MANKATO — An engineering firm hired by Jordan Sands says an April 25 quarry blast at Jefferson Quarry was not responsible for the tremor that shook the Mankato area.
The report, compiled by Barr Engineering, points to evidence an earthquake took place about seven seconds after the blast.
At the time, the U.S. Geological Survey said the incident measured 2.8 on the Richter scale and was likely due to a quarry blast based on seismological data. Several geologists since then have said the raw data shows an explosion caused the Mankato area to shake.
"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.
Barr Engineering experts say the USGS didn't include some data in their initial estimates and misidentified the earthquake's epicenter as the quarry. They also pointed out the energy released by the quarry blast likely wouldn't have been felt more than 100 miles away — monitoring stations as far away as western South Dakota and northern Missouri reportedly recorded the Mankato quake on April 25.
Experts with another company, Lettis Consultants International, used additional data from the USGS and on-site vibration readings to determine an earthquake likely occurred seven seconds after the blast, about 2 miles below ground and more than a mile south of the quarry.
It's unclear whether the USGS will reexamine the incident.
Company officials revealed the report to more than 60 residents during a meeting Monday night at the Eagles Club.
A Jordan Sands spokesperson confirmed a meeting took place Monday night but said the company wouldn't comment until it issues a press release later this week.
Jordan Sands CEO Scott Sustacek, who said last month the company contracted two independent firms to analyze the quake, was reportedly traveling Tuesday and was unavailable for comment.
Mankato officials released Barr Engineering's study Tuesday night. The city found no evidence Jordan Sands was negligent or deviated from its scheduled April 25 blast plan.
Yet the city has put stipulations on the company's future operations at Jefferson Quarry. Jordan Sands won't be able to blast at the quarry after Dec. 29 unless a committee of residents, city and company officials sign off on future operations. Jordan Sands will also have more regulations to follow once it resumes blasting, which could take place starting next week.
The company is already winding down operations at the quarry.
Residents at the meeting were skeptical of the report's findings.
"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.
Jordan Sands officials worked with city officials to send out mailers to some parts of Mankato in advance of Monday night's meeting.
An uncommon event
The quake caused momentary panic throughout the Mankato area when it hit.
Residents on North Fourth Street said they felt like their homes were going to collapse and headed outdoors.
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.
Area officials initially announced a combination of atmospheric pressure and cold weather caused the quarry explosion to reverberate throughout the Minnesota River Valley. The city of Mankato and Jordan Sands agreed to suspend blasting at the quarry until geologists and geophysicists could review what happened.
Geologists say a 2.8 earthquake would normally start between 5 to 15 kilometers below the ground, but at the time there was no evidence such a quake took place. Several said it was unusual for a quarry blast to cause such a large event.
Earthquakes in Minnesota are rare, but not impossible. Barr