MILWAUKEE — A minor earthquake occurred this week near the eastern Wisconsin city where researchers have been investigating a series of unexplained booming sounds, federal geologists said Thursday.
The U.S. Geological Survey said the 1.5-magnitude earthquake struck Tuesday just after midnight in Clintonville, a town of about 4,600 people about 40 miles west of Green Bay.
Geophysicist Paul Caruso told The Associated Press that loud booming noises have been known to accompany earthquakes. It's possible the mysterious sounds that town officials have been investigating are linked to the quake, he said.
Earthquakes can generate seismic energy that moves through rock at thousands of miles per hour, producing a sonic boom when the waves come to the surface, Caruso said.
"To be honest, I'm skeptical that there'd be a sound report associated with such a small earthquake, but it's possible," he said.
Those reservations didn't stop Clintonville City Administrator Lisa Kuss from declaring "the mystery is solved" at a news conference Thursday evening.
She said USGS representatives described the event as a swarm of several small earthquakes in a very short time.
"In other places in the United States, a 1.5 earthquake would not be felt," she said. "But the type of rock Wisconsin has transmits seismic energy very well."
The U.S. Geological Survey says earthquakes with magnitude of 2.0 or less aren't commonly felt by people and are generally recorded only on local seismographs. Caruso said the Tuesday earthquake was discovered after people reported feeling something, and geologists pored through their data to determine that an earthquake did indeed strike.
Local residents have reported late-night disturbances since Sunday, including a shaking ground and loud booms that sound like thunder or fireworks.
City officials investigated and ruled out a number of human-related explanations, such as construction, traffic, military exercises and underground work.
Clintonville resident Jordan Pfeiler, 21, said she doubted an earthquake caused the noises. She said the booms she experienced were in a series over the course of several hours and not continuous as she might have expected if they were caused by an earthquake.
Still, she said, "It's a little scary knowing Clintonville could even have earthquakes."
Steve Dutch, a geologist at the University of Wisconsin-Green Bay, said a 1.5 magnitude earthquake produces the energy equivalent of 100 pounds of explosives and could produce loud sounds.
But he was reluctant to describe Tuesday's event as an earthquake, saying the term is generally used to refer to widespread stress in the earth's crust. What happened in Wisconsin could be near the surface, perhaps caused by groundwater movement or thermal expansion of underground pipes, he said.
Still, Dutch said it was possible that the event could produce a series of sounds over time.
"If you've got something causing a little bit of shifting underground, it may take a while for whatever is causing it to play itself out," he said
Caruso, the U.S. Geological Survey scientist, said Tuesday's event was confirmed as an earthquake because it registered on six different seismometers, including some as far as central Iowa.
Jolene Van Beek, 41, had been jarred awake several times by late-night rumbling this week. When asked by telephone Thursday whether she thought the noises were caused by an earthquake, she joked that she was at a nearby lake "waiting for the tsunami to hit."
"Anything to do with earthquakes is going to freak people out," she said. "You'd never expect it in Wisconsin."
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Dinesh Ramde can be reached at dramde(at)ap.org.
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By DINESH RAMDE 03/22/12 10:14 PM ET