Earthquakes are striking the heartland from Alabama to Montana at an unprecedented rate -- and human activity is probably to blame.

Area over which increased seismic activity has been observed.

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 new work is being presented at the Seismology Society of America's conference later this month. An abstract for the presentation is available online.

In some regions, the increase in earthquakes is even greater than six fold. For example, in Oklahoma over the past half-century, there were an average of 1.2 quakes of greater than 3.0 magnitude per year. Since 2009, there have been more than 25 per year.

"A naturally-occurring rate change of this magnitude is unprecedented outside of volcanic settings or in the absence of a main shock, of which there were neither in this region," the scientists write.

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.

That, at least, was the United States' Army's experience in doing deep well injection during the 1960s. "If you are doing deep well injection, you are altering the stress on the underlying rocks and at some point, the stress will be relieved by generating an earthquake," seismologist Dave Wolny explained back in 2007. "The events are generally small, but there is no way to predict how the injection process has altered stresses on the fault system in the area, and thus, no way to predict how large the events may get."

The USGS scientists aren't willing to draw the causal connection between fracking and earthquakes. "While the seismicity rate changes described here are almost certainly manmade, it remains to be determined how they are related to either changes in extraction methodologies or the rate of oil and gas production," they conclude.

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.


Image: Philip Bump.

Via @jdsutter

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