The number of earthquakes in the U.S. has risen dramatically since the advent of widespread hydraulic fracturing for the recovery of oil and natural gas. In both North America and Europe, concern that such activity could cause damaging earthquakes is rising and the debate shows signs of becoming another unscientific brouhaha fueled by ignorance and fear. It has been known for many years that fracking can cause seismic activity, as can damming rivers, mining minerals and pumping oil from underground. Is fracking being subjected to unfair criticism? Several new reports and a multi-national study, just published in the journal Science, attempt to take an objective, scientific view of the problem.
The July 12 issue of Science contains a research report by a tri-national team suggesting that sizable U.S. earthquakes may have been induced in part by nearby wastewater injection wells. Such wells are used to dispose of used fluids associated with oil and gas hydraulic fracturing operations. Adding fuel to the fire, the new study, “Enhanced Remote Earthquake Triggering at Fluid-Injection Sites in the Midwestern United States,” seems to establish a link between deep injection wells and sympathetic seismic activity triggered by earthquakes in other areas of the globe.
A team of researchers, led by Nicholas van der Elst of Columbia University's Lamont-Doherty Earth Observatory, linked a severe Chilean earthquake in February, 2010, to a significant earthquake in Prague, Oklahoma less than a day later. Unusual seismic activity continued near Prague for almost two years until late November 2011, when an earthquake destroyed 14 homes and injured two people. In April 2012, a major earthquake in Sumatra triggered another earthquake near Prague. Prague is the site of multiple wastewater injection wells, which the researchers say created the conditions for the remotely triggered seismic events.
Earthquakes can be induced by underground fluid injection, which increases pore pressure and allows faults to slide under preexisting shear stress. The increase in wastewater disposal from natural gas development and other sources has been accompanied by an increase in fluid-induced earthquakes in recent years. These earthquakes include widely felt earthquakes in Oklahoma, Arkansas, Ohio, Texas, and Colorado. Although most injection wells are not associated with large earthquakes, the converse is not true. At least half of the 4.5 moment magnitude (Mw) or larger earthquakes to strike the interior of the United States in the past decade have occurred in regions of potential injection-induced seismicity. In some cases, the onset of seismicity follows injection by only days or weeks, and the association with pumping at particular wells is clear. In others, seismicity increases only after months or years of active injection.
As the authors state above, Oklahoma is not the only place that injection well rrelated earthquakes have been triggered. The paper refers to earthquakes that are promoted by anthropogenic activity as induced and to earthquakes that are initiated by transient natural stresses as triggered. By this definition, there can be triggered induced earthquakes. “A search of the Advanced National Seismic System (ANSS) earthquake catalog gives preliminary evidence that induced seismic zones are sensitive to dynamic triggering by surface waves,” they report.
The figure above, taken from the paper, catalogs earthquakes above 3.0 M between 2003 and 2013 (A). Earthquakes in red occurred during the first 10 days after the February 2010, Maule; March 2011, Tohoku-oki; or April 2012, Sumatra earthquakes. Triggering occurs almost exclusively in three injection fields, labeled Prague, Trinidad, and Snyder. (B) Stacked earthquake counts in the 10 days before and after the three ≥8.6 Mw remote earthquakes.
In the end, the authors conclude that some areas of fluid injection become sensitive to small changes in stress. This allows the passage of seismic waves from remote large earthquakes to trigger more earthquakes near the well sites. They list a number of criteria for such sensitized regions: “First, all of the triggered sites in this study had a long history of regional subsurface injection over a period of decades. Second, each triggered site was near to hosting a moderate magnitude earthquake, suggesting critically stressed faults. Last, each site had relatively low levels of seismicity rate in the immediate vicinity (10 km) before the first triggering episode.”
Remote triggering indicates that conditions within an injection field have crossed a critical threshold. Furthermore, the occurrence of a triggered quake means that larger earthquakes are possible or even likely. The authors call for improved seismic monitoring in areas of subsurface fluid injection. But this paper is not the only recent journal article regarding earthquakes and fracking.A review of the subject in the same issue by William J. Ellsworth, of the U.S. Geologic Service, points out that U.S. earthquakes of greater than magnitude 3 have climbed sharply in the last decade, from an average of 21 per year before 2000 to 188 in 2011. Ellsworth's article, “Injection-Induced Earthquakes ,” is a review paper, and it provides an excellent overview of the topic. Here is Ellsworth's abstract:
Earthquakes in unusual locations have become an important topic of discussion in both North America and Europe, owing to the concern that industrial activity could cause damaging earthquakes. It has long been understood that earthquakes can be induced by impoundment of reservoirs, surface and underground mining, withdrawal of fluids and gas from the subsurface, and injection of fluids into underground formations. Injection-induced earthquakes have, in particular, become a focus of discussion as the application of hydraulic fracturing to tight shale formations is enabling the production of oil and gas from previously unproductive formations. Earthquakes can be induced as part of the process to stimulate the production from tight shale formations, or by disposal of wastewater associated with stimulation and production. Here, I review recent seismic activity that may be associated with industrial activity, with a focus on the disposal of wastewater by injection in deep wells; assess the scientific understanding of induced earthquakes; and discuss the key scientific challenges to be met for assessing this hazard.
