The series of small earthquakes that have slightly rocked the Greenbrier area this month are no cause for alarm, according to Scott Ausbrooks, geohazards supervisor for the Arkansas Geological Survey.
The strongest of the recent tremors happened at 1:53 p.m. Oct. 15, registering a magnitude of 3.0 on the Richter scale — enough to notice in an otherwise still environment, but not enough to cause damage, Ausbrooks said. At 9:54 Sunday night, another minor earthquake, registering a magnitude of 2.9 on the Richter scale, occurred in an area about 2 miles north-northwest of Greenbrier and 4 miles south-southeast of Twin Groves.
Several other small earthquakes, all between 2.0 and 2.6 in magnitude, have also recently occurred in the area west and northwest of Greenbrier, according to Arkansas Geologic Survey data.
It’s similar to the phenomenon known to geologists as the "Enola Swarm," Ausbrooks said, when a 1.2-magnitude event near Enola in January of 1982 heralded a week of near-constant seismic activity that left geologic survey maps of the area peppered with small seismic events and peaked Jan. 21 with a 4.5-magnitude earthquake roughly at the center point between Enola and Holland. This largest event was at the magnitude level for those near the epicenter to experience "stuff falling off shelves," Ausbrooks said, but not strong enough to damage buildings.
According to Ausbrooks, the geologic structure beneath the area that is now experiencing the minor earthquakes is similar in its composition to that beneath the "Enola Swarm," and he said he would be surprised if earthquakes there reached a higher magnitude than those in the swarm.
Measurable earthquakes in both areas, he said, are thought to be caused by naturally-occurring fractures in relatively shallow rock layers about a mile to a mile-and-a-half down. There are several fault lines that cross the area, he said, but these are considered dormant by geologists, though they do produce some "microquakes" that can only be detected through instruments.
"There you just don’t have the kind of strain that build up real fast and causes larger events," he said.
Some in Faulkner County have speculated, as did the residents of Cleburne, Texas, earlier this year, that natural gas drilling and exploration may be triggering the seismic activity. The connection seemed clearer to Cleburne residents, according to reports, because no such activity had been recorded in their area prior to natural gas exploration there.
Those who believe there is a connection often cite the gas recovery process known as "fracking," in which a liquid is injected into a bore hole at high pressure to break up rock layers, thereby giving trapped natural gas an avenue to escape. Ausbrooks said that, as he understood it, the drilling and "fracking" occurs at a shallower depth than the geologic structures thought to be causing the earthquakes. There may be a connection, he said, but "at this moment we just don’t know what it’s related to."
Dr. Charles Langston, another geologist studying the phenomena and director of the University of Memphis’ Center for Earthquake Research and Information, said it was "kind of unusual that it decided to start popping off like this," but wasn’t ready to say that the theory that natural gas exploration was behind the earthquakes was more than plausible.
"In the area right there we’ve had lots of earthquakes in the past, so these could be naturally occurring," he said. "They seem oddly constant — right around 2.5. I guess the bigger question would be is there something bigger coming?"
If there is something bigger in the works, Langston said it probably wouldn’t be the work of local drillers. In the 1940s, there was a small earthquake in the Los Angeles Basin that was determined to have been caused by the removal of a great deal of petroleum, which resulted in "subsidence of the ground," or the "sinking" of an area due to the material that had supported it being removed, and in Rangely, Colo., he said, it was shown that the high-pressure fluid technique being used by drillers there did produce a measurable increase in measurable — but likely not felt — seismic activity.
Though the theory that the "fracking" operation triggers small earthquakes "seems reasonable," Langston said he didn’t "think well injection would cause an overly large earthquake," because the fractures the process creates run only as far as 100 meters, which he said isn’t enough to unsettle the structures to the point of producing damaging earthquakes.
For purposes of compiling data to better study the recent seismic activity, Langston urged citizens who feel — or think they feel — an earthquake to go to www.ceri.memphis.edu and follow the "did you feel it" link to complete a "felt report" describing the activity.
(Staff writer Joe Lamb can be reached at 505-1238 or by E-mail at firstname.lastname@example.org. Send us your news at www.thecabin.net/submit.)