In October 2010, Eileen Heider was in the living room of her home just north of Menominee Township in Michigan’s Upper Peninsula when things got a little weird.
"I was sitting watching TV on my recliner and I started moving," she told Fox11. "It maybe only lasted 15 seconds, but I was moving."
Heider thought it was an earthquake, but the area isn't known to have them. The next day, she discovered that a large crack had opened up in the woods on her property. It was a huge gash in the ground — the length of a football field and as deep as six feet in some spots — and it left geophysicists and the media buzzing.
Wayne Pennington, now dean of the College of Engineering at Michigan Technological University, heard about it while he was at a conference in Boulder, Colorado. To Pennington, the crack seemed relatively common, but the email chatter hinted otherwise.
"I thought it sounded like something that happens very often, but is always a surprise to people whose property it's on," Pennington tells MNN. "There's some downslope movement, there and at the uphill end where the soil slides down, you can often see a crack."
But as more information surfaced and more academics weighed in, Pennington became more intrigued. On his way home to the university from the airport, he decided to take a firsthand look.
"There I was in my dress shoes and nice clothes and no equipment and when I saw it — instantly, it was like nothing I had ever seen before. I didn’t know what it was."
More than just a crack
Pennington kicked into action, taking notes on a pad of paper he had handy and using his phone to take GPS measurements. He paced off measurements, getting mud all over his dress shoes.
Although it was the crack itself that had made the news, it was what was underneath that raised Pennington's curiosity.
"The exciting part was the ridge that the crack was on top of. People said that ridge didn’t exist before. The trees were at crazy angles on both sides of it. They were tipped away from the crack," Pennington says. "The crack at the top of the ridge is just the expression in the top soil above the flexing harder rock, in this case limestone. Think of it as a stretch mark."
Trying to figure out what could have caused the ridge and the resulting crack, Pennington says the ideas he had in the field just didn't make any sense. He took several photos and, when he returned to his office, started circulating information and pictures to colleagues around the country.
Stanford University geophysicist Norm Sleep suggested a new theory: Perhaps what had happened in the woods outside of Menominee was a geological pop-up.
How a geological pop-up works
But that theory created a new puzzle. Pop-ups can occur when shallow layers of rock spring up after having been weighed down heavily by rock or ice.
They often occur at the base of a quarry or they happen when the earth rebounds after a glacier retreats, but there aren't any quarries in the area and "the glaciers retreated here 11,000 years ago!" says Pennington.
"Take the quarry example: You might have 200 feet of rock pushing down and that's a lot of weight," says Pennington (pictured right). "A rock can't squeeze out because the rock next to it is pushing back, also wanting to squeeze out, and the rock next to that … and so on, but there's just no room.
"If we were to unload one part of it, taking a large amount of weight off of it, then those rocks where the load has been removed can respond to the stresses applied by the rocks to their side by popping up."
Seismic refraction tests
The Michigan Tech researcher and his team still had questions about the pop-up theory. They knew the limestone, which is hard rock, couldn't be very deep beneath the soil or the ridge would have looked different. They wanted to measure how deep the soil and the sand was on top of the limestone, but they didn't want to use a bulldozer to do it.
They chose to do seismic refraction experiments, which measure the speed of sound as it travels within the layers of the earth. They found that the sound was slower perpendicular to the crack because the sound waves have to cross a lot of fractures. That made the researchers believe they had found a pop-up.
So did the local residents feel an earthquake?
"The answer is, 'Well yes, but…,' " Pennington says. "Technically, it was recorded by a seismograph and it was a sudden event in the earth apparently resulting from natural causes, not an explosion or mine cave-in, which fits the definition of an earthquake. But it wasn’t motion of rock on one side of the fault relative to the other side of the fault. It wasn’t that. It's just not what we usually think of when we think earthquakes."
Learning from the research
The researchers recently published their study in Seismological Research Letters, a journal published by The Seismological Society of America. In the paper, Pennington says they purposely included some speculation about what may have influenced the timing of the pop-up. The events may or may not have had an impact, but they hope that when other scientists are researching similar events years from now, they can benefit from all the observations.
For example, the day before the pop-up was formed, a large white pine tree that had blown over was being collected for firewood. "About two tons of material had been taken away," Pennington says. "That's not a lot — a garbage truck weights more than that — but it happened the day before, so the coincidence is remarkable."
In addition, when looking at older aerial photographs, researchers noticed an unusual feature along the nearby road that ends where the pop-up starts. Maybe it was a drainage adjustment that rerouted rainwater, Pennington says, and perhaps it weakened the limestone, eventually causing the pop-up.
In studying the research, Pennington and his team have found no reported instance similar to the Menominee Crack, but that doesn't mean it won't happen somewhere else.
"Here, this area is done. Those stresses are relieved," Pennington says. "Something similar could happen in some place, but we have no idea where or why."
As you can see from this local news update, the story is still making headlines years later: