The state of Michigan is considered aseismic because it doesn't typically experience earthquakes. So when a strange, one-magnitude quake left a large crack on Michigan's Menominee Forest in 2010, residents and scientists were all baffled.

About six years later, geologists believe they have now identified what the geological event was inside the Menominee Forest. In a new study, a team from Michigan Technological University attempted to find out how the mysterious crack possibly formed.

Ripping The Ground

In the morning of Oct. 4, 2010, residents near the Menominee Forest heard a loud noise and felt the ground shaking. Looking at the part of the woods where a pine tree was blown over in a windstorm, people discovered that a large crack had split the Earth.

Wayne Pennington, dean of Michigan Tech's College of Engineering, decided to investigate the crack. Upon inspection, he said the crack was more or less 361 feet long, up to two feet wide, and as deep as four or five feet in some spots.

What Pennington found strange was the presence of a six-foot high ridge beneath the crack which had appeared at the same time as the geological opening.

"The crack itself was not as important to note as the ridge," said Pennington. The presence of the ridge meant a deeper structure. The underlying structure is called a pop-up, which is sometimes called an A-tent because of its shape.

The Mighty Sledgehammer

To confirm that the crack was indeed a pop-up, Pennington and his colleagues conducted seismic refraction tests. Seismic refraction measures the speed of sound as it travels within the ground. The path of the waves changes depending on the types of rocks, the fractures, and other invisible geologic features.

The Michigan Tech research team used a sledgehammer to strike a huge metal ball lying on the ground, and then captured the sound waves it produced. Sound travels faster in broken rock and moves parallel to its cracks. It travels slower when it moves perpendicular to the cracks and has to move across the fractures.

Pennington and his team found a pattern that appeared to be consistent with the intense bending and the fracturing of the pop-up's limestone.

After much analysis, the group concluded that the crack was indeed a pop-up feature. The cause of the pop-up, however, was still uncertain.

Pop-ups usually form where downward pressure is low. Glacial zones and quarries where the overlying rock or ice is removed are good examples. In the Menominee Forest, there are no nearby quarries or glacial zones.

"There was a large tree that had been removed after it fell over," said Pennington. "The timing certainly provides an interesting coincidence."

In the meantime, Pennington said the unique crack still remains a mystery, but they wanted to examine it because there was no available data on the literature about pop-ups.

"As far as we can tell, this is a one-of-a-kind event," added Pennington. "But in case it is not, we wanted the information about it to be available for other investigators."

The findings of the study are featured in the journal Seismological Research Letters.

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