12 July 2013, News Wires – Powerful earthquakes halfway around the world can trigger swarms of minor tremblors near wastewater-injection wells, sometimes months after the initial event, according to new research.
The findings, published in the journal of Science by one of the world’s leading seismology labs, come as the US Environmental Protection Agency conducts a study of the effects of hydraulic fracturing – particularly the disposal of wastewater – that could form the basis of new regulations on oil and gas drilling.
The occurrence of “induced” earthquakes – those triggered by injecting fluid into fault lines – has been well understood for decades. A spike in drilling and fracking across the US in recent years has led to an increase in the amount of fluid being injected underground. Studies have linked this practise to spates of small earthquakes in places like Oklahoma and Arkansas.
In the latest study, seismologists at Columbia University said they have identified three quakes – in Oklahoma, Colorado and Texas – that were triggered at injection-well sites by major earthquakes a long distance away.
“The fluids (in wastewater injection wells) are driving the faults to their tipping point,” said Nicholas van der Elst of Columbia’s Lamont-Doherty Earth Observatory in Palisades, New York, who led the study, which was funded by the National Science Foundation and the US Geological Survey.
The findings could bolster the claims of fracking foes who blame the controversial completion method on water and air pollution as well as earthquakes, although the Columbia study says fracking itself, as currently practised, “appears to pose a low risk of inducing destructive earthquakes”.
Fracking proponents reacted cautiously to the study.
“More fact-based research … aimed at further reducing the very rare occurrence of seismicity associated with underground injection wells is welcomed, and will certainly help enable more responsible natural gas development,” said Kathryn Klaber, chief executive of the Marcellus Shale Coalition, according to a Reuters report.
Quakes with a magnitude of 2 or lower, which can hardly be felt, are routinely produced in fracking, said geologist William Ellsworth of the US Geological Survey, an expert on human-induced earthquakes who was not involved in the study.
The largest fracking-induced earthquake “was magnitude 3.6, which is too small to pose a serious risk”, he wrote in Science.
But van der Elst and colleagues found evidence that injection wells can set the stage for more dangerous quakes, Reuters reported. Because pressure from wastewater wells stresses nearby faults, if seismic waves speeding across Earth’s surface hit the fault it can rupture and, months later, produce an earthquake stronger than magnitude 5.
What seems to happen is that wastewater injection leaves local faults “critically loaded”, or on the verge of rupture. Even weak seismic waves from faraway quakes are therefore enough to set off a swarm of small quakes in a process called “dynamic triggering”.
“I have observed remote triggering in Oklahoma,” said seismologist Austin Holland of the Oklahoma Geological Survey, who was not involved in the study. “This has occurred in areas where no injections are going on, but it is more likely to occur in injection areas.”
Once these triggered quakes stop, the danger is not necessarily over. The swarm of quakes, said Heather Savage of Lamont-Doherty and a co-author of the study, “could indicate that faults are becoming critically stressed and might soon host a larger earthquake”.
For instance, seismic waves from an 8.8 quake in Maule, Chile, in February 2010 rippled across the planet and triggered a 4.1 quake in Prague, Oklahoma – site of the Wilzetta oilfield – some 16 hours later.
That was followed by months of smaller tremors in Oklahoma, and then the largest quake yet associated with wastewater injection, a 5.7 temblor in Prague on 6 November 2011. That quake destroyed 14 homes, buckled a highway and injured two people.
The Prague quake is “not only one of the largest earthquakes to be associated with wastewater disposal, but also one of the largest linked to a remote triggering event,” said van der Elst.
The Chile quake also caused a swarm of small temblors in Trinidad, Colorado, near wells where wastewater used to extract methane from coal beds had been injected.
On 22 August 2011, a magnitude 5.3 quake hit Trinidad, damaging dozens of buildings.
The 9.1 earthquake in Japan in March 2011, which caused a devastating tsunami, triggered a swarm of small quakes in Snyder, Texas – site of the Cogdell oilfield. That autumn, Snyder experienced a 4.5 quake.
The presence of injection wells does not mean an area is doomed to have a swarm of earthquakes as a result of seismic activity half a world away, and a swarm of induced quakes does not necessarily portend a big one.
Long-distance triggering is most likely to occur where wastewater wells have been operating for decades and where there is little history of earthquake activity, the researchers wrote.
“The important thing now is to establish how common this is,” said Oklahoma’s Holland, referring to remotely triggered quakes. “We don’t have a good answer to that question yet.”
University of Utah mechanical engineer Sidney Green called the results interesting but “rather speculative” and said they need more study, according to the Associated Press.
Julie Shemeta, a geophysicist and president of Colorado-based consultant MEQ Geo said that if the observations bear out, it could help oil and gas operators know “where it’s safe to inject and where it’s not”.