CRESCENT (United States) • The central US state of Oklahoma has gone from registering two earthquakes a year to nearly two a day, and scientists point to a controversial culprit: wastewater injection wells used in fracking.
Located in the middle of the country, far from any major faultlines, Oklahoma experienced 585 earthquakes of a magnitude of 3.0 or greater last year. That is more than three times as many as the 180 which hit California last year.
"It's completely unprecedented," said Mr George Choy, a seismologist at the US Geological Survey.
As of last month, Oklahoma has experienced more than 600 quakes strong enough to rattle windows and rock cars. The biggest was a 4.5-magnitude quake that hit the small town of Crescent on July 27.
Hydraulic fracturing, or fracking, is the process of shooting water, mixed with sand and chemicals, deep into the earth to crack rock formations and bring up oil and natural gas trapped inside. The process has unlocked massive amounts of oil and gas in Oklahoma and other states over the past decade.
But along with the oil and gas comes plenty of that brackish water, which is disposed of by injecting it into separate wells that are dug less than 2km below ground.
The unnatural addition of the water can change pressure along faultlines, causing slips that make the earth shake, said Mr Choy.
Oklahoma has about 4,500 disposal wells, with about 3,200 operating on any given day.
From 1975 to 2008, the state experienced anywhere from zero to three earthquakes a year that registered at 3.0 or higher. Then, the numbers jumped: There were 20 in 2009, 35 in 2010, 64 in 2011, 35 in 2012, 109 in 2013 and 585 in 2014.
There is debate among scientists over how large a fault could be reawakened, and how hard that fault might shake.
One camp believes Oklahoma will not see an earthquake bigger than a 4.0 to 5.0 magnitude, which would be enough to break windows and knock things off shelves. Others believe a 7.0-magnitude earthquake could occur, which would be strong enough to topple buildings.
"We are the only state where once this problem came up, we just kept going (with fracking)," said Mr Johnson Bridgwater, executive director of the Oklahoma chapter of the Sierra Club, a prominent environmental group. "We want public safety to come first, rather than treating this state as a giant lab."