At 8.10 am local time (2.10 pm GMT) on the morning of 25 December 2011 the small town of Cromwell in Seminole County, Oklahoma, was shaken by a small Earthquake. The quake measured 3.3 on the Richter Scale, which is fairly small, but occurred at a depth of only 10.4 km, directly beneath the town, enough to give the town quite a jolt, though there are no reports of any injuries or serious damage.
Earthquake location map from the United States Geological Survey.
Oklahoma is a long way from any major tectonic boundaries, but it does have a number of fault systems, and small Earthquakes are not unusual. Up until 2009 Oklahoma averaged about 50 Earthquakes a year, but in each of the two years 2010 and 2011 Oklahoma suffered over 1000 quakes, which requires an explanation.
Map of the geological fault lines of Oklahoma, from the Bureau of Economic Geology at the University of Texas at Austin.
This is major increase in seismic activity over a short period of time, and requires an explanation. In the absence of any known natural phenomenon that could have caused this (such as the appearance of a volcano in central Oklahoma), many residents have started to question whether the state's hydrocarbon industry could be to blame. Oklahoma has long been a producer of both natural gas and oil, but as reserves have shrunk producers have increasingly turned to Hydraulic Fractionation (Fracking) as a means of maintaining production levels.
Fracking is a process by which water and chemicals are forced into buried strata in high pressure blasts in order to produce shock waves, which will hopefully cause the release of hydrocarbons. Shock waves in buried strata are, in layman's terms, Earthquakes, so an increase in Earthquakes in areas where Fracking is being practiced is not surprising. The Fracking industry have always maintained that their aim is to provoke quakes with magnitudes of less than 1.0 on the Richter Scale, but in Oklahoma, as in other places the practice has been linked with much larger quakes, often in excess of 3.0; which is more than a hundred times as large (the Richter Scale is logarithmic, 2.0 is ten times as large as 1.0, 3.0 is ten times as large as 2.0 and one hundred times as large as 1.0), something the industry disputes, refusing to accept responsibility for such large quakes without further evidence, even though they cannot offer an alternative explanation for the increase in quake activity.
The American National Academy of Sciences is currently looking into the subject, and is expected to produce a detailed report on Fracking and Earthquakes sometime in 2012, but in the meantime the Oklahoma Geological Survey commissioned a report from Austin Holland, a tectonics expert at the Department of Geosciences, University of Arizona, into the conetection between a fracking operation at the Eola Field in Garvin County, to the southwest of Seminole.
Holland came to the conclusion that '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'. This has been widely reported in the press as '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', or even 'Seismologist Cannot Link Drilling To Oklahoma Earthquakes', which is a long way from what Holland actually reports.
Holland examines seven criteria which need to be met in order to establish that the quakes are the responsibility of the Fracking operation in Garvin County. Five of these can be met, and the remaining two cannot be answered either way given the evidence available when making the report, which had not been gathered with the specific intention of answering these questions.
1. Are these events the first known earthquakes of this character in the region?
Unknown. Oklahoma does not have a long history, and earthquake monitoring has never been a priority as the state has never suffered a seriously debilitating quake. Therefore it is impossible to say with confidence that quakes on this scale have never occurred, merely that they have never been recorded.
2. Is there a clear correlation between injection and seismicity?
Yes. There was a clear correlation in time and space between deep fracking operations at Eola Field and earthquakes, but not between shallow operations and any quakes.
3. Are the epicenters near wells (within 5 km)?
Yes. Nearly all the earthquakes have epicenters (the point on the surface directly above the centre of the quake) within 5 km of a well, and the majority much closer.
4. Do some earthquakes occur at or near injection depths?
Yes. Most of the earthquakes occur near injection depths.
5. If not, are there known geologic structures that may channel flow to sites of earthquakes?
Yes. There are numerous vertical block faults within the Eola Field, which have excellent potential for channeling flow.
6. Are changes in fluid pressure at well bottoms sufficient to encourage seismicity?
Yes. If this was not the case then hydraulic fractionation would not work in these rocks.
7. Are changer in fluid pressure at hypocentral locations sufficient to encourage seismicity?
Unknown. The study was conducted using data that was collected for other purposes; it would simply not be possible to say from this data if this criteria was correct.
Map from Holland's report showing the extent of the Eola Field (cross-hatched area), pre-existing geological faults (green lines), the site of the well where Hydraulic Fractionation is taking place (black hexagon), an the location and depth of known quakes that have taken place since fractionation began (coloured circles, key to depths bottom right).
Thus when Holland states that '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' he means just that; he is saying that using the data available, which was not gathered for the purpose of this study, it is not possible to be 100% certain that the quakes are being caused by the fracking, but this is still quite likely. In legal terms the balance of probability is that fracking is causing earthquakes, but that this cannot be proven beyond all reasonable doubt. Unfortunately Holland states this with dry scientific reserve rather with a salesman's hyperbole, which has enabled a degree of (sometimes willful) misinterpretation.