Geologists raise fault’s estimated size

— Geologists say a previously unmapped fault that is the source of a swarm of earthquakes around Guy and Greenbrier in Faulkner County is longer and potentially more destructive than they initially believed.

Prompted by growing knowledge of the fault, the Arkansas Oil and Gas Commission last week ordered a halt to operations at two disposal wells in Faulkner County that commission staff think might be linked to the seismic activity.

The injection wells operated by Chesapeake Operating Inc. and Clarita Operating LLC contain discarded fluid used to drill for natural gas in the Fayetteville Shale.

In the five days since the injections were stopped Thursday, the number of the biggest quakes - those over magnitude 2.5 - has decreased in the area, said Scott Ausbrooks, geohazard supervisor for the Arkansas Geological Survey. Two such quakes occurred between Friday and press time Tuesday, while nine such quakes struck in the five days before that.

Commission staff members have said the injection wells are near the fault. The order to stop injections lasts until a March 29 meeting, at which point it may be extended.

Scientists as recently as January thought that what they’re informally calling the “Guy-Greenbrier Fault” spanned 3.7 miles, but now estimate it to be 6 to 7.5 miles long.

Ausbrooks said the length is a concern because a longer fault could trigger bigger earthquakes.

Theoretically, if an entire 3.7-mile fault were to rupture, it could trigger a magnitude-5.0 to magnitude-5.5 earthquake. A 6-mile fault, meanwhile, could trigger a magnitude-5.5 to magnitude-6.0 quake.

Ausbrooks said a magnitude-6.0 quake releases 32 times more energy than a magnitude-5.0 quake.

In general, poorly built structures begin to sustain structural damage with magnitude-5.5 quakes, Ausbrooks said. Structures at the epicenter of such a quake would have broken windows and cracked masonry, he said, and the quake would be felt outside the state.

A magnitude-4.7 quake struck Feb. 27 roughly three miles from Greenbrier, and was Arkansas’ largest earthquake in about 35 years. It was one of hundreds that have been felt in the area since September.

Scientists, including those at the state geological survey and at the Center for Earthquake Research and Information at the University of Memphis, discovered the Guy-Greenbrier Fault by tracking the pattern of recent seismic activity in the area. The scientists then confirmed the fault’s existence by using imaging of the subsurface obtained from companies operating in the shale. The companies hadn’t noticed the fault because “it’s much deeper than what they are concerned about,” Ausbrooks said.

The center says on its website that they are concerned there is a risk of triggering more and larger earthquakes if the injection wells continue to operate.

While the fault is the source of the quakes, the question remains whether the injection wells are triggering or magnifying the seismic activity.

“The theory is that the well increases the ... pressure in the rock formation sufficiently to overcome friction, to allow the fault to move, which generates the earthquake,” Ausbrooks said.

Ausbrooks said the fault is deep and old. Its last vertical movements are believed tohave been 400 million years ago, he said.

The fault doesn’t cut to the surface, and the uppermost part is about 8,000 to 10,000 feet deep. Clarita’s well is 7,800 to 10,900 feet deep while Chesapeake’s is 5,700 to 6,200.

The Clarita well is “injecting at the same layer of rock” that the fault cuts up into, Ausbrooks said.

Operators of the well are arguing that the earthquakes are occurring because of natural causes. Clarita said in a release last week that the quakes began before its well was operational.

The general area - northern Faulkner County - has seen natural seismic activity in the past, including a series of small quakes that began in 1982 in Enola, which is about 10 miles away from the current swarm.

However, the Enola earthquakes originated from a different fault than the current swarm, Ausbrooks said.

Ausbrooks said there were “no known earthquakes in this Guy-Greenbrier swarm area” before the first of the two wells went into operation.

Since the injections stopped Thursday, “we’re still seeing micro-seismicity,” Ausbrooks said. He added that Arkansas saw a “couple of” magnitude-1 and magnitude-2 quakes on Tuesday. “There’s no reason why that shouldn’t continue for a period of time.”

Scientists at the center in Memphis and with the survey are basing their research on a 1960s case in Denver that showed injection wells could cause earthquakes. Ausbrooks said that the quakes diminished dramatically after the injections were stopped and started once the injections began again. However, the largest quakes struck about two years after the injections stopped, he said.

“Once you start that pressure front moving out [through the rock], it takes time for it to dissipate and to move out.”

Ausbrooks said he couldn’t comment on what the survey’s position is on a potential link between the wells and the seismic activity because it is awaiting the March 29 hearing.

Business, Pages 25 on 03/09/2011

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