A series of 11 small earthquakes in Youngstown, Ohio, may have been caused by wastewater injection wells used by oil and natural gas drillers, according to AP.

A U.S. Geological Survey team is preparing to publish a report that has drawn a link between hydraulic fracturing and an increase in earthquakes in the U.S.

The study's findings will be a topic of discussion later this month when the Seismological Society of America meets in San Diego.

Only the study's abstract is available to the public, but its findings suggest hydraulic fracturing has helped increase the number of earthquakes near the Raton Basin along the Colorado-New Mexico border west of Trinidad, Colo.

Hydraulic fracturing involves the high-pressure injection of thousands of gallons of water, sand and drilling chemicals deep underground to fracture natural gas-yielding rocks. The drilling technique has been used for decades but has, in the past several years, been revolutionized when drillers learned how to drill and fracture rocks horizontally.

The technique has spurred an energy boom in the Northeast and in Ohio, Wyoming, Colorado and Texas. Opponents, however, say it is harmful to the environment and to people.

Kara Capelli, a spokeswoman for the USGS, said the study's findings do not explicitly indicate the hydraulic fracturing of shale rocks itself as the source of earthquakes. Rather, injection wells, dug into the ground to hold hydraulic fracturing wastewater and drilling chemicals, are responsible, according to the study's abstract.

A remarkable increase in the rate of [Magnitude 3] and greater earthquakes is currently in progress in the US midcontinent, the abstract reads. The average number of M >= 3 earthquakes/year increased starting in 2001, culminating in a six-fold increase over 20th century levels in 2011.

According to the findings, as outlined by what is available to the public, there were an average of 21 magnitude 3 earthquakes a year between 1970 and 2000. That number started climbing between 2001, and by 2011, the USGS was registering roughly 134 magnitude 3 earthquakes on average.

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, read the report. 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.

The Seismological Society of America will meet in San Diego on April 17 through 19.