Wastewater Disposal and Earthquake Generation
Wastewater disposal involves injection of large volumes of water at high pressures into the ground: such lubrication of the rock is thought to increase the likelihood of slippage on a fault. Dr. McGarr told Decoded Science that ‘these waste water disposal activities are the important causes of induced earthquakes’.
The recently-unveiled USGS research has provided a methodology – albeit yet to be refined – for predicting the maximum possible size of an earthquake which could be released in any given operation. (Fracking, incidentally, is only one of the processes considered which may trigger earthquake events – other, more environmentally friendly procedures such as carbon storage and geothermal technologies, which also involve fluid injection at depth, also carry a risk).
Based upon studies of previous events, including minor tremors generated in Australia, Europe and, most recently, Oklahoma, the study established a relationship between the amount of fluid injected and the size of tremor generated. It found that for 10,000 cubic metres of fluid injected, the maximum earthquake size would be M3.3 and, given that the magnitude scale is logarithmic, each doubling of fluid would increase the size by M0.4.
Though useful, the model is simplistic. It is based upon the maximum size of a tremor (in many cases, any event would be smaller than predicted) and involves certain key assumptions – most notably, that the rock is strong (weaker rock will fail at lower stress levels). Many key variables are not incorporated and the model doesn’t take account of specific geological conditions. Dr. McGarr told Decoded Science, for example, that it is not applicable where rock is highly permeable (i.e. water flows easily through it) or is able to deform relatively easily and so can accommodate the stress.
Where Does the Research Lead?
The extent of earthquake generation by fluid injection currently seems limited (very few of the disposal sites in the U.S. have caused any reported tremors at all). The USGS research provides a basis for further development, possibly involving local geological conditions – but many issues relating to the relationship between fluid injection and earthquakes remain unresolved.
British Geological Survey. Blackpool earthquake. (2011). Accessed 20 December 2011.
Choi, Charles Q. Did fracking cause Oklahoma’s largest earthquake? Scientific American. (14 November 2011). Accessed 20 December 2011.
Corbyn, Z. Method predicts size of fracking earthquakes. Nature. (December 2011). Accessed 20 December 2011.
Environmental Protection Agency. Natural gas extraction – hydraulic fracturing. Accessed 20 December 2011.
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