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April 11, 2011 / Dave Gorham

Guy, Arkansas: What's With all the Fracking Earthquakes?

Guy, Arkansas has experienced more than 700 earthquakes over the past several months. Generally these quakes have been less than a 4.0 on the Richter scale, with most being in the 2.0 to 3.0 range. Geologists are not blaming magma (magma near the surface of the earth is the typical cause of multiple earthquakes). Instead, they’re blaming hydraulic fracking.

ImpactWeather’s resident geologist* Fred Schmude, Manager, ImpactWeather StormWatch team has been watching the Guy quakes and reports: “Arkansas sits atop a large unstable fault zone called the New Madrid which runs from northeast Arkansas northward across southeast Missouri, southern Illinois, southern Indiana, western Kentucky, and western Tennessee. Underneath the fault zone runs a subsurface rift zone called the Reelfoot rift. The New Madrid seismicity is thought to be associated with compressive stresses on the Reelfoot rift from geologic plates which in turn create the earthquakes.”

Fred continues: “Typically with any geologic rift zone, molten rock (magma) will be forced upward along the rift. The famous rift valleys of Iceland and eastern Africa are good examples of rift zones where magma occasionally  percolates up toward the surface of the earth. In Arkansas, the Reelfoot rift is no longer active and not capable of such a dynamic process as we see with other active rift zones. Any magma generated from the Reelfoot cooled many years ago in the form of large plutonic rock, as can be seen in the diagram.”

Crystallized magma (black) indicates the Reelfoot rift is no longer active. Image: Wikipedia

If subsurface magma isn’t the cause of the Guy quakes, then what is? To have so many quakes in such a small area indicates either a local geological phenomena or something man-made associated with drilling or mining.

Scott Ausbrooks, geohazards supervisor for the Arkansas Geological Survey,  said the quakes are part of what is now called the Guy earthquake swarm, a series of mild earthquakes that have been occurring periodically since 2009.  A similar swarm occurred in the early 1980s when a series of quakes hit nearby Enola, Arkansas.

Ausbrooks indicated, “It could just be a naturally occurring swarm like the Enola swarm, or it could be related to ongoing natural gas exploration in the area.”

Note that a major source of natural gas in Arkansas is the Fayetteville Shale, an organically-rich rock formation in north-central Arkansas. Drillers release the gas by using hydraulic fracturing or “fracking” – injecting pressurized fluid to create fractures deep in the ground.

Ausbrooks continued, “Geologists don’t believe the production wells are the problem, but rather the injection wells that are used to dispose of ‘frack water’ when it can no longer be re-used. The fracking fluid is pressurized and injected into the ground.”

Deep injection well for disposal of hazardous, industrial and municipal wastewater. Image: Wikipedia

“We see no correlation between natural gas production wells and earthquakes, but we haven’t ruled out injection wells,” he said, adding that if production wells were the cause, the earthquakes would be scattered all over the region underlain by the Fayetteville Shale formation and not in just one area.

“The earthquakes are occurring in the vicinity of several injection wells,” said Ausbrooks.

Based on this information, it does appear there could be some correlation with the injection wells rather than the production wells. The injection wells produce the pressure and that could be the reason why the quakes are occurring.

Only time will tell since a moratorium has been put into place on injection wells in the Guy area to see if there is a correlation between these and the earthquakes. If there is a significant reduction of the quakes over the next month, then we may see future restrictions on this type of drilling in the years to come, especially over those unstable geologic areas where even only small stresses on the local fault zones can result in earthquakes.

* As is the case with many on the ImpactWeather meteorology staff, Fred is cross-trained in a field other than meteorology.  In addition to a BS in meteorology from Texas A&M, he also obtained his BS in geology at Texas Tech.

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