The strange reason this part of Arkansas gets so many earthquakes

Ten earthquakes in northern Arkansas in the past five days?

Blame it on Bull Shoals Lake, swollen by spring floods.

The lake, which straddles the Missouri-Arkansas border, has risen 42 feet since the first of March, adding more than 6 trillion pounds of water weight to the lake basin.

That crushing weight triggered a 3.6-magnitude earthquake north of Harrison, Arkansas, on Sunday and at least nine more in the following days, according to David Johnston, earthquake geologist with the Arkansas Geological Survey.

"That's a whole lot of weight — 6 trillion pounds of weight added to the local geologic column," Johnston said Friday. "That much weight can cause a tremendous amount of stress."

The Harrison earthquake swarm — 10 quakes ranging from 3.6 magnitude to 1.5 magnitude on the Richter Scale — aren't connected to the huge New Madrid fault zone at the juncture of Arkansas, Missouri and Tennessee, which produced massive earthquakes when it unzipped in the early 1800s.

Johnston said the lake-induced quakes near Harrison likely will remain small, but they might not end anytime soon.

"It's the same process when you let the water out of the lake," he said. "When you lower the lake, you can then relax that pressure and cause small earthquakes again."

Johnston said a team from Arkansas Geological Survey will be placing more seismometers (ground-motion detection devices) in the quake zone to better record what is happening and to get a better feel for the depths underground where the quakes are happening.

The deepest so far has been about 3.7 miles, while the shallowest is estimated at just over a half mile underground.

Johnston said there are no reports of damage from a 3.6 magnitude quake early Sunday morning. But some people reported houses creaking, china shaking on shelves, pictures rattling on walls and glasses of water sloshing on tables.

"It shook the whole house, and the first thing I thought was that it was the sinkhole," Will Presley said. "It was just so loud. The shaking went on for 15 or 20 seconds or so. I just knew my house was off in that sinkhole."

Harrison resident Terri Willis said aftershocks from the bigger Sunday quake "sounded like thunder rumbling," far different from the first one.

"That first one was like a big boom that shook the house," she recalled. "I thought something had hit the other end of our house, or a tree had hit the house."

Despite the rumbling beneath her feet, Willis said she isn't worried.

"My sister used to live in California, and she said it was like this every day," Willis said. "We had dropped our earthquake insurance, since we never needed it. But now it might be worth it."

Johnston, the Arkansas Geological Survey geologist, said there's a reason many people heard an explosion with that first earthquake.

He said earthquakes transmit energy through the ground with two different kinds of waves: a primary wave and a secondary wave.

"The noise you hear is the P wave being converted to an acoustic (sound) wave when it hits the surface," he said.

"Booming" earthquakes are common in Oklahoma, where fracking and injection wells have created some quakes big enough to cause minor damage.

Johnston emphasized that the Harrison earthquake swarm has nothing to do with fracking or injection wells, just the tremendous extra weight of nearby Bull Shoals Lake.

"We call it reservoir-induced seismicity," he said.