|Location of M4.7 earthquake, Feb. 28, 2011|
Earthquakes have been noticed in Arkansas as far back as 1699 (by missionaries traveling down the Mississippi River). This region was affected by the 1811-1812 New Madrid earthquakes, and intermittent earthquakes have been felt since then. Current earthquake activity (available on the Arkansas site), which is not on the New Madrid fault zone, generally consists of several M2 - M2.5 earthquakes on most days, although on April 8, a pair of M3.9 earthquakes hit central Arkansas, about 35 miles north of Little Rock. About 1000 quakes have hit the region since September, 2010. The largest was a M4.7 on Feb. 27, the biggest quake to hit the state in 35 years.
Concerns have been raised that injections of wastewater from natural-gas production were related to the seismicity. The wells are owned by Chesapeake Energy and Clarita Operating. The region is underlain by an "organically rich rock formation", a major source of natural gas. To extract the gas, pressurized water, containing a slew of chemicals, is injected to create fractures deep in the ground, a process known as 'fracking.' When the fracking water is no longer useable, it is disposed of by injecting it into the ground. At the request of the Arkansas Gas and Oil Commission, the owners ceased operations of the injection wells on March 4, well before these April 8 M3.9 earthquakes. Scott Ausbrooks, a geo-hazards supervisor for the AGS says that it may be too early to rule out a cause-and-effect with the injection process.
Is the seismicity related to fracking? It seems like there is not a definitive experiment. Will fracking cause permanent changes in the shallow crust? Likely--once you've fracked a rock, you can't unfrack it. It takes energy to fracture rocks (this energy comes from the motors of the engines that drive the fluids into the rocks), and you can't un-frack the rocks easily. There are many questions and concerns raised about fracking, including not only the stability of the rocks and seismicity, but water quality as large volumes of water filled with chemicals are injected into the rocks. Fracking has been going on for a long time, and so the energy extraction industries have a lot of experience with this technique, but public awareness and concern has increased dramatically the past couple of years because of the increased emphasis on extraction of natural gas reserves.