The recent surge in earthquakes across the mid-section of the country is almost certainly man-made, according to a group of researchers at the U.S. Geological Survey.
According to a study the researchers will present next month, there were 50 earthquakes greater than magnitude-3.0 in mid-part of the country between Alabama and Montana in 2009. In 2010, there were 87. Last year's 134 represents a sixfold increase over 20th century levels.
That timeline, of course, neatly corresponds with the rapid expansion of natural gas drilling, which necessitates the disposal of hydraulic fracturing fluid in underground wells.
From the abstract of the study:
The acceleration in activity that began in 2009 appears to involve a combination of source regions of oil and gas production, including the Guy, Arkansas region, and in central and southern Oklahoma. Horton, et al. (2012) provided strong evidence linking the Guy, AK [sic] activity to deep waste water injection wells.
Last year, the Arkansas Oil and Gas Commission voted to shut down one injection well near Greenbrier. Another company operating in the area voluntarily shut down two wells.