of historic evidence and the fact that these faults are deeply buried under deformable sedimentary layers.
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In mid-March, Oklahoma's oil and gas regulator ordered well injection volumes cut in half for wells that might be injecting into the deep basement rocks below the sedimentary cover.
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It is not clear if this moderate step will be sufficient to slow the number of earthquakes. It took many years to build up the pressure of fluids below confining sedimentary layers, to activate the deep faults. If these confining layers continue to hold the pressure in the crystalline rock below, the earthquakes could continue for years.