A strange and sudden buckling of the earth in Michigan five years ago is now being explained as a limestone bulge, researchers reported Feb. 9.

The upheaved rock and soil was discovered after a deep boom thundered through the forest near Birch Creek on Michigan's Upper Peninsula, north of Menominee. The sound shook nearby homes with the strength of a magnitude-1 earthquake on Oct. 4, 2010, at about 8:30 a.m. Central time, residents said at the time. The next day, locals discovered a long crack atop a narrow ridge.

The crack was 360 feet long and about 5 feet deep; and the ridge was nearly 7 feet high and about 30 feet wide at its largest point. Tilted trees leaned away from the crack at about 14 degrees on either side — proof the ridge was new. Torn roots stretched for their former companions, now stranded on the other side of the crack. [See Photos of the Weird Crack and Uprooted Trees]

"It was interesting to see that the crack seemed to ignore the roots," said senior study author Wayne Pennington, dean of the College of Engineering at Michigan Technology University in Houghton. "The forces were stronger than the roots."

Based on a seismic study, the most likely explanation for the ridge is a pop-up in the upper layers of limestone beneath the clay soil, Pennington and his co-authors, all MTU students, concluded in a study published in the journal Seismological Research Letters.

If I had a hammer

Even though the researchers can't say for sure what caused the pop-up, they now have a better picture of what happened underground.

The teams surveyed the underground rock by creating sound waves with a sledgehammer. The researchers slammed a sledgehammer into a metal ball sitting on the ground, and tracked how the waves passed through the soil and rock layers below. The analysis revealed a sharp buckle in the limestone below the crack.

That picture suggested the bedrock limestone violently heaved upward when the pop-up appeared, displacing the overlying clay layer. The clay soil is about 5 feet deep along the ridge. The crack resulted from the stretching of the surface clay as it bent upward, much as a crack forms in the top of a loaf of bread as the dough rises.

The survey confirms there is no earthquake fault underlying the ridge. Besides, it would take a tremendous earthquake to move the rock and soil several vertical feet, Pennington said.

Pop goes the bedrock

Pop-ups are common in quarries in eastern North America, where rock removal releases pent-up strain in the underlying rocks. Pop-ups also appear after glaciers retreat; however, the last glaciers retreated from Menominee 11,000 years ago, and there is no quarrying in the area.

Rocks in the area are squeezed by plate tectonics, the researchers said. The Midwest is under pressure from squeezing coming from the West Coast and the East Coast.

Yet the region is not experiencing increased stress that would result in future larger earthquakes, Pennington added. The pop-up appeared in the uppermost bedrock, whereas large earthquakes strike miles deep. There have been two moderate earthquakes in Michigan since 2010, which were in different areas and unrelated to the crack, the scientists said.

One final clue was the loss, to lightning, of a giant white pine tree in the week before the crack appeared. "The timing is remarkable, and it leads us to be suspicious, but the tree weighed less than a fully loaded dump truck," Pennington told Live Science.

"The earth is still full of surprises," Pennington said. "It's just a little surprise, but it's still interesting and we're always learning more."

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