Oklahoma Earthquakes

john's picture

Last saturday, November 5, 2011, a magnitude 5.6 earthquake occurred northeast of Shawnee Oklahoma. The question on some minds is whether the event was related to fluid pumping (for example, hydrofracking for hydrocarbon production) in the area.  While we cannot know for certain, there is a historical record of fluid pumping leading to moderate earthquake occurrence. 

A famous example is the August 9, 1967 magnitude 5.3 earthquake near Denver.  This and a similar magnitude 5.0 event on April 10, 1967  were likely associated with the injection of fluids into a deep (~12,000 foot) well at the Rocky Mountain Arsenal [1].  Fluid injection began in 1961, and within a short time, sequences of small earthquakes began to be felt in the area.  Magnitudes of the events generally increased in time, culminating with the large magnitude 5 events.

Another more recent example was the Basel, Switzerland earthquake sequence that was associated with the drilling of a geothermal well within the boundaries of the city.  The well was drilled as part of a hot dry rock energy recovery project.  Drilling and fluid injection began in 2006, with the largest earthquake to date having a magnitude 3.4.  The event occurred on 6 January 2007.  Drilling in the project was suspended shortly afterward. [2]

 A final example is the Geysers geothermal field in northern California [3].  Swarms of small earthquakes having magnitudes between 0.5 and 4.6 have occurred since operations at the geothermal field began in 1960.  Recovery of steam and heated fluids is accompanied by re-injection of "waste" fluids. 

While the connection betwen the fluid injection and the earthquakes has not been precisely established, it may be that the pressurized fluids act to "lubricate" subsurface fractures and faults, allowing them to slip in response to pre-existing regional stress fields.  Physical effects such as this have been proposed to act in association with natural, or tectonic earthquakes as well.

[1] http://www.onepetro.org/mslib/servlet/onepetropreview?id=00002558&soc=SPE

[2] http://www.treehugger.com/corporate-responsibility/geothermal-power-plan...

[3] http://en.wikipedia.org/wiki/The_Geysers

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