Distant quakes 'can trigger wastewater-site temblors'
Earthquakes can be triggered at the sites of wastewater injection by quakes on the other side of the world, research suggests.
The injection of wastewater from underground operations such as oil drilling is known to increase local seismic activity.
Now a study in Science suggests that waves from the most distant temblors can cause quakes at wastewater sites.
Researchers suggest this can act as a kind of "stress meter" for the sites.
The notion of natural earthquake triggering is not new; in hydrothermal and volcanic areas, tremors can be triggered by large, distant earthquakes. But the new study suggests what is in effect a new category: natural triggering of seismic events primed by human activity.
Injection of wastewater from operations such as drilling, geothermal, or hydraulic fracturing ("fracking") is banned in the UK and many European countries, but it has become increasingly prevalent in the US.
In the state of Texas alone, more than 7,000 such wells are in operation and the link between injection wells and even large seismic events is strengthening.
In March, researchers linked a 5.7-magnitude event in Oklahoma to waste water injection that had been going on for nearly two decades.
"In some cases of induced earthquakes you drill a well, you start pumping, and a week or two later you start having earthquakes on a very nearby fault - we saw this in Arkansas in 2011, and a site in Ohio," explained lead author of the study Nicholas van der Elst of the Lamont-Doherty Earth Observatory at Columbia University, New York.
The team scoured the seismic records for three wastewater injection sites in the US states of Oklahoma, Colorado and Texas, looking for signs of the smallest earthquakes that could be detected.
"In the sites we looked at, the connection hadn't been quite as straightforward, so we were looking for additional evidence that fluids were bringing these regions to the tipping point," Dr van der Elst told BBC News.
When compared with global records of larger earthquakes, a pattern became clear: there was a pronounced increase in earthquakes of magnitude 3 or greater following large events elsewhere in the world, such as the February 2010 event in Chile or the March 2011 Tohoku event off Japan.
"When you have a really big earthquake somewhere else on the planet, this sets up big seismic waves that spread out like ripples over the surface of the Earth. When these seismic waves pass faults that are already very near to failure, these seismic waves can give that additional push that sets off an earthquake."
Dr van der Elst said that these correlations of mid-size earthquakes with distant events could be used as a useful test of a site's integrity.
"If you've had a quiet injection site in the past, you'd like to be able to know if that site has transitioned, reached some critical threshold where larger earthquakes are possible," he said.
"If you can use this method as a kind of stress meter to show where the stresses are building, that might be really useful for making policy decisions about whether to keep pumping there or whether to try a different site."
Richard Davies, director of the Durham Energy Institute at the University of Durham, called the paper "an exciting, interesting result".
"Seismologists have known for some time that there are transient stresses from earthquakes that can potentially cause other faults to slip, causing an earthquake," Prof Davies told BBC News.
"But this paper is a very interesting contribution, as it proposes that mankind can artificially 'prime the faults' by injecting wastewater over long periods under the ground.
"Mankind is essentially lubricating the faults enough so that they are eventually triggered by a distant, natural earthquake. Think of a hovercraft - the air pumped into the base of the craft means that even small forces allow the heavy vehicle to move - the physics is the same."