A new United States Geological Survey study has found that middle America between Alabama and Montana is experiencing an "unprecedented" and "almost certainly manmade" increase in earthquakes of 3.0 magnitude or greater. In 2011, there were 134 events of that size. That's six times more than were normally seen during the 20th century.
While the changes in the area's seismicity began in 2001, the trend has really accelerated since 2009, the geologists note. That happens to coincide with increased oil and gas production using new extraction techniques in some parts of the area.
The conclusion that at least one environmental group has drawn from this data is that fracking, in one way or another, has caused these earthquakes. The Environmental Working Group notes that more than 400,000 wells were drilled between 2001 and 2010, a 65% increase over the previous ten-year period. They also note that the new extraction techniques require vast amounts of water to be injected into the ground: major producer Chesapeake estimates that it uses about 5 million gallons of water per well. Lots of wells plus lots of water injected underground could change the subterranean conditions and lead to more earthquakes.
But if it is not fracking, then ... What is it? At the moment, we don't have a whole lot of other hypotheses, just a lot of unexplained earthquakes in places where they don't normally strike.