Massive injections of wastewater from the oil and gas industry are likely to have triggered a sharp rise in earthquakes in the state of Oklahoma.
Researchers say there has been a forty-fold increase in the rate of quakes in the US state between 2008-13.
The scientists found that the disposal of water in four high-volume wells could be responsible for a swarm of tremors up to 35km away.
Their research has been published in the journal, Science.
Sudden swarm
There has been increasing evidence of links between the process of oil and gas extraction and earthquakes in states like Arkansas, Texas, Ohio and Oklahoma in recent years.
In 2011, a small number of people were injured and 14 houses were destroyed in the town of Prague, Oklahoma by a 5.7 tremor.
Investigators linked it to the injection of wastewater from the oil industry.
The US Geological Survey (USGS) has also reported on the question of seismicity induced by wastewater disposal.
This new research goes further, linking a large swarm of Oklahoma tremors with a number of specific water wells, distantly located.
More than 2,500 earthquakes greater than magnitude 3.0 have occurred around the small town of Jones since 2008. This represents about 20% of the total in the central and western US in this period.
There has been a significant increase in the number of tremors in central Oklahoma since 2008
Researchers have now linked this increase to a near doubling in the volumes of wastewater disposed of in the central Oklahoma region between 2004 and 2008.
Water is never far away in the energy extraction process. It is used not just for hydraulic fracturing, but also to squeeze more oil out of conventional wells.
Large amounts of naturally occurring water are often released with the oil and gas - and this briny liquid needs to be separated from the fuels, using a method called "dewatering".
"There is a high ratio of water to oil," said the study's lead author Dr Katie Keranen from Cornell University.
"It differs for each well. The typical nationwide ratio is five to one. We're seeing much higher ratios, in the hundreds, at the beginning of the well."
According to Dr Bill Ellsworth from the USGS, the high price of oil has driven this water-based approach. But the law says that drinking water has to be protected from the salty flow.
"As part of the business model, you have to be able to dispose of these very large volumes of saline water. You can't treat it; you can't put it into the rivers. So, you have to inject it underground."
Pressure points
Four of the biggest of these wells in Oklahoma have been pumping around 4 million barrels of water a month to a depth of 3.5km beneath the surface.
To determine the impact of this water, the scientists developed a model that could calculate the way the underground wave of pressure from these wells spread out.
By comparing this to seismic data from the Jones cluster, it was concluded that the injection of wastewater is "likely responsible" for the swarm.
"It is possible that pressure looks to have risen in the places where the earthquakes are occurring," said Dr Keranen.
"That pressure increase is what we see in natural triggering. So, if a fault is close to failure, the amount that the pressure is going up at these locations in our model is enough to push them over the edge."
The four wells that are the subject of the study are owned by a company called New Dominion. It insists that it operates its wells (named Sweetheart, Chambers, Flower Power and Deep Throat) safely and within permitted parameters.
"The company notes the author did not consult with New Dominion's geologist and engineers to determine whether her premises are in any way correct," the company said in a statement.
"At best, these incorrect assumptions are irresponsible."
Bigger triggers
The authors say that they are uncertain about the potential for the large-scale disposal of wastewater to trigger events of larger magnitude.
They point to an incident in 2010 when an earthquake ruptured a portion of a 7km long fault. If the entire fault had gone, the authors write, it could have led to a magnitude 6.0 tremor.
"We often see more larger earthquakes when we see a lot of smaller ones," said Dr Keranen.
"But this is new situation with induced seismicity and we still have a lot of questions that we are trying to address."
This view is echoed by Dr Bill Ellsworth from USGS.
"There are thousands of these wells in the US, so only a few appear to be problematic. The difficulties can be avoided but we need to know more about the process so we can give proper guidance to the authorities."
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