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.
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 and the United Kingdom, and 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 also very strong interests in statistical computing, and in automation projects. He is also a director of the University of Auckland's Bioinformatics Institute. See all posts by James Curran »