Last week the National Research Council published a comprehensive study of the seismic hazards and risks of a variety of energy-related drilling activities. Despite widely publicized reports of drilling-related quakes in Ohio and Arkansas, the report concluded that such events are very rare, compared to both the total number of wells drilled and to naturally occurring earthquakes. Nor are the technologies with the highest rates of induced seismicity necessarily the ones that come first to mind. Rather than ignoring these risks because of their rarity, the committee of university and industry experts that produced the report recommended the development of new protocols for monitoring and managing these risks, as well as further research into the potential for induced seismicity from emerging technologies like carbon capture and storage (CCS.)
The study encompassed four categories of energy-related drilling, including oil & gas exploration and production, geothermal energy, liquid disposal wells, and CCS. Within oil & gas, they looked at conventional production and "enhanced recovery", along with hydraulic fracturing or "fracking". The latter two techniques involve pumping water or some other fluid into a reservoir to stimulate production. For geothermal, they considered conventional geothermal, both liquid- and vapor-dominated reservoirs, and "enhanced" or engineered geothermal systems, which pump fluid into hot, dry rock to extract useful heat. They found recorded seismic events in all categories and sub-categories, though again the numbers are small, particularly for quakes large enough to cause damage: Fewer than 160 recorded events globally over magnitude 2.0 within a period of about 30 years from a well population in the millions, and against a natural annual background of 1.4 million small earthquakes of 2.0 or greater and more than 14,000 larger quakes of 4.0 or greater.
In assessing the incidence of seismic events attributed to or suspected to have been caused by energy activities, the committee set a threshold for what they called "felt seismic events". This is crucial, because all of these technologies routinely cause minuscule events--"microseisms"--that can be detected by a seismometer in close proximity, but would go unnoticed by anyone standing on the surface. Magnitude 2.0 seems to be the lowest level event likely to be felt by an observer in the vicinity, while an event of 4.0 would be accompanied by more shaking over a larger area, and thus felt by many more people. Having grown up in earthquake country, I can attest to this. Anything below about 4.0 would often be mistaken for a train or large truck passing by, while most damage was due to quakes of 5.0 or greater. For comparison, last year's quake in Mineral, VA that affected the Washington Monument and National Cathedral registered 5.8. Only about a dozen of the induced seismic events included in the study were larger than that.
It's important to note that the mechanisms by which various energy-related drilling and injection processes trigger felt seismic events are fairly well understood. Scientists and engineers have known since the 1920s that human activities can trigger quakes, and the geosciences have advanced enormously since then. The main contributing factors identified in the report were the effect of fluid injection on increasing the pressure in the pores of subsurface rocks near faults, along with the "net fluid balance", which they defined as the "total balance of fluid introduced into or removed from the subsurface." As a result of these factors, drilling approaches in which the net fluid balance isn't materially altered, such as in waterflood enhanced oil recovery, or for which the changes are short-lived, as in hydraulic fracturing, tend to have very low rates of inducing felt seismic events. In particular, the study found only one documented felt seismic event, of magnitude 2.8, attributable to shale fracking, out of 35,000 fracked shale gas wells.
By contrast, liquid disposal wells, which steadily increase subsurface pore pressure over time, along with several types of geothermal production, exhibit somewhat higher rates of felt seismic events, though these are still relatively rare and generally minor in impact. At least theoretically, CCS seems to have a somewhat higher potential for causing seismic events, although this has apparently not been manifested in the substantial number of wells injecting CO2 for enhanced oil recovery--cited in the report as 13,000 as of 2007 and many more today. Surprisingly, the largest quakes attributed to human activities were associated with conventional oil production, including a couple of 6+ quakes in California and one measuring 7.3 in Uzbekistan.
One of the most interesting findings in the report was that there is no single government agency in the US with jurisdiction over induced seismic events associated with energy production. Responsibility--and capabilities--appear to straddle the Environmental Protection Agency, US Geological Survey, Forest Service and Bureau of Land Management, along with various state agencies. The committee proposed the development of new coordination mechanisms to address these events, as distinct from the ad hoc cooperation that has taken place to date.
I'm not sure what policy makers--the report was commissioned by the Chairman of the Senate Energy and Natural Resources Committee--and the public will make of these findings. At least from a statistical perspective the technologies assessed here look safe in terms of their seismic risks, and it would be hard to justify sweeping new regulations on the basis of this report. (I don't know how practical the "traffic light" monitoring system the authors propose would be.) On the other hand, with the exception of a few people in naturally quake-prone areas--including one neighbor back in California who thinks they are "fun"--earthquakes are fear-inducing, in both anticipation and experience. Arriving at a consensus on how low a risk of felt seismic events is acceptable might not be easy, especially where natural earthquakes are rare. Although the public's appetite for reassurance seems to be fairly low these days, it's clear that the National Research Council, an arm of the private, non-profit National Academies chartered by Congress during the Lincoln administration, sees no reason to panic about the seismic hazards and risks entailed in energy-related drilling.