Researchers with the US Geological Survey are developing an estimate of earthquake hazards that for the first time includes hazards posed from earthquakes triggered or suspected to have been triggered by waste-water disposal wells used by the oil and gas industries.
The effort stems from two earthquakes in 2011 that topped magnitude 5 and were suspected to have been triggered by waste-water injection – one in southern Colorado, at magnitude 5.3, and another in Oklahoma, at magnitude 5.7. The Oklahoma quake was preceded by a magnitude 5.0 foreshock.
The quakes are part of an increase in earthquake activity in the central and eastern US in recent years.
Between 1967 and 2000, the areas experienced an average of 21 quakes a year with a magnitude of 3.0 or higher. Between 2010 and 2012, the average exploded to some 300 quakes a year of magnitude 3.0 or higher, according to a 2013 analysis by USGS researcher William Ellsworth and published in the journal Science.
Prior to the 2011 quakes in Oklahoma and Colorado, "people had really believed that waste-water-injection induced earthquakes could not exceed magnitude 5," said Justin Rubinstein, a USGS researcher working on the new assessment, during a briefing Thursday at the Seismological Society of America's annual meeting in San Francisco. "These really demonstrate that this is a significant hazard."
Nor is the risk restricted to injection wells, noted Gail Atkinson, an earthquake scientist at Western University in London, Ontario.
"Waste water is the dominant cause," she said, "but what we are seeing as time goes on is that there are also events being induced from hydraulic fracture operations."
These operations, known as fracking, inject fluids into oil- or gas-bearing rock formations to liberate oil and gas from reserves that otherwise would be hard to exploit.
The new hazard assessment, which would augment the USGS's 2014 National Seismic Hazard Map, will key off the recent increase in quakes east of the Rockies.
For now, the project isn't trying to determine whether the quakes are natural or human-triggered, Dr. Rubinstein said, although he and his team, as well as other researchers, are trying to find ways to tell the difference.
Data are sparse on fault locations, underground fluid flows, the pace and volume of water being pumped into the ground at any given site, and how readily nearby faults can in effect be lubricated by waste fluids, he said.
"To some degree, it doesn't actually matter whether or not these earthquakes are induced," he added. "The increased rate indicates there is an increase in hazard."
The number of wells suspected of triggering moderate quakes is small compared with the total number of injection wells in the US, researchers note. But while the number of suspects may be small, they can have a long reach, Dr. Atkinson explained.
She has been studying a swarm of more than 200 earthquakes of magnitude 3 or larger that has been shaking central Oklahoma since 2009. The trigger appears to be a group of injection wells pumping high volumes of fluid deep underground – fluid that then travels to facilitate the rupture of faults tens of kilometers from the wells.
As Rubinstein and colleagues work on the new assessment, "we're trying to leave open the possibility that these earthquakes are natural,” he said. “We need to consider all the possibilities.”
Those possibilities include the prospect that induced quakes of even greater magnitude than those recorded so far are possible, he added, as well as the possibility that if the quakes are induced, actions can be taken to end the activities triggering them in that location.