April tremor in Mankato was an earthquake, company says

That April blast that shook Mankato, Minn., that officials said was caused by blasting at a quarry and not an earthquake? It was an earthquake. At least, that’s the claim of an engineering consultant hired by Jordan Sands quarry, the Mankato Free Press says.

That’s set off a dispute with the U.S. Geological Survey which insists the blast, registering 2.8 on the Richter Scale, was caused by blasting.

But the report says that an earthquake occurred seven seconds after the quarry blast, two miles below the ground and a mile south of the quarry.

“I would say it looks fairly blasty,” Jana Pursley, a geophysicist at the USGS, said Tuesday.

Pursley said there only appeared to be one singular event in Mankato on April 25 — either an earthquake or a blast — and not two events separated by a few seconds. While raw seismological data suggests an explosion caused the ground to shake, Pursley said there could be room for error as the nearest USGS station is more than 31 miles away from the Mankato area.

“For me to really be 100 percent certain, I would need a station within a few miles,” she said.

Quarry officials released the reports to a group of residents but wouldn’t talk to the Mankato Free Press about it until it releases a press release later this week, the paper says.

Some residents aren’t buying it.

“Mankato’s never had an earthquake, so for them to tell us it was unrelated to the blast it was ridiculous,” said Megan Dell, owner of the Mecca Tattoo building on Broad Street.

Elly Zaragoza said in an email she wasn’t surprised by the residents like her who were upset with the company.

“A company reviewing itself can hardly be objective,” she wrote.

Zaragoza said she was concerned the company would only hold one public meeting to discuss its investigation.

The company is winding down operations at the quarry and after December, no blasting can take place without a committee of residents and officials approving the plan.