Was Mankato boom a mine blast or an earthquake?

MANKATO – A blast at a locally owned quarry here two months ago struck with surprising power. An earsplitting boom was followed by a violent trembling that shook homes and garages to their foundations, toppled lamps off tables and knocked one man over as he tended his backyard garden.

In the investigations that followed, a government scientist said the ground shook because of an explosion at the Jefferson Quarry. But the mining company said its own research found that an earthquake had struck the city seven seconds after it set off charges.

The resulting standoff has left homeowners wondering who will pay for all the cracked plaster and other damage, and whether it will be months or even years before legal action can resolve who's at fault.

"Nobody wants to take responsibility for anything," said Ann Helgeson, who bought her house adjacent to the quarry just weeks before the ground shook. She was home with her young son that day when the house started to shake from the cellar up.

Now, the freshly painted walls are cracked and portions of the once-flat floor dip. Gaps can be seen where the living-room ceiling meets the wall. "We've got cracks upstairs everywhere," said her 6-year-old son, Gage.

Helgeson's insurance company inspected the damage in early May and left her thinking that it would be covered. It wasn't until a letter arrived in the mail last week that she learned the company had determined the cracks were caused by natural settling and therefore were not insured. What's more, the company wrote, they were dropping her completely after noticing that a portion of her home's exterior had asbestos shingles, which they don't cover.

Left without other options, Helgeson said she's hoping someone from the city will step in to help her and some of the other 128 homeowners who reported damage.

Mankato community service officer Sandy VanEman was dispatched to Jordan Sands' Jefferson Quarry at 621 Cleveland St. in Mankato on April 25 to witness what the company said would be a controlled blast, a routine procedure. It's common for the Department of Public Safety to send someone to the quarry for a blast, and VanEman had witnessed others.

On that day, the explosion was "extremely loud; much louder than the controlled blasts I've experienced in the past," she wrote in her report. As homeowners called 911 to complain, VanEman asked to speak to someone at the blast site. The person who's usually in charge of the blasts was off that day, she learned, so someone else had handled it.

About half a mile away, neighbors in the Tourtellotte Park neighborhood were just emerging from their homes.

Tana Marthaler said she thought a truck had crashed into the front of her house. Her neighbor Morgan Lenhoff thought the same thing.

"There was a very loud boom," said Lenhoff.

"I looked up and the house was wobbling away," he said.

On June 14, the company said its own investigation found that a magnitude-2.8 earthquake struck Mankato just seven seconds after it set off 4,580 pounds of charge at its limestone and sand quarry, which is within city limits. The determination was made by Ivan Wong of Lettis Consultants International, who was hired by Jordan Sands. His calculations put the epicenter of the quake 1.6 miles to the quarry's south and about 2 to 3 miles below ground.

The company's report concluded that the earthquake was not caused by quarry blasting, that the quake would have happened eventually, and that the chance of more quakes is low.

But a U.S. Geological Service monitoring station 40 miles east of Mankato recorded the explosion; a government scientist said the vibrations that day were from quarry blasting, not an earthquake.

"The location is right at a mine site, so that's a big clue," said USGS geophysicist Paul Caruso. "Second, an experienced seismologist can look at the wave forms, and explosions look a lot different than real earthquakes."

"There are certainly differing opinions among scientists about a variety of things," he said. "It may be one of those things that get reviewed over time."

For now, Jordan Sands has asked homeowners to send them information about any damage to their homes. Sustacek said the mining company's insurance will review the claims, but it's too soon to tell where that might lead.

The city examined the company's blasting records and interviewed company employees and found no negligence.

Still, the company's explanation hasn't convinced witnesses and homeowners, who have learned that most insurance companies won't cover earthquake damage.

"Seven seconds after the blast we have an earthquake?" said Marthaler, one of the homeowners claiming property damage.

Matt McKinney • 612-217-1747