Surge in Oklahoma earthquakes linked to fracking for oil and gas

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.