Ellsworth points out that approximately 110,000 UIC class II wells are used for enhanced oil recovery. In addition, 30,000 class II wells in the United States are used for wastewater disposal. Of these wells, most have no detected seismicity within tens of kilometers, only a few are correlated with seismicity. As the previous paper discussed, seismic activity seems more probable around wastewater injection wells.
There has been a growing realization that the principal seismic hazard from injection-induced earthquakes comes from those associated with disposal of wastewater into deep strata or basement formations. Earthquakes release stored energy when a fault slips. A fault remains locked as long as the applied shear stress is less than the strength of the contact. The problem here is that injection wells can help promote a slip, which results in an earthquake. The reason this happens is that injecting fluid into a rock formation increases what is called pore pressure. Complicating matters, changing the loading above a fault can also cause a slip. This can happen from removing oil from the ground. These mechanisms are shown in the figure above.
Ellsworth goes into great detail, including an analysis of three fairly significant induced quakes going back a half century. There is no escaping the fact the the number of earthquakes detected has risen. Below is a plot of the cumulative earthquake count for magnitude 3 and greater quakes across a portion of the U.S. As can be seen, the rate has risen in recent days above the long-term rate of 21.2 earthquakes/year. States experiencing elevated levels of seismic activity included Arkansas, Colorado, New Mexico, Ohio, Oklahoma, Texas, and Virginia. The greatest rise in activity occurred in 2011 when 188 M ≥ 3 earthquakes occurred. But is this a reason for concern?
In environmental circles and on the political left fracking is portrayed as just as bad as nuclear energy. Hollywood has already generated movies trying to do to fracking what The China Syndrome did to nuclear energy. And it is not just oil and gas production that have fallen under suspicion. Efforts to increase geothermal power production are being tainted by concern over induced earthquakes.
Often touted as a renewable green energy source, geothermal's dark side is examined in an article, “Anthropogenic Seismicity Rates and Operational Parameters at the Salton Sea Geothermal Field,” by Emily E. Brodsky and Lia J. Lajoie, both from the Department of Earth and Planetary Sciences, University of California, Santa Cruz. “Although increased seismicity commonly accompanies geothermal production, induced earthquake rate cannot currently be forecast based on fluid injection volumes or any other operational parameters” they write. “We show that at the Salton Sea Geothermal Field, the total volume of fluid extracted or injected tracks the long-term evolution of seismicity.” Regardless of what green energy advocates say, no source of energy is free and all have their risks.
Geothermal – green energy with a few free earthquakes thrown in.
Perhaps even more telling is one of the sites used in the Ellsworth study. The U.S. Bureau of Reclamation extracts saline groundwater from shallow wells where the Dolores River crosses Paradox Valley to prevent its entry into the Colorado River system. Since 1996, the brine has been disposed of by injection into a 4.3-km-deep UIC class V well. Injection has induced more than 1500 earthquakes with M ≥ 1, including the MW3.9 earthquake on 25 January 2013, which was located 8 km northwest of the well. Sometimes earthquakes are caused by conservation efforts, not those evil oil industry types.
In the end, Ellsworth concludes that the earthquake risk from fracking itself is minimal, and even the threat from wastewater injection wells is not very great: “Long-term and high-volume injection in deep wells clearly carries some risk, even though most wells are apparently aseismic. In contrast, earthquakes induced during hydraulic fracturing have lower risk because of their much smaller magnitudes. The largest fracking-induced earthquakes have all been below the damage threshold for modern building codes.”
In the final analysis, a few small tremors from fracking is nothing more than the price of our modern technological civilization. When compared to the damage caused by burst dams and unregulated air pollution the fracking threat is pretty minimal. Of course, there are those who demand there be absolutely no possibility of any danger, regardless of how slight. Those fools would have us all return to living in trees, noble savages hunting and gathering among our fellow animals.
No thanks, I like technology: fresh plentiful food year round, heating and air conditioning, city water and sewage, automobiles and airplanes, all the benefits of our hydrocarbon based infrastructure. If my bed occasionally shakes gently in the night (which it does here in central Arkansas) so be it. In fact, I find it rather reassuring. Life entails risk, only the dead are free from it.
Be safe, enjoy the interglacial and stay skeptical.