I’m going to step a bit outside of my comfort zone for this, my fourth and final article in a series on the environmental impacts of natural gas. (Click for part I, part II, and part III.) The question under consideration today is whether fracking causes earthquakes? Fortunately, there is some available science within the published literature to draw on. As I’ve consistently tried to emphasise during this series, we can do no better than make peer-reviewed science our ultimate arbiter for dealing with contentious issues like shale gas drilling.
The notion that human activity can induce seismic events is not new. Geologists have known for decades that mining and other drilling activities can trigger earthquakes. (I use “trigger”deliberately. Rather than causing these events outright, they are more catalysts for releasing inherent stresses within the tectonic system.) However, the link between fracking and earthquakes is a fairly recent concern. This is certainly true in the public sphere, where it was brought to prominence by various media reports in early 2012. This coverage followed the release of an abstract that was intended to preview a talk at a U.S. Geological Survey (USGS) gathering: Are Seismicity Rate Changes in the Midcontinent Natural or Manmade? Perhaps unsurprisingly, media interpretation of this abstract was rather more excitable than the text itself… So much so that David Hayes, Deputy Secretary of the U.S. Department of the Interior, saw fit to publish a clarifying statement:
“While it appears likely that the observed seismicity rate changes in the middle part of the United States in recent years are manmade, it remains to be determined if they are related to either changes in production methodologies or to the rate of oil and gas production.
We also find that there is no evidence to suggest that hydraulic fracturing itself is the cause of the increased rate of earthquakes.”
A larger point from Hayes’ statement is that the observed seismic events need to be placed into context. The overwhelming majority are far too small to be of economic and humanitarian significance.
Tremors? Yes. Noticeable? Not likely.
A similar debate has been taking place in Britain, where higher population densities have also caused people to worry about the dangers of fracking-induced earthquakes. Two micro/minor quakes (< 3.0 ML on the Richter Scale) at a fracking location near Blackpool in 2011 prompted an inquiry into their origin. The British Geological Survey (BGS) reports that further analysis of these events did indeed pinpoint exploratory fracking operations as the immediate cause, and that further small quakes cannot be ruled out. However, the BGS goes on to suggest that “the risk from these earthquakes is low, and structural damage is extremely unlikely”.
More recently, Davies et al. (2013) have just published an important new study in the journal Marine and Petroleum Geology. An ungated version of the paper is here. The researchers compare 198 known incidents of (human) induced earthquakes since 1929 and similarly conclude that the seismic risks from fracking are minimal:
“It should be noted[…] that after hundreds of thousands of fracturing operations, only three examples of felt seismicity have been documented. The likelihood of inducing felt seismicity by hydraulic fracturing is thus extremely small but cannot be ruled out.”
Figure 1: Frequency vs. magnitude for 198 published examples of induced seismicity. (Source: Davies et al., 2013)
The study’s lead author, Prof. Richard Davies of Durham University, has emphasised the need for a balanced perspective on the matter in various press interviews following its release. Other human activities like mining, oil recovery and even the generation of geothermal energy are responsible for far greater incidences of induced seismicity (both in terms of occurrence and magnitude). In contrast, the tremors caused by fracking are most likely to be undetectable to humans, or at least “no greater than the impact of a man jumping off a ladder”.
Wastewater injection, context and other takeaways
In researching this article, I was rather surprised to discover that the “link” between fracking and seismicity is also a question of injecting wastewater deep into the earth for disposal, not necessarily the fracturing of the shale rock formations itself. (Mother Jones has a nice graphic showing this here.) Thus, another recent paper published in the journal Geology by Keranen et al. (2013). The authors of this study tie a 5.7 Mw Oklahoma quake in 2011 to a local wastewater injection site, in the process suggesting some modifying criteria for how we classify future incidents of induced seismicity. It should be noted that wastewater disposal of this type applies to many forms of energy production, such as advanced oil recovery. Indeed, some overzealous fracking opponents appeared to miss the fact that the injection site under evaluation in the above study was actually an oil field – not a location for fracturing of natural gas. A nice summary from Columbia University makes this point explicit: “The water linked to the Prague [Oklahoma] quakes was a byproduct of oil extraction at one set of oil wells, and was pumped into another set of depleted oil wells targeted for waste storage.”
All this is not to say that frackers should be free to inject wastewater at any location of their pleasing. Clearly, we have evidence that fault formations should play a prominent role in determining the viability of deep disposal wells. (My reading of the situation leads me to believe that most energy companies have long been engaged in taking such steps.) However, I fail to see why fracking should be uniquely singled out on that score. If we further consider some of the other research mentioned in this article, I don’t regard earthquakes as any kind of insurmountable obstacle to fracking activity at this stage.
The takeaway is consistent with the overarching theme of my series. Yes, there are environmental trade-offs to securing the benefits of fracking and natural gas at large. Placed into the right context, however, these are relatively benign and often much better than the immediate alternative. Economics teaches us that there is no such thing as a free lunch, but fracking looks like a pretty good deal to me.
Image: Earthquake Damage via Shutterstock