A huge increase in the number of minor earthquakes across the middle of the US is almost certainly caused by wastewater injection from oil or gas drilling, including the controversial technique known as fracking.
US Geological Survey (USGS) scientists say that the number of magnitude 3 or above earthquakes was pretty steady at about 21 quakes a year until around 2000. Since then, though, the number has shot up, with 151 events per year by 2004.
And, they say, there's a correlation with the disposal of wastewater through deep-well injection, particularly in Colorado, Texas, Arkansas, Oklahoma and Ohio.
The findings are particularly sensitive given the recent increase in the use of 'fracking', or hydraulic fracturing, of shale beds in order to release natural gas.
After numerous anecdodal examples of a link, it's been widely suggested that fracking could increase the chances of earthquakes; but this is happening only indirectly, says the Geological Survey.
"USGS’s studies do not suggest that hydraulic fracturing, commonly known as 'fracking', causes the increased rate of earthquakes," says David Hayes, deputy secretarty for the US Department of the Interior.
"USGS’s scientists have found, however, that at some locations the increase in seismicity coincides with the injection of wastewater in deep disposal wells."
Nevertheless, says the USGS, more research needs to be carried out - particularly into the question of whether wastewater injection could cause more serious earthquakes.
"Evidence from some case histories suggests that the magnitude of the largest earthquake tends to increase as the total volume of injected wastewater increases," says Hayes. "Injection pressure and rate of injection may also be factors."