earthquakeNatural gas has been promoted for decades now as the cleanest-burning of the fossil fuels – in terms of both conventional, health-threatening air pollutants as well as climate-altering greenhouse gases.

Environmental and public-health complaints have spread in recent years, however, because of unwanted side-effects of a boom in drilling for natural gas using the method known as hydraulic fracturing, or fracking.

The resulting controversies have focused largely on concerns that fracking causes water and air pollution. But there are also questions about earthquakes – essentially, whether they may result from fracking’s injection of fluids including water and chemicals into shale formations deep underground (and from the later injection of waste fluids that come back to the surface).

Oklahoma’s biggest earthquake ever recorded – which occurred on Saturday night, Nov. 5, damaged 14 homes and was followed by a series of aftershocks – is the latest event to fan that aspect of the multifaceted fracking debate and generate news coverage.

Like Texas, Pennsylvania and other states, Oklahoma has witnessed an increase in fracking in recent years. And it has seen an increase in earthquakes. Time reported this week:

Oklahoma isn’t California — this is a state that is usually pretty seismically stable, one with about 50 small quakes a year until 2009. But the number of quakes spiked in 2009, and last year 1,047 tremors shook Oklahoma.

The Associated Press reported on Tuesday that experts were skeptical that fracking, linked in some studies to smaller tremors, was related to the bigger earthquake on Saturday, which had a 5.6-magnitude measurement and was centered 44 miles east of Oklahoma City:

In Oklahoma, home to 185,000 drilling wells and hundreds of injection wells, the question of man-made seismic activity comes up quickly. But so far, federal, state and academic experts say readings show that the Oklahoma quakes were natural, following the lines of a long-known fault.

“There’s a fault there,” said U.S. Geological Survey seismologist Paul Earle. “You can have an earthquake that size anywhere east of the Rockies. You don’t need a huge fault to produce an earthquake that big. It’s uncommon, but not unexpected.”

The Oklahoma Geological survey tentatively concluded in a study published in August, but not reported by the news media until last week, that fracking may have triggered a cluster of about 50 small quakes in the state in January:

The strong correlation in time and space as well as a reasonable fit to a physical model suggest that there is a possibility these earthquakes were induced by hydraulic fracturing. However, the uncertainties in the data make it impossible to say with a high degree of certainty whether or not these earthquakes were triggered by natural means or by the nearby hydraulic fracturing operation.

Also last week, it was reported that the energy company Cuadrilla, which conducted the first fracking operation in the United Kingdom, had concluded in its own study that the drilling technique had probably caused two small earthquakes in northwest England earlier this year.

The ScienceInsider website of the American Association for the Advancement of Science reported:

Seismologist Brian Baptie of the British Geological Survey, which was not involved in the report, says this is the first time hydraulic fracturing has been shown to cause earthquakes, although it is not entirely surprising. Injection of fluid waste from shale gas and oil drilling has caused small quakes in the past. Fluid injection, he explains, creates small fissures in the rocks that allow them to slide more easily past each other, “lubricating” them in a sense. In the case of the Cuadrilla site, the report stated, this occurred near an already-stressed fault. The fluid spread over a period of 10 hours after injection and caused the quakes.

In July, Arkansas’ Oil and Gas Commission unanimously voted to ban disposal wells for waste fluids from natural gas drilling because of concerns about earthquakes. The Associated Press reported at the time that the ban was being enacted in “a region where hundreds of earthquakes have struck, a move officials said was necessary to prevent a potential catastrophe.”

The AP added:

Gas companies have tapped reserves of natural gas in the Fayetteville Shale by injecting water and chemicals under high pressure to fracture the shale, a process known as fracking. The fluids that emerge in the process have the potential to taint rivers and drinking water.

Separate disposal wells are used by drillers to dump the fluids.

With a moratorium, companies would have to truck the fluids to injection wells elsewhere in Arkansas or in Oklahoma or Texas, Commission Deputy Director Shane Khoury told The Associated Press after [the commissioners’] vote. About 730 disposal wells are active in the state, he said.

Concerns were voiced in North Texas about the possibility that natural gas drilling in that region’s Barnett Shale formation might have been linked to several small earthquakes in the Dallas-Fort Worth area on three occasions in 2008 and 2009, and that still-larger quakes might be in store.

Researchers at Southern Methodist University and the University of Texas concluded [PDF] in a study published last year that it was “plausible” that fluid injection in one saltwater disposal well in the area could have caused the Dallas-Fort Worth earthquakes:

Prior to 31 October 2008 [when the first of the 2008-09 quakes occurred], there had been no local felt earthquakes known in Dallas and Tarrant Counties, which have been settled since about 1850. About seven weeks before the DFW quakes began, injection commenced in a [saltwater disposal] well only a few hundred meters from DFW epicenters and with an injection depth of 3.1 – 4.1 km [near the 4.4 – 4.8 km depth of 11 monitored earthquakes].

The SMU and UT researchers sketched this broader context for the DFW-area earthquakes, noting the need for more scientific understanding about the possible links between manmade earthquakes (“induced seismicity”), fracking and other activities related to efforts to provide cleaner energy sources and fight global warming:

Fracture stimulation [fracking] and saltwater disposal are critical components of the current development strategy for shale gas, which provides a large, relatively clean source of energy that can be tapped until renewable sources become more viable. Enhanced geothermal projects rely on fracturing and fluid injection and have also raised concerns about induced earthquakes. Geological carbon sequestration is one approach being researched to combat global climate change. It involves pumping large volumes of carbon dioxide into targeted geo- logic formations. To allay public concerns, more needs to be known about how these activities interact with in-situ stresses and possibly affect seismic activity. Energy producers, policy-makers, and researchers would all benefit from an improved understanding of induced seismicity.

– Bill Dawson

Image credit: Baris Onal,