Quarry blast truck

A truck leaves the Jefferson Quarry Tuesday afternoon. The quarry was the site of a planned blast at 11 a.m. Tuesday that had aftershocks that shook much of Mankato and North Mankato. Photo by Pat Christman

MANKATO β€” Forceful enough to shake the fortress-like Blue Earth County Courthouse and to rattle buildings across the Minnesota River Valley, the tremor that rolled through the city Tuesday morning made more than a few Mankatoans think "earthquake."

What it actually was, according to Mankato police and officials at the Jefferson Quarry, was a scheduled blast at the quarry that was amplified by atmospheric conditions to create the equivalent of a 2.8 Richter scale temblor.

"We've been overwhelmed with 911 calls," a Blue Earth County dispatcher told law enforcement officers shortly after the 11:04 a.m. event. "We've been telling people, we pretty much don't know what's going on."

Residents on North Fourth Street said they felt like their homes were going to collapse and headed outdoors.

Geri Kragh was in her home on Lafayette Street near the intersection with North Sixth Street when she felt her sofa move. She went outdoors where she met neighbors who said they had similar experiences, with glassware trembling on the shelves. The shock wave was even more intense in the neighborhood adjacent to the quarry (see accompanying story) and a process was set up by Tuesday afternoon for property owners to report damage to buildings.

Here's how the blast registered at Rochester Community and Technical College:

Workers in downtown businesses reported buildings that shook or felt like they were rocking back and forth. The explosion even shook the Blue Earth County Historic Courthouse, which has 4-foot-thick outer walls, temporarily halting a Blue Earth County Board meeting as county officials openly wondered whether an earthquake happened or a bomb from North Korea had detonated nearby.

"This is a pretty sturdy building. It's been around for 125 years," County Administrator Bob Meyer said. "This is not a building that typically would move."

Board Chairman Mark Piepho made an assumption that was common in Mankato in the moments after 11:04 a.m.

"I've never been in an earthquake before," Piepho said. "I realize it was small scale, but boy, I didn't think the building would shake like that."

It wasn't an earthquake, though. At least not a natural one.

Jordan Sands, a subsidiary of the Coughlan Companies, had set off a blast at the Jefferson Quarry on Mankato's northwest side to loosen sand for extraction. In size and technique it was no different than hundreds of others the company has done, including 40 this year alone, said Jordan Sands CEO Scott Sustacek.

"This was business as usual, but obviously it didn't turn out that way," Sustacek said.

The company announced it would do no further blasting until it thoroughly investigates why the explosion was felt so broadly across the community. The current theory is that weather conditions, which city officials referred to as "overpressure," caused the unexpected impact.

"There's some anomaly that took place, so we're digging in to figure out what that is," Sustacek said.

In using explosives to loosen the sand, which is sold to the fracking industry, it's inevitable that some energy from the blast moves upward, he said. Typically, that energy dissipates into the atmosphere. On Tuesday, the barometric pressure or low-hanging clouds may have bounced that energy downward.

"We're doing a thorough review to know if our initial assessment proves out or doesn't," Sustacek said.

The company's permit requires law enforcement to be notified of any blast, and a community service officer is routinely sent to a monitoring location near the Jefferson Quarry, which sprawls over 54 acres between the Minnesota River and Third Avenue just south of Highway 14, said Mankato Police Cmdr. Dan Schisel.

So the blast was not a surprise to local law enforcement. But the widespread repercussions β€” including an explosion of 911 calls from shocked residents β€” was enough of a surprise that the connection wasn't immediately made.

"It took a little bit to put the two together β€” that the blast occurred and this was the result," Schisel said.

BEC Emergency Management Director Mike Maurer said there were no initial reports of injuries or significant damage. The community services officer felt the blast but saw no debris flying, Schisel said.

By afternoon, Mankato city officials were asking people to call the city's 311 information line (or 387-8600) if they believe the event damaged the structural integrity of their building. People were urged to call their insurance company if there was damage to personal property.

The event, which likely would have been a "ho-hum" moment in earthquake-prone areas such as California, did rate a report by the U.S. Geological Survey, which monitors quakes around the nation. The agency rated it at a magnitude of 2.8 on the Richter scale and placed it 2 kilometers north of the city center.

β€œWhat you felt was shock waves moving through dirt,” said Thomas Brown, physics department chairman for Minnesota State University, after learning most reports did not mention loud noise.

Jim Welsh, chairman of the Department of Geology at Gustavus Adolphus College, said earthquakes and quarry blasts have similarities.

β€œThey are basically the same process, one is natural, the other man-made,” Welsh said.

And Tuesday's event is as close to a building-shaking earthquake as Mankatoans are ever going to feel, said Bryce Hoppie, an MSU geography professor. It wouldn't be unthinkable for northern Minnesotans to feel an earth-moving event, but the fault line below Mankato hasn't moved in 400 million years.

"And it's not likely it would move again in the span of humanity, of our race," Hoppie said.

In the Mapleton area studying river erosion Tuesday morning, Hoppie was disappointed to have missed the man-made quake. But he's experienced major quakes in California, and he said Mankatoans who felt their buildings were shifting back and forth probably were just showing they were temblor virgins.

A 2.8 magnitude quake isn't likely to do bring anything that dramatic.

"If a Schwan's truck drove by, that could be a 2.8 β€” depending on how much ice cream was in it," Hoppie said.

Still, the blast was strong enough to be detected by the USGS seismic monitoring station just south of Lead, South Dakota.

"I think anyone that's ever driven to the Black Hills can appreciate just how far that blast was 'felt,'" Hoppie said.

Tuesday morning's event will be memorable for people who experienced it, and it comes as the Coughlan Companies are in the process of winding down operations at their in-town quarries after more than 130 years of stone mining.

There are two Coughlan quarries in the heart of the city, although they are largely unseen and unnoticed by most residents. The Mankato Quarry located just west of the Madison Avenue-Riverfront Drive intersection has already ceased mining. The still operating Jefferson Quarry begins just north of Pine Street and parallels the Minnesota River to the southern edge of the Highway 14 corridor.

The Coughlan family announced last year it was preparing to redevelop the quarries and was planning to offer the land, at least in part, for community use.

Staff Writers Edie Schmierbach, Trey Mewes and Deanna B. Narveson contributed to this story.

Here's some of what people were saying on Twitter after the blast:

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