According to the Arkansas Geological Survey, there remains no clear evidence that natural gas exploration is causing the small earthquakes recorded — and sometimes felt — in the Greenbrier area over the last few months.

"Right now, the majority of the microtremors that we’ve seen are much deeper than any of the production wells nearby," AGS geohazards supervisor Scott Ausbrooks said Monday. "At this time, we don’t see a causal relationship between the drilling and the earthquakes."

Relatively low-level seismic activity in the geologic formations under Faulkner County is nothing new. In 1982, a phenomenon that became known as the "Enola Swarm" got the attention of AGS with a magnitude-4.5 earthquake on Jan. 21, 1982 — the most powerful earthquake recorded in the area. Since the start of the Enola Swarm, more than 40,000 earthquakes have been recorded in the Enola area, including a magnitude-4.4 earthquake on May 4, 2001.

The recent earthquakes, which have been centered in the Greenbrier area, are happening in formations of rock that Ausbrooks said are similar to those under Enola.

"Looking at the geologic structure and the geology of the area, it is very similar — the faulting, the sedimentary structures; everything is very similar," Ausbrooks said. "You have a change in direction of faults there. In this part of the state, you have east/west trending faults, but when you get into Faulkner County they change direction. At a certain point there they kind of get a kink in them, if you will, and they go in a more northeast direction and ... these tight points are where we see these events occurring."

"If you think about it, there’s over 2,000 production wells in the shale area; if there’s a relationship between the two, these earthquakes should be all over where the wells are, but this isn’t the case," Ausbrooks added.

It wouldn’t be the natural gas production that would cause earthquakes, Ausbrooks said, because the horizontal drilling and high-pressure fracturing occur at depths between 2,000 and 8,000 feet. The only human activity recorded to have a role in triggering seismic activity, he continued, is deep well injection, and the natural gas industry does do this to an extent as a way to dispose of wastewater, but there hasn’t been any evidence that this practice is causing earthquakes in Faulkner County.

What may cause the number of reported earthquakes to increase is a new permanent seismometer in Wooley Hollow State Park that came online last week and is linked with five other new seismometers in the Arkansas Seismometer Network.

The permanent instrument replaces several less accurate portable units set up in Faulkner County after the earthquake activity again showed itself in October.

(Staff writer Joe Lamb can be reached at 505-1238 or by E-mail at joe.lamb@thecabin.net. Send us your news at www.thecabin.net/submit.)