November 9, 2011

What the frack?

The New Zealand Herald (09-Nov-2011) has a very interesting article about earthquakes in Oklahoma. Scientists from the Oklahoma Geological Survey plan to investigate whether the process of “fracking” has led to an increase in earthquake activity.

Fracking is a controversial fossil fuel recovery method whereby high pressure water is injected into rock, fracturing it, and then send is forced into the cracks allowing the substance of interest, in this case gas, to escape. This process has been known about for quite sometime, but it is the depletion of existing reserves, and the subsequent increase in the price of oil and gas that has made it exceptionally popular in recent times.

In Oklahoma, the principal fracking area is known as the Devonian Woodford Shale. According to Wikipedia, the first gas production was recorded in 1939, and by late 2004, there were only 24 Woodford Shale gas wells. However, by early 2008, there were more than 750 Woodford gas wells. Another site reports that currently over 1,500 wells have already been drilled with many more to come. The wells cost $US2-3 million, and there are more than 35,000 shale gas wells currently in the United States.

One of the nice things about the US Geological Survey, and its state based constituents, is that it is usually relatively easy to get data from them. I say relatively, because it required some searching and programming to speed the process up, but the data is all there for someone willing to spend sometime getting it.

To show fracking is causing an increase in seismic activity would require proper experimentation. However, it may be possible to show correlation at least between the increase in fracking wells and the number of seismic events. I don’t have enough clout, or time, to extract the information about the number of wells, and their location. However it is still interesting just to take a look at the data we can get regarding the number of earthquakes ourselves.

Time series plot of earthquakes in Oklahoma

The black line in time series plot above shows the number of seismic events from January 1977 to October 2011. The rise at the start of 2010 is certainty indisputable. The blue line a form of exponential smoothing called Holt-Winters smoothing (or Holt-Winters triple exponential smoothing). It is a simple statistical technique that attempts to model the trends (among other things) in time series data. The green line is the predicted number of earthquakes using this smoothing model (calculated on the pre-2010 data) for the time period starting January 2010 to October 2011, and the red line is the upper confidence limit on this prediction. This is a very simple modelling attempt, and undoubtedly the “real time-series analysts” could do better (and here is the data for you), but what I would like to think this shows is that the increase in quake count is so far off the charts that it definitely qualifies for further investigation.

Some of you will no doubt be grumbling that I have not accounted for the magnitude, or depth, or location, or in fact many other things, and indeed I have not. However, I do think the data is interesting, and the association with the increase in fracking should be explored further – which is what the Oklahoma Geological Survey plans to do.

I have made the uncleaned raw data available here.


James Curran's interests are in statistical problems in forensic science. He consults with forensic agencies in New Zealand, Australia, the United Kingdom, and the United States. He produces and maintains expert systems software for the interpretation of evidence. He has experience as an expert witness in DNA and glass evidence, appearing in courts in the United States and Australia. He has very strong interests in statistical computing, and in automation projects. See all posts by James Curran »


  • avatar

    Definitely worth further investigation, but wouldn’t a graph for Christchurch look pretty similar?

    6 years ago

    • avatar
      James Curran

      I had pretty much the same thought when I got home last night. I have asked a friend if they can get similar data for Canterbury.

      Interestingly, Oklahoma has had some large earthquakes, including a 5.6 magnitude quake last week.

      Austin Holland, the geologist mentioned in the article, has said that he doesn’t believe that fracking is associated with events of that size. He also did a smaller study in a specific location. His research found most of those tremors happened within 24-hours after fracking ended. While Holland says there is a “strong correlation,” he also says it’s “impossible to say with a high degree of certainty” whether those earthquakes were natural or man-made.

      As you have thought about, and Thomas and I both agree, a large seismic event like the Christchurch earthquake will have many hundreds, if not thousands of aftershocks. I’m assuming, perhaps naively, that Dr. Holland has taken this into account, and that he knows infinitely more about it that me since he is a geologist and I am not.

      I would also point out again that this is an immensely controversial issue, and reading around one can find “experts” who are at polar extremes on the hypothesis. There is also an awful lot of emotive, uninformed comment.

      6 years ago

  • avatar
    Thomas Lumley


    A graph for ChCh might look similar, but Oklahoma is rather larger than ChCh. And in fact, in ChCh an analysis like this would correctly tell us that something significant happened in early September 2010 that affected future quake risk.

    A useful refinement would be to look at magnitudes, to see if there are many separate quakes with their own aftershock series or just one set. And to look at location in more detail.

    6 years ago

  • avatar
    john pearson

    I’m not a geologist but am living in Canterbury, my thoughts: Location hides a lot, I suspect it needs to be accounted for on multiple scales, depth also as a simple proxy for fault effects (as a manifold inside a low curvature sphere, at least at the surface) or just strength. Critical to interpretation is whether the nett seismic activity changing (and how would that be best measured and over what scale?) or is the location of the activity changing, in particular to more densely inhabited areas. If the hypothesis that fracking increases local seismic activity is true, then it is not a huge leap to see that fracking could be used to shift seismic stress from sensitive zones or induce low impact events prior to a natural tipping point.

    6 years ago

  • avatar

    Bottom line, I’d want to see conditional probability of quakes given fracking rather than just a time plot showing number of quakes in a couple places that happened to have quakes subsequent to fracking. Fracking’s moving into a whole pile of places in North America. I’d be really really surprised if places that rarely got earthquakes didn’t look exactly like this after any random draw moderate-to-large seismic event.

    I’m still agnostic about the effects of fracking on quakes. Certainly possible, and there’s a plausible causal mechanism for it. But a quake in one spot subsequent to fracking isn’t enough to do it for me. I’d probably put better than 50/50 odds on that fracking is causally linked to quakes. But I wouldn’t go a ton higher than 50/50.

    6 years ago

  • avatar
    Brad Luen

    For Canterbury I’m fairly confident you’d get a qualitatively different pattern: a big spike for Sept 2010, a decay for the next few months, an even bigger spike in Feb 2011, and then decay again. That there isn’t (after scanning the data by eye) a large earthquake mear the beginning of the increased activity in Oklahoma makes the sequence different from any that immediately comes to my mind.

    We know a few empirical things about earthquakes — that they cluster in space and time, that large earthquakes are followed by exponential more further quakes than are small earthquakes, and that the rate of aftershocks after a large event decays as something like 1/t. The trouble is that none of these things are exact enough to be useful as null hypotheses. You could fit one of the existing stochastic process models, like the ETAS model, and show that for some goodness-of-fit criteria the model fits better with a change-point, but these models don’t fit especially well at the best of times so it wouldn’t IMO prove anything beyond reasonable doubt.

    6 years ago