CHEROKEE — The Cherokee Nitrogen plant explosion sent waves of sound and tremors that shook homes and abruptly jolted seismographs throughout northwest Alabama.
Steve Jones, an amateur seismologist who has been studying earthquakes since 1986, said he detected a “two-phase seismic signature” from the explosion on the seismograph at his Huntsville residence.
“One (of the signatures or waves) was from the ground and the second from the air,” said Jones, who operates the Alabama Quake website. “It was like a sonic boom. The air wave was going slower; it likely came about two minutes later.”
He said most likely the people who felt tremors from the explosion live in a home with a slab foundation. They would have felt it first, then heard the boom. People whose houses have a crawl space likely just heard the boom without feeling the tremor, he said.
Sandy Ebersole, of the Geological Survey of Alabama, wasn’t surprised about reports of the explosion being heard or felt several miles from the site. She said seismic waves can travel far distances and carry well in the eastern part of the U.S.
“It’s like small earthquakes or rock quarry blasts — they can be heard and felt sometimes miles away,” Ebersole said.
David Brommer, an assistant professor in the geography department at the University of North Alabama and a climatologist, said an inversion in the atmosphere could have something to do with how the sound traveled.
In meteorology, an inversion is a change in the atmospheric properties.
“Temperatures in the atmosphere decrease with height,” Brommer said. “Energy is emitted from the ground and passes into the atmosphere, and the higher that energy goes, the cooler it gets.
“These inversions are usually 1,000 to 4,000 feet above the ground. They act like a lid, so it traps things in the lower atmosphere.”
Brommer said Tuesday there was a inversion of about 2,000 feet — “which would help trap that sound and keep it lower to the ground and help it travel farther,” he said. “And the fact that it was a cool, clear night helped the sound travel.”
He said an example would be like picking up a short wave radio or an AM radio signal.
“You can pick up an AM signal better at night,” Brommer said. “(Those waves) bend and pass through the atmosphere better at night. The inversion helps things pass through the atmosphere better.”
George Grabryan, Lauderdale Emergency Management Agency director, was returning from a meeting in Waterloo when the blast occurred.
“I got a call from someone in the Waterloo area who was outside and said he felt the ground wave and heard the noise,” Grabryan said.
“Our center was starting to receive calls,” he said. “We had some folks who asked if we had an earthquake. A number of calls came from the Greenhill area (and the western part of Lauderdale County).”
Grabryan said his first thought was that a blast occurred at one of the western Colbert County plants, so he called Colbert EMA Director Mike Melton, who confirmed it.
Ebersole said it’s difficult to explain why some people heard the explosion and felt its effects and others didn’t.
“There’s so many variables that go into who hears the sound and who feels it,” Ebersole said. “Explosions like these are similar to earthquakes — some hear them, some feel them and some feel them and hear them. It all depends where it happened and how the waves travel.
“It’s really hard to explain, but it makes for interesting discussion and research.”