Clintonville, Wisconsin (CNN) -- The mystery behind four days of unexplained shaking and odd sounds rattling Clintonville, Wisconsin, has been solved.
The cause? A "swarm" of minor earthquakes amplified by the unique bedrock beneath the state of Wisconsin.
The strange sounds -- variously described as rattling pipes, clanging metal, thunder or firecrackers -- have continued on and off since early Sunday night in just one part of the small town of 4,600, located about 180 miles northeast of Madison. They were loud enough Monday morning that a CNN journalist could hear them during a cell phone conversation with city administrator Lisa Kuss.
Speaking to Clintonville residents Thursday night, Kuss said the U.S. Geological Survey has determined that "our community did in fact experience an earthquake that registered 1.5 on the earthquake magnitude scale." That minor quake was measured on Tuesday night by several mobile earthquake monitoring stations that were dispatched to the region, she said.
Based on all the data, the USGS believes the shaking and strange sounds are the result of "a swarm of several small earthquakes in a very short amount of time," Kuss said.
While these small earthquakes normally don't cause such commotion, Kuss said the location of the shallow temblors helped amplify the shaking.
"In other places in the United States, a 1.5 earthquake would not be felt," she said. "But the type of rock that Wisconsin has transmits seismic energy very well."
When the shaking began last Sunday, hundreds of residents began calling 911. Kerry Danley said she hear noises around midnight that sounded like a paintball gun.
"It was just pop-pop-pop," she said. "So I woke up -- just jumped out of bed actually -- ran downstairs, looked outside, nothing.
Since Sunday, the shaking has happened nearly every night, quieting down during the day. Absent of any explanations, residents were left to their own devices to come up with explanations.
"My bet is on gremlins," one Facebook user jokingly posted to WLUK's Facebook page. Alien machinery buried for millennia, countered another.
No, said one one tongue-in-cheek Twitter user. It's clearly mole men launching their attack on the surface dwellers.
Others suggested huge earthworms or sewer cats. Some Clintonville residents were even holding "shake" parties at night, waiting for the rumbling.
As city officials ruled out electrical explosions, gas leaks and sewer collapses, they started consulting geological experts around the country. Based on the data from eight seismic monitoring stations, Kuss said the USGS finally determined on Thursday that earthquakes were to blame.
While the cause of the shaking has been solved, it's still not clear if the rattling in Clintonville is over, Kuss said.
"There is no way to say for certain whether our area will ever again experience an earthquake," she said. "But it still very likely, although not guaranteed, that any future earthquakes that we experience would again be on the low end of the earthquake magnitude scale."
Tuesday's 1.5 tremor is only the second recorded earthquake in Wisconsin since 1947, according to USGS geophysicist Paul Caruso. The last quake happened south of Milwaukee, and was too minor to record a definitive magnitude.
Caruso explained that the rock underneath Wisconsin and in much of the country east of the Rocky Mountains is "very consolidated" and without fault lines. And that means small quakes are actually felt by residents, unlike in California where the energy is absorbed.
For example, the August earthquake in Virginia that rattled Washington, was felt as far away as New York and Florida.
"All throughout the eastern United States, even small earthquakes are felt great distances," Caruso explained. "It's because the rocks are just really old. They transmit the energy really well."
He said earthquake swarms are actually quite common in the region east of the Rockies.
Caruso said there was no hydraulic fracturing or "fracking" going on in Wisconsin, a drilling process that some believe may have caused a series of quakes in Ohio earlier this year.
When asked why Clintonville residents heard such strange noises during the apparent quakes, Caruso said all seismic shifts generate noise but these sounds cannot be heard during major quakes.
"When seismic waves travel through the ground, they're moving ... faster than the speed of sound and when they hit the surface," Caruso explained.
"(It) rattles the ground like a speaker ... so it's common for people to hear what they describe as sonic boom sounds accompanying earthquakes. But usually when there's a big earthquake, people either don't hear the sounds because the frequency is lower than the threshold of what humans can hear. Or other sounds going on (like) things falling down."
CNN's Jim Spellman reported from Clintonville, Wisconsin, and CNN's Tricia Escobedo reported from Atlanta, Georgia.