Mounting evidence suggests small earthquakes in Oklahoma, Texas, and Arkansas are linked to fossil fuel extraction | Bread for the World/CC BY 2.0
SAN JOSE, California — On Friday, two earthquakes greater than magnitude 2.5 rattled Oklahoma. The day before that it was five, the day before that seven. Oklahoma should be one of the least likely focal points in the United States for earthquakes, and yet there's one almost every day in this state that's a thousand miles from the nearest tectonic plate boundary.
Substantial evidence now connects these earthquakes to human activity — particularly the energy industry. Oklahoma and neighboring Texas, Arkansas, and Kansas, all experiencing seismic upticks of their own, are hotbeds of fossil fuel extraction.
But according to a panel of geologists at the AAAS Annual Meeting, the culprit isn't hydraulic fracturing, or "fracking," in which geologists crack open subsurface rocks to extract oil and gas; instead, it's the processes associated with pumping wastewater and other fluids back into the ground.
"The public's concerned about hydraulic fracturing. It's something they've been led to fear," said Stanford geophysicist Mark Zoback. "But in terms of triggering earthquakes it's probably the least problematic of all the oilfield activities that are going on out there."
Zoback and his fellow panelists are seeking to understand the causes of induced seismicity, as well as to measure their associated risks and consider their regulatory implications.
Wastewater is the biggest byproduct of the oil and gas industry. In the United States, it's estimated that the average oil well extracts 7.5 barrels of water for every barrel of crude oil. That water is saltier than seawater and may contain naturally occurring arsenic and radioactive elements trapped inside rock formations deep underground. It's nasty, and it needs somewhere to go.
So oil producers force it back into the ground, injecting it into wells drilled into porous, fractured rock formations. This is a one-way operation: as more and more fluid is injected into the ground, pressure builds. And it's this rising pressure that is triggering new motion along faults.
Zoback wants to know why. As co-director of a consortium on induced and triggered seismicity, one of his goals is "to try to lay down some of the scientific principles so that those regulations are in fact addressing the root cause of the problem."
He explained that better monitoring and subsurface mapping are helping to identify the subset of faults that are seismically active — and therefore the biggest threats.
"The public's concerned about hydraulic fracturing. But in terms of triggering earthquakes it's probably the least problematic of all the oilfield activities that are going on out there."
Stanford geophysicist Mark Zoback
In early 2014, the United States Geological Survey (USGS) released its most recent national seismic hazard map, a vital data source for policymakers guiding building codes and other quake-related regulations. But according to USGS geologist William Ellsworth, induced seismicity in Oklahoma and its neighbors was omitted from the map until the long-term hazard is better defined.
Ellsworth and his colleagues have been working toward quantifying it, especially as compared to known seismic areas nearby. The New Madrid Seismic Zone and the Charleston region of South Carolina, for instance, have seen larger historical earthquakes but don't shake as often as the midcontinent. According to their data, Ellsworth said, the hazard "is certainly higher in those parts of the country where we see earthquakes happening every day" — like Oklahoma.
California Geological Survey chief John Parrish added that induced seismicity is also rising in California. As in the midcontinent, these quakes are tied to the energy industry — but in this case the source of the problem is geothermal energy production, not wastewater injection by the petroleum industry.
Much like hard-to-extract petroleum reserves trapped in shale, geothermal resources that are deeper and drier than ideal require their own subsurface cracking to maintain high pressure underground.
That this kind of pressure results in induced seismicity isn't surprising, Parrish said; but one question he hopes to answer is why that seismicity hasn't also been correlated to wastewater injection from the state's petroleum industry.
"Do we have a situation in California where the geology is not allowing the induced seismicity from oilfield operations to occur," he asked, "or have we just been lucky so far?"
Researchers still have much to do before they fully grasp the causes and consequences of induced seismicity. But should the situation call for eliminating fluid injection altogether, alternative solutions do exist. For these, Zoback looks to other states.
"If [the seismicity] were still in an area of active drilling or fracking, we could simply recycle that water" — like in Pennsylvania, where wastewater is reused for drilling and fracking elsewhere in the oilfield.
In Arkansas, regulators banned wastewater injection in an area where they suspected it had triggered an earthquake swarm. "They chose to create an exclusion area and say you're not allowed to inject within, I think it was 1000 square kilometers," he said. "They haven't had problems since then."