Scientists think a 5.7 earthquake was caused by mining wastewater being injected into underground 'compartments'

Population density

Aftershocks 5 -12 November

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Geology,March 2013,Keranen et al,Potentially induced earthquakes in Oklahoma, USA: Links between wastewater injection and the 2011 Mw 5.7 earthquake sequence:

Abstract

Significant earthquakes are increasingly occurring within the continental interior of the United States, including five of moment magnitude (M_w) ≥ 5.0 in 2011 alone. Concurrently, the volume of fluid injected into the subsurface related to the production of unconventional resources continues to rise. Here we identify the largest earthquake potentially related to injection, an M_w 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 M_w 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.
Received 18 September 2012.
Revision received 23 January 2013.
Accepted 23 January 2013.

Mother Jones March/April 2013:

Such seismic activity isn't normal here. Between 1972 and 2008, the USGS recorded just a few earthquakes a year in Oklahoma. In 2008, there were more than a dozen; nearly 50 occurred in 2009. In 2010, the number exploded to more than 1,000. These so-called "earthquake swarms" are occurring in other places where the ground is not supposed to move. There have been abrupt upticks in both the size and frequency of quakes in Arkansas, Colorado, Ohio, and Texas. Scientists investigating these anomalies are coming to the same conclusion: The quakes are linked to injection wells. Into most of them goes wastewater from hydraulic fracking, while some, as those in Prague, are filled with leftover fluid from dewatering operations.

* Mapping images from United States Dept. of the Interior/US Geological Survey