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.