Since the early fall, there have been thousands of them…none of them very large — only a fraction have been felt. The Arkansas Geological Survey is trying to unravel a mystery: What is causing earthquakes in the town of Guy, Arkansas?
So what is a swarm? According to the United States Geological Survey, an earthquake swarm is a localized surge of earthquakes with all of them about the same magnitude – often so small they go unnoticed. These are shallow quakes between two and eight kilometers (between one-and-a-quarter and five miles) below the surface.
There are several geologic faults in the area, but none associated with the New Madrid fault, the large seismic fault in the region and one that was the source of an estimated 7.0-magnitude earthquake in 1811. And there was another historic flurry of earthquakes in 1982, 15 miles south of Guy. Geologists know it as the Enola Swarm, responsible for 15,000 quakes within a year’s time, followed by more shaking in 2001.
At first, town officials assumed the current wave of shakes came from work at a gravel company on the outskirts of town. Others have blamed the gas companies that arrived several years ago to drill for gas in a geological formation called the Fayetteville shale. The gas companies are engaging in hydraulic fracturing, whereby water, sand and chemicals are injected at high pressures into underground formations to open pockets of gas. The state Geological Survey has no idea whether the current swarm is a natural or man-made event, but the office is seriously exploring the latter.
Earthquakes occur on faults. Most earthquakes occur miles deep. At well-studied plate boundaries like the San Andreas Fault System in California, often seismologists can determine the specific fault on which an earthquake occurred. East of the Rockies, far from plate boundaries, that is rarely the case. Most of the known faults are deep, and probably there are other faults that have not been discovered. It is hard to link an individual earthquake to an individual fault. In most areas, the best guide to earthquake hazards is the earthquakes themselves.
Until the mystery is solved, residents of Guy will continue to ride out the swarm.
Download the USGS map shown above by clicking on the link below.