State to add 2 seismometers

Devices part of tracking, explaining spate of temblors

Two additional seismometers are set to be deployed in northern Arkansas after more than a dozen small earthquakes recently shook the region, including five Thursday.

The installation of the data-collecting devices near Bull Shoals Lake, northeast of Harrison, is intended to present a clearer picture of what has spurred an increase in seismic activity, said Scott Ausbrooks, assistant director of the Arkansas Geological Survey.

“The more stations you have, the better control you will have in monitoring,” Ausbrooks said, noting that officials have been able to get a general idea of where the tremors originate.

Three permanent seismometers were set up at state parks in north Arkansas. One is at Hobbs State Park in Hindsville, another at Ozark Folk Center State Park in Mountain View, and the third at Lake Charles State Park outside Powhatan.

Those devices are part of a network of six permanent broadband seismic stations that was installed in the spring of 2010, according to the Arkansas Geological Survey website.

Since June 11, 13 earthquakes have been recorded about 10 miles northeast of Harrison near Bergman in Boone County. Seven of those shook north Arkansas residents Sunday, another hit Tuesday and five occurred Thursday.

The largest quake, a 3.6-magnitude tremor, rattled the northern part of the state around 7:40 a.m. June 11, according to U.S. Geological Survey data. It was centered 5 miles north of Bergman.

An aftershock measuring 2.4 magnitude shook the same area about an hour later, and a third quake with a magnitude of 2.3 was recorded around 2:25 p.m. that day.

The Center for Earthquake Research and Information, which also is involved in the deployment of the additional instruments, said the largest aftershock was a 2.9-magnitude quake.

It was recorded around 9:30 p.m. Thursday nearly a mile northeast of the 3.6-magnitude tremor.

“So far this appears to be a normal aftershock sequence, and the size and frequency of earthquakes should decrease over the next several days,” according to a statement from research center, which is based at the University of Memphis.

There has been no determination of what may have been a factor in the recent quakes, but the source is believed to be natural — either through a fault line or as the result of an influx of water in the lake spurred by recent rainfall, Ausbrooks said.

“Given the proximity to the lake, the recent significant rise may be a contributing factor, but we don’t know for sure,” he said.

As of Friday afternoon, Bull Shoals Lake sat at an elevation of about 690 feet above sea level. That is about 30 feet above its normal seasonal pool level, according to data from the U.S. Army Corps of Engineers.

The agency said it had ruled out fracking as a cause of the recent seismic activity.

“There is no drilling or wastewater disposal in the area,” Ausbrooks said.

Ausbrooks noted that while they are uncommon in the state’s north-central region, quakes have happened in the area in previous decades.