2.8 magnitude shake follows Mankato quarry blast

MANKATO, Minn. (KMSP) - It was the talk of Mankato--and beyond. The ground started shaking around 11 a.m.,enough to trigger a 2.8 magnitude reading on the United States Geological Survey’s sensors.

The quake was felt as far away as South Dakota and Rochester, where Rochester Community and Technical College instructor John Tacinelli posted seismic monitor graph on Twitter.

"Might not be an #earthquake but it was cool," Tacinelli posted.

Officials believe the shaking originated in a Mankato quarry when Jordan Sands conducted a routine blast of silica sands. However, within milliseconds, workers and residents could feel a secondary shake.

“Everything about today’s event happened consistent with every other event that we’ve conducted so far this year,” said Brett Skilbred, vice president of Jordan Sands. “Normally, you’ll feel it close to the site. But having the type of widespread effect as today is very abnormal.”

Jordan Sands conducts these blasts regularly, more than 40 times this year. The company is investigating, along with the Mankato Department of Public Safety. State geologists have been called in to help.

A couple theories to explain the widespread vibrations have emerged. One theory: The sky’s low, dense clouds reflected the blasts’s energy back toward the ground, causing a sensation similar to shaking experienced by strong thunder. Another theory: The blast triggered a fault to move. Both theories would be highly unusual in relatively stable Minnesota.

“The rocks are cold and they’re old and they’re thick and they’re strong. There are faults that go through their rocks, but they don’t pop very often,” said Steve Losh, a geology professor at Minnesota State Mankato. Losh told Fox 9 the widespread shaking would have required “a very efficient transfer of energy from the blast to the rocks.”

Cindy Mead, who lives across the street from the quarry said the blast felt typical—at first. “It’s a blast like any other blast. I’ve been out here since 2000, but there was a thump after the blast that was pretty pronounced. In fact, we went out to see if a rock landed in our yard.”

Farther away from the blast, Chris Ellison said he thought a truck hit his building. “My wife was walking the dog around the neighborhood, and she said the trees shook and the dogs started howling.”

The city of Mankato has received several calls of cracked walls. “We are getting some reports of damage from throughout the community, and the city’s interest is in any structural damage at this point,” said Jeff Bengtsom, deputy director of Public Safety.

The city is asking those who want to report “damage affecting a building’s structural integrity” to call 311 or 507-387-8600, or to call the insurance company to report any damage to personal belongings.

There will be no blasts at the quarry pending the investigation.

Check out the USGS data on the Mankato quarry blast here.