Oklahoma has seen a boom in two things in recent years: oil and gas production and earthquakes.
To many residents of the Midwestern US state, the timing says it all. Before the oil and gas industry started drilling so many underground injection wells, they say, it was rare to feel an earthquake. Today, Oklahoma is the second-most seismically active state in the continental United States, behind California.
Now they have some fresh scientific evidence to back up their observations. Researchers from Cornell University and the University of Colorado say a large swarm of earthquakes in central Oklahoma was probably caused by activity at a few highly active disposal wells, where waste water from drilling operations - including hydraulic fracturing, or fracking - is forced into deep geological formations for storage.
Four high-rate disposal wells in southeast Oklahoma City probably induced a group of earthquakes known as the Jones swarm, which accounted for 20 per cent of the seismicity in the central and eastern United States between 2008 and 2013, the team reported in the journal Science.
The Jones swarm, named after a small town east of Oklahoma City, included more than 100 earthquakes of magnitude 3.0 or more during that five-year period.
Cornell's team reported that earthquakes could be induced nearly 30km from a disposal well, beyond the current range of about 5km currently used to diagnose induced earthquakes.
Fracking involves shooting a high-pressure mixture of water, sand and chemicals deep underground to extract oil and natural gas. The resulting waste water is often forced underground as well.
When fluid is injected into rock formations, it increases pressure in the pores of those formations, said study senior author Shemin Ge, a hydrogeology professor at the University of Colorado, Boulder. The increased pressure can trigger earthquakes in pre-existing faults or other areas of geological weaknesses, she said.
Katie Keranen, a geophysics professor at Cornell and the study's lead author, said even a small change in pressure can cause a fault to rupture and trigger an earthquake.
The Oklahoma Independent Petroleum Association (OIPA) said because oil and gas activity was so prevalent in the state, seismic activity was likely to occur near industry operations, but that did not prove a correlation.
Some states lacking significant oil and gas work also saw increased seismic activity, OIPA president Mike Terry said.
Terry said disposal wells had been used in the state for more than 50 years. Oil and natural gas are produced in 70 of Oklahoma's 77 counties, so "any seismic activity within the state is likely to occur near oil and gas activity", he said.
"A rush to judgment based on one researcher's findings provides no clear understanding of the causes," he said.
For the first half of this year, Oklahoma recorded 241 earthquakes of 3.0 on the Richter scale or greater, up from 109 of that level last year - itself a level more than 5,000 per cent above normal - and nearly at the five-year total of 278 recorded from 2008-2013, according to state data. From 1978 to 2008, the state on average recorded only two earthquakes of magnitude 3.0 or larger a year.
Additional reporting by Reuters