Even if you’ve been following the subject of increased earthquake activity in America’s oil and gas basins, with fracking as the probable cause, you might still be impressed by the hockey-stick trend line above.
In the 36 years from 1973 through 2008 (blue line), there were 858 earthquakes in the central and eastern United States with a magnitude of 3 or greater.
These are quakes that are plenty strong enough to make you feel the earth moving underfoot, but not strong enough to cause much damage to buildings. They averaged about 24 per year.
In the six years and four months from 2009 through this past April (red line), there were 1,570 quakes in the same magnitude range — nearly twice as many — with both the running total and the rate climbing sharply.
The yearly average for the red-line period stands at 193. Last year alone there were 688. And already this year, through the end of May, there have been 430 quakes above magnitude 3.
If that pace continues through December, the level of seismic activity recorded in 2015 will exceed the entire 36-year period of the blue line.
What in the underworld is going on?
Oil and gas production development is driving this trend, according to new analysis from the U.S. Geological Survey (USGS), aimed at separating myths from facts about the rising tide of earthquakes across much of the country. And, yes, fracking plays a role, but kind of a minor one.
It’s complicated, as chief author Justin Rubinstein of the USGS office explained to me yesterday afternoon.
How to trigger an earthquake
“People think fracking, they think earthquakes,” he said, “and that was really part of the reason I wrote this article in the first place. I wanted to have something that I could point the public to, point the media to, to explain what’s going on and why.”
What’s going on is injection of fluids into the earth, in volumes and at pressures sufficient to cause rock on either side of a fault to shift into a new alignment. This shift, or failure of the fault, is the definition of an earthquake, and when the failure results from human activity, we have an “induced” earthquake.
But fracking — the injection of hydraulic fluid to break up underground rock and let oil and gas flow to the wellhead — is only one of three distinct activities that are inducing earthquakes in oil and gas country, Rubinstein’s paper explains. It was published this week in the journal Seismological Research Letters (paid access).
Fracking by definition causes micro-earthquakes by creating or enlarging faults, Rubinstein said. But the data show that fracking operations can be associated with “felt earthquakes” — detectable by humans with their normal senses, with magnitudes of around 2.0 or 2.5 are greater — in only a small percentage of cases. (His analysis set the bar at magnitude 3 to be sure of having a complete “catalog” of quakes above a common threshhold.)
But injection for fracking lasts a few hours, or maybe a few days, and the volume of hydraulic fluid is comparatively small.
Another cause of induced earthquakes — but, again, a comparatively small share — is “enhanced oil recovery” through the injection of water, steam or carbon dioxide into existing wells to bring more product to the surface. But because the injected material is mostly filling space vacated by earlier extraction, the pressure changes aren’t as significant.
So the most important cause by far, Rubinstein explained, is the injection of industrial waste water, sometimes called “produced water,” from oil and gas operations into permanent storage underground. He writes:
Produced water is the salty brine from ancient oceans that was entrapped in the rocks when the sediments were deposited. This water is trapped in the same pore space as oil and gas, and as oil and gas are extracted, the produced water is extracted with it.
Produced water often must be disposed in injection wells because it is frequently laden with dissolved salts, minerals, and occasionally other materials that make it unsuitable for other uses.
And the volumes are huge: some injection wells receive a million barrels of produced water per month, and the injection may continue for many months. These scale factors — the larger volume of fluid injected, the longer duration of injection, the much larger underground area through which the fluid distributes — have a strong influence on both the frequency and size of induced earthquakes.
They have also been shown to create quakes more than 10 kilometers from the injection point, and at depths exceeding four kilometers.
Most wells not problematic
In our conversation, as in his article, Rubinstein made clear that it’s only a minority of injections for any purpose that cause problems.
“Most of the time, these wells are not problematic,” he said. “There’s 35,000 or so injection wells in the West, and it’s really only a few dozen that have been directly linked to induced seismicity.”
So far, none of the industrially induced quakes have caused fatalities or catastrophic damage. But that has something to do with their being located generally away from densely settled areas.
An induced quake centered in Prague, Oklahoma, in 2011 damaged some 14 buildings, he said, notably including the Benedictine Hall at St. Gregory’s University in Shawnee, where a multimillion-dollar campaign was needed to restore the building’s four toppled towers.
Another, centered near Trinidad, Colorado, caused similarly light damage.
But there is concern that impacts of induced quakes could be intensified if they occur in larger cities, especially where building codes and construction styles didn’t anticipate shaking foundations.
“I live in San Francisco, for example, and we don’t have a lot of brick buildings anymore. But in other areas, with unreinforced brick structures, there could be problems.
“The earthquake in Napa last summer didn’t knock down a whole lot of buildings. But it did knock down brick buildings.”
No alternative for disposal
The problem of wastewater injection isn’t going to away, he said, because there’s really no other way to dispose of it in such huge volumes. Instead, he hopes the findings of research like his may help manage the risks a little better.
Earthquakes remain essentially unpredictable, he acknowledged, but with more robust networks of seismic sensors, early detection could be improved — and potentially preventive measures could perhaps be developed.
“In most of the central U.S., there might be one sensor every 100 miles. In Texas, I think there are about 25 for the whole state.
“But in Oklahoma, in response to the increased activity, they’ve been adding a lot more. And we’ve seen places where they’ve slowed down wastewater injection, and the earthquakes have slowed down in response.”
Another hurdle, he said, is getting better access to industry data about the disposal practices.
“There’s really good data about activity at fracking wells and enhanced-recovery wells,” he said, “because the results are taxed. The companies have to report on what they’re doing and they have to report promptly. That’s all pretty public.
“With wastewater injection, most of them collect data to meet the requirements, but they’re not very stringent — the states don’t really care.”
Maybe the rising tide of earthquakes can begin to change that.
“We really need more data if we want to prevent earthquakes,’ he said. “Industry often has a lot more information than we do about what’s below the surface, what the layers are like, where the fault lines area. This is all very relevant.
“I really do think we can lower the risk of induced earthquake. But it will require a level of coordination between academic scientists and government agencies and industry that isn’t there yet.”