More than 230 earthquakes with a magnitude greater than 3.0 have shaken the state of Oklahoma already this year. Before 2008 the state averaged one such quake a year. The surge in seismic activity has left residents and experts alike wondering about the underlying cause. 
 
 
 
 Past research has shown that processes such as wastewater injection at oil drilling and fracking sites throughout the state could induce a small number of earthquakes but scientists have never been able to specifically link some of the more distant or stronger earthquakes with these sometimes faraway wastewater wells. That is, until now. 
 
 
 
 A study published today in Science explains how wastewater injection sites—areas where toxic water left over from oil drilling and fracking processes is injected into the ground between impermeable layers of rocks to avoid polluting freshwater—could be driving the sharp increase in the sometimes-disastrous earthquake events. “It really is unprecedented to have this many earthquakes over a broad region like this,” says study co-author Geoffrey Abers of Cornell University. “Most big sequences of earthquakes that we see are either a main shock and a lot of aftershocks or it might be right at the middle of a volcano in a volcanic system or geothermal system. So you might see little swarms but nothing really this distributed and this persistent.” 
 
 
 
 
 
 
 
 
 
 
 
 Abers and his colleagues dug up data on the rates and volumes of liquids associated with the wastewater injection sites. They then modeled the flow of the water and calculated the physical properties of the rocks into which the water was injected. In so doing the team determined that a relatively small number of wastewater injection sites used in oil drilling and fracking in Oklahoma may have the ability after all to induce relatively strong earthquakes a long distance away, throughout the state. “The important thing is that we are seeing earthquakes that are much more widely distributed, much farther from wells and in a lot of different directions,” Abers observes “Some of these earthquakes are as much as 20 miles away from what seems to be the primary wells that are increasing the pressure.” 
 
 
 
 Scientists have known since the 1960s that wastewater injection, during which millions of barrels of wastewater are forced into a disposal well, can induce earthquakes by increasing the fluid pressures underground. That increasing pressure reduces the frictional strength of faults, Abers explains, allowing them to slip. 
 
 
 
 Seismologist Austin Holland of the Oklahoma Geological Survey, who was not involved in the study, says there could be a large number of factors playing into the quakes. But like the authors of the new paper, he, too, has found evidence linking the Oklahoma quakes specifically to oil and gas activity. “We certainly do have a contribution from oil and gas, but the question is how much, how extensive is this and how is this occurring,” he says. “This study will certainly help improve our understanding in the scientific discussion of what is occurring in Oklahoma.” 
 
 
 
 The number of wastewater injection sites also seems to keep increasing, according to Abers, who notes that the permitted rate of wastewater doubled between 2004 and 2008, and has most likely increased since then. He says several earthquakes in Texas over the past few years have been linked to wastewater injection but the scale of these events pales in comparison with what is happening in Oklahoma. There have also been well-documented cases of wastewater injection-driven quakes in Ohio, Utah, Colorado and British Columbia, all within the last year or so. 
 
 
 
 Holland added that these events demand political and social discussion. “Just how important is it to produce oil and gas in Oklahoma, and are we willing to deal with the issues of these disposal wells in order to produce the oil and gas that we are accustomed to producing?” he asks. 
 
 
 
 Abers notes that ongoing research on this topic will be needed to better understand the complexity of these processes as they continue into the future. “I think this rate of earthquake increase in the midcontinent is really extraordinary and is continuing, but this isn’t the last word on this in any means,” Abers remarks. “There is clearly something important going on there that we need to keep an eye on and that we need to be trying to understand.” ||||| Image copyright Getty Images Image caption As the oil and gas industry has expanded in parts of the US, so have the number of earthquakes 
 
 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. 
 
 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 Dr Katie Keranen, Cornell University 
 
 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. 
 
 Image copyright Cornell Image caption 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." 
 
 Follow Matt on Twitter @mattmcgrathbbc.
– Earthquakes in Oklahoma are up more than a hundredfold in recent years, and a new study spies a pretty clear link between the shaking and the fracking that has given the state's economy a huge boost. Researchers took a close look at four specific sites where wastewater from oil and gas extraction was injected into the ground and found that the process could be linked to swarms of quakes in areas up to 20 miles away from the sites, reports the BBC. The four wells examined have been pumping four million barrels of water a month to a depth of around two miles underground. At one site linked to the wells, a small town called Jones, there have been more 2,500 earthquakes greater than magnitude 3.0 since 2008—a fifth of the total in the central and western US during that period. "It really is unprecedented to have this many earthquakes over a broad region like this," a study co-author tells Scientific American, explaining that wastewater injection can cause quakes by sending out waves of fluid pressure, causing faults miles away to slip. "Most big sequences of earthquakes that we see are either a main shock and a lot of aftershocks or it might be right at the middle of a volcano in a volcanic system or geothermal system. So you might see little swarms but nothing really this distributed and this persistent," he says. (In Texas, several small towns troubled by quakes are considering banning fracking.)