Geological Mystery Called the Menominee Crack Has Been Solved, Mostly

February 16, 2016 | Joanne Kennell

Long, jagged crack in the forest floor. Earthquake
Photo credit: Michigan Tech, Wayne Pennington

It was the first ever recorded earthquake in Michigan’s aseismic region… weird!

In 2010, a large cracked formed north of Menominee, Michigan, located in the state’s Upper Peninsula.  However, Michigan is not known for its earthquakes; in fact, it is considered aseismic, or earthquake-free. The Menominee Crack was the region's first ever recorded earthquake, measuring less than one on the Richter scale.

However, the feature and its cause have remained a mystery.

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Now, researchers from Michigan Technological University (MTU) think they have identified what the structure is: a geological pop-up.  This is a feature usually found at the base of quarries or where glaciers have recently receded, but the Menominee Crack is one-of-a-kind according to the team.

“We wanted to look into the crack because we could not find information in the literature on pop-up structures forming outside specific areas,” said Wayne Pennington, dean of the College of Engineering at MTU in a press release.  “As far as we can tell, this is a one-of-a-kind event; but in case it is not, we wanted the information about it to be available for other investigators.”

The team not only surveyed the site, but they also conducted seismic refraction tests to make sure that the crack was indeed a pop-up structure.  By capturing sound waves, the team was able to confirm that the limestone underneath the crack was fractured and bowed upwards.

Diagram of a geological pop-up structure
Cross section reveals how the thin layer deformed on top of the brittle limestone that broke into a pop-up structure. Credit: Michigan Tech, Ian Repp

A pop-up structure usually forms where the downward pressure is lessened — something that is very common at quarries and glacial regions where overlying rock and/or ice has been removed.  “There are no nearby quarries, however,” Pennington explained.  

And glaciers left more than 11,000 years ago.  However, Pennington added, “There was a large tree that had been removed after it fell over and the timing certainly provides an interesting coincidence.”

The cause of the crack still remains a mystery considering it did form in an aseismic zone, but at least they now know what the feature actually is!

All in all, it is definitely something unique to the Michigan landscape.

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