Monday night levee blast felt 170 miles away

While the large amount of explosives used in the first Birds Point-New Madrid levee breach this week may be reason enough to feel the blast more than 150 miles away, experts in earthquake studies said Wednesday there may have been other factors that contributed to the vibrations felt in cities in four states.

The United States Geological Survey received reports of vibrations felt in 22 ZIP codes, the furthest away from the blast point being Maryland Heights, Mo. -- around 170 miles away -- and Walnut Ridge, Ark., about 120 miles from Monday night's levee breach. The most intense vibrations were reported northeast of the blast site, and the USGS labeled the "shaking" as moderate. Weak vibrations were felt in Cape Girardeau, consistent with comments on a Facebook page created by the U.S. Army Corps of Engineers to notify the public about the Birds Point operation. The Corps of Engineers used 150 tons of explosives for the first two breaches, the first which took place Monday evening and the second around 12:30 p.m. Tuesday. The USGS received reports of vibrations felt in Tuesday's blast in three ZIP codes, the furthest in Almo, Ky., about 110 miles from the explosion.

The third breach was delayed Tuesday after the government announced they used too much of the explosives.

The colored areas on the graphic show where the Monday levee blast at Birds Point was felt and the intensity at which they were felt. (Source: U.S. Geological Survey)

Chuck Langston, director of the Center for Earthquake Studies, and Brian Blake, earthquake program coordinator at the Central United States Earthquake Consortium, both in Memphis, Tenn., both said the explosion may have been felt from long distances due to the type of soil in Southeast Missouri.

Blake said the soils throughout the Mississippi River Valley transmit vibrations from earthquakes, or explosions, over a long distance because they're unconsolidated, loose materials.

"We've got sand and clay, so, we're like a big bowl of gumbo in that we sit in there so when you shake it tends to shake for further distances than it would, say, in California, a state that sits really close to bedrock," he said.

Corps spokesman Jim Pogue said the overcast skies in the area Monday contributed to the long distance the sound carried.

"The overcast skies experienced in the area would have caused the sound from the Birds Point-New Madrid Floodway blasts to carry much farther than on a clear day," Pogue said. "It would not have been unusual to hear the blasts for miles. It is also not uncommon for people to often perceive the sound or rattling of windows from a blast as ground motion, when it was probably the sound wave."

The colored areas on the graphic show where the Tuesday afternoon levee blast at Birds Point was felt and the intensity at which they were felt. (Source: U.S. Geological Survey)

Langston said the time of day each breach took place contributed to the distances in which the vibrations were felt. Shaking would have been felt at longer distances during the Monday night's breach, when atmospheric conditions are different from during the day. An air wave, Langston said, caused the shaking, not the seismic waves from the explosion.

"Naturally, at night, the channels for sound waves does form in calm conditions and the ground cools off so you get this lower velocity of air near the ground that increases as it goes up in the atmosphere," he said. "When the wind blows correctly at night the waves can go large distances."

Dr. Christine Aide, a Department of Physics and Engineering Physics instructor at Southeast Missouri State University, agreed with Blake, claiming so many people could feel the explosion because much of the ground materials in Southeast Missouri is alluvium.

"The material amplifies the waves, giving them greater amplitude, which corresponds to greater ground movement," Aide said.

Both Aide and Langston expressed doubts about the explosions having an affect on the New Madrid fault. Aide said it's unlikely the blasts would trigger movement.

While the blasts are big, Langston said they wouldn't put stress on the system.

"It sounds like a big deal, but these blasts are in sand and statured mud, basically. So, the spree from the waves isn't really that large," he said.

ehevern@semissourian.com

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