Earthquakes are normal occurrences along the boundaries of major plate margins, such as along the San Andreas fault system of California, and are less common within plate interiors. Try telling that, however, to the citizens of Oklahoma who have recently experienced one of the largest recorded earthquakes in the states history.
Over the past decade the region has seen an increase in natural gas wells that extract gas through a process of hydraulic fracturing (hydrofracking or fracking). The method injects fluids into the subsurface to assist in the extraction of natural gas. A host of environmental issues related to fracking is better left for another post.
Recent articles in Reuters, the Colorado Independent, and a study by the Okalahoma Geologic Survey have questioned whether the rise in fracking in the region is related to the increased earthquake activity. The USGS, in fact, reported on an earthquake driven by fluid injection near Denver, CO in 1967.
Any undergraduate student of geology can tell you that increased pore fluid pressure can lead to slip on faults that would otherwise be stable under dry conditions. An analogy would be sliding a pint glass across a wet bar versus a dry one. Rather than rehash Mohr circle construction and Terzaghi’s law of effective stress we can use some open source software and online datasets to look at the spatial and temporal correlation of natrual gas wells and earthquakes.
The figures below were constructed using the open source software R and the packages RGoogleMaps, and ggplot2 (with their dependcies). The first figure is a map of Oklahoma with the locations of natural gas wells in blue (taken from the Oklahoma Geologic Survey) and earthquake epicenters in red (from the USGS).
The majority of earthquakes are clearly clustered in regions bounded by wells. But is there a temporal relationship between drilling and earthquakes? Glad you asked…
Before large scale drilling there were less than 25 earthquakes per year. With the start of increased drilling circa 2000 the region has seen an order of magnitude increase in the number of earthquakes.
A recent op-ed in Scientific American casts the right sentiment with it’s title “Safety First, Fracking Second: Drilling for natural gas has gotten ahead of the science needed to prove it safe”
2011-11-09: Update: A link to the R code as well as earthquake and well locations used to create the above plots can be found here.