In the 30 years leading up to the wastewater injection, the basin straddling the border of Colfax County in New Mexico and Colorado was seismically quiet, with just one significant earthquake. But since energy companies expanded coal-bed methane operations in 1999, seismic activity has gone way up, according to the study.
Starting in August 2001, there have been 16 magnitude 3.8 or stronger earthquakes – including one magnitude 5.0 and another magnitude 5.3. That compares to the one earthquake during the previous three decades, a magnitude 4.0.
The increase in earthquakes is limited to the area of industrial activity and lies within 3.1 miles of wastewater injection wells, the study found. Moreover, the greater the rate of wastewater injection, the greater the seismic activity.
Professor Mousumi Roy of the Department of Physics and Astronomy at the University of New Mexico said the study does not jump to conclusions outside its scope, but limits its findings to those earthquakes that were fully catalogued.
“That’s commendable,” she said, of what she described as “a fairly rigorous statistical study.”
Roy said she was not particularly surprised to read that the injection of fluids underground can bring earthquake faults to slip – “that’s something we’ve known for some time” – but the study should give scientists a better picture of how fluids move underground, the “mechanisms” behind earthquakes.
The U.S. Geological Survey study, “The 2001-Present Induced Earthquake Sequence in the Raton Basin of Northern New Mexico and Southern Colorado,” was published online and will appear in the October issue of the Bulletin of the Seismological Society of America. It was conducted and written by Justin Rubinstein, William Ellsworth, Arthur McGarr and Harley Benz.
The Raton Basin extends about 90 miles north to south and 50 miles east to west, with roughly equal areas in each of the two states. It gets its name from the city of Raton.
The injection of wastewater is not the same thing as hydraulic fracturing. It is, however, a widely used method of disposing of the liquid byproduct of methane mining operations. There are 21 high-volume wastewater disposal wells in the Colorado section of the basin and another seven in New Mexico.
According to the study, the injection of fluids deep underground can raise pressure in nearby fault zones.
This, in turn, lowers the faults’ ability to resist friction, which makes the phenomenon known as “earthquake slip” more likely.
The researchers said the statistical likelihood of the observed rate change occurring naturally – “if earthquakes behaved randomly” – is 3 percent.
Energy companies began producing coal-bed methane in 1994 in Colorado and extended the practice to New Mexico five years later. Injection data for New Mexico before June 2006 are not available, but the researchers were able to estimate the amount of injection by the amount of water produced.
The study specifically does not link any one earthquake with injection. “Although there are many lines of evidence showing that the seismicity in the Raton Basin has been induced by wastewater injection activities in the area, it is very difficult to say whether an individual earthquake was caused by injection because nature seismicity has also been recorded there.”
Geophysics professor Susan Bilek of the New Mexico Institute of Mining and Technology said she had not seen the study and could not comment on it.
However, she said in an email, “there have been other documented cases which do seem to strongly suggest that wastewater injection can lead to earthquake activity, depending on a host of factors such as injection volumes and regional geology.”