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The U.S. Geological Survey (USGS) in partnership with the Oklahoma Geological Survey has conducted a study on the causes of increased occurrence rates of earthquakes in the state over the past few years and a possible link between seismic activity and wastewater disposal from oil and gas drilling sites.

Since 2009, earthquakes have increased disproportionately when compared to previous periods. Between 1975 and 2008 there were one to three earthquakes of a magnitude of 3.0 or more each year, while between 2009 and 2013 the average grew to 40. Overall, more than 200 earthquakes with a magnitude of 3.0 and above occurred in Oklahoma over the past four years, data from the USGS showed.

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Scientists think that this dramatic rise could be linked with the injection of wastewater from fracking sites into deep wells underground. Their study suggests that the rates at which earthquakes have occurred lately cannot be down to natural fluctuations in seismic activity.

Bill Leith, USGS seismologist, explained that the research concluded that there must have been changes both in the background rate of events and in the earthquake triggering properties to have caused such an increase in seismicity. The USGS and the Oklahoma Geological Survey will now be looking at whether there is evidence that wastewater injection can be a trigger for such events, scientists said.