The U.S. Geological Service has reported at least 40 earthquakes in the northern Arkansas towns of Greenbrier and Guy this week -- 18 on Wednesday alone. Since October, the area has seen nearly 700 earthquakes. No injuries or damage have been reported.
The earthquakes, although small -- generally ranging from 1.8 to 3.5 in magnitude -- appear to be increasing in strength. This morning, residents in the Greenbrier, about 50 miles north of Little Rock, woke up to a 3.8-magnitude earthquake.
"They are getting stronger," Scott Ausbrooks, a geologist with the Arkansas U.S. Geological Survey, told AOL News today in a phone interview. "We had a 3.8 this morning at 4:49 a.m. It woke me up, actually. We've seen an uptick since the weekend."
Geologists say the tremors are too small to cause any major damage. The experts are uncertain about what's causing them.
"It's unusual. It's a very unique activity. There's no indication of when they're going to come or when they're going to just die out," Haydar Al-Shukri, director of the University of Arkansas' Arkansas Earthquake Center, told AOL News today by phone.
The region's New Madrid fault is responsible for generating an earthquake in 1812 that remains one of the largest ever recorded in North America. But the Arkansas towns affected by the recent swarm of earthquakes are not along the fault -- or on any major one -- raising questions about why the area has seen so much activity in the past year.
Ausbrooks says the earthquakes are occurring along a 3.7-mile-long line so tiny that it has no name and was unknown until the previous spate of earthquakes hit the area in October.
Some experts said natural gas exploration, which is extensive in the region, may be triggering the seismic surge, but they noted that no direct correlation between the drilling and the earthquakes has been proven. "The whole region has been drilled, which fractures the rock formation," Al-Shukri said. "But we don't have a definitive correlation between that and the recent earthquakes."
Ausbrooks said that toxic water created by the process of extracting the gas is later injected deep into the earth through disposal wells, which may also cause seismic activity. "If you take a map and you plot the earthquakes on there and then you superimpose it with the gas wells, you don't see any direct correlation. But we do see that with the disposal wells," he said.
Calls for comment to natural gas companies that use oil shale extraction in Arkansas were not immediately returned today.
Geologists said it was extremely unlikely that any major earthquakes would strike in the area in the near future. They said the recent activity is more of a nuisance than anything particularly dangerous. "People may see stuff get knocked off shelves or experience strong shaking, but we're not going to see anything really destructive," Ausbrooks said.