Series of tiny quakes hits central Arkansas
Posted on Friday, November 28, 2008
A series of small earthquakes that rattled central Arkansas in the past few weeks could be the sign of something much bigger to come, seismologists say.
By this weekend, seismologists hope to install three portable measurement devices to gather information about future temblors in the area. That information could show whether the rumbles come from heatrelated geological changes or from an undiscovered fault — which could mean a risk of a substantial earthquake in the future.
“The potential for generating a high-magnitude earthquake is real,” said Haydar Al-Shukri, the director of the Arkansas Earthquake Center at the University of Arkansas at Little Rock.
Five earthquakes have hit central Arkansas since Nov. 1. The quakes haven’t been that powerful, ranging in magnitude from 2. 2 to 2. 7. Those with a magnitude of 2. 5 to 3 are typically the smallest felt by people.
While hundreds of earthquakes occur each year and several happen in Arkansas, the location of the recent quakes give Al-Shukri pause.
Arkansas earthquakes generally occur in the state’s northeast corner, part of the New Madrid Seismic Zone. During the winter of 1811-12, a series of three earthquakes with magnitudes of about 8 struck the zone, and much smaller temblors continue to this day.
But central Arkansas does not have any seismic history, Al-Shukri said.
“It is abnormal. It is significant,” he said. “We need to carefully watch this activity.”
The area does not have any permanent seismograph, meaning researchers had to ask the University of Memphis to provide its portable equipment to put in the area. The nearest seismographs aren’t close enough to provide the detailed readings scientists need to determine what could be causing the tremors or properly locate where they may be coming from, said Scott Ausbrooks, the geo-hazard supervisor for the Arkansas Geological Survey.
“I don’t know if you’ve looked at a map of where these events are located, but they’ve got a scatter on them,” he said. “We’re thinking this is probably the inherited error built in when you try to locate events of this small a magnitude from that far away. We’re kind of thinking this is coming from one central area.”
Residents of Magnet Cove, near where the 2. 7 magnitude earthquake hit on Nov. 1, told police dispatchers they heard something that sounded like an explosion when the quake hit. Ausbrooks said officials would install the three seismographs in the region around the Hot Spring County community.
One possible culprit could be a hydrothermal quake, caused by a source of extremely hot fluid pushing into rocks under the surface. The hot fluid percolates into the cracks of the rocks and causes movement, triggering an earthquake, Al-Shukri said.
That cause could match the geologic history of the area. Central Arkansas is home to Hot Springs, a city that grew up around its namesake spas. The springs have 143-degree waters rushing to the surface continuously.
If that’s the case, the earthquakes likely wouldn’t pose a drastic danger to the area, Al-Shukri said. At their strongest, such earthquakes have a magnitude of about 5, just reaching the U. S. Geological Survey’s threshold for a “moderate” quake.
However, if the earthquakes are caused by a previously unknown fault, that could mean a more powerful earthquake in the region’s future. A recently discovered fault in east Arkansas near Marianna caused an earthquake with a magnitude of between 7. 2 and 7. 5 in the past 5, 000 years, Al-Shukri said.
A quake of that magnitude is capable of causing widespread, heavy damage.
“Now, it’s not active, but in geologist time, that’s yesterday,” he said.
Ausbrooks repeatedly said he didn’t want to speculate on what could be causing the earthquakes, saying he wanted to see what data the new seismographs capture. However, he acknowledged an unknown fault could be running through the area.
“There are numerous faults across the state, both known and unknown,” Ausbrooks said. “This area has got a lot of faults associated with it from the mountain building of the Ouachitas, but they’re considered inactive.”
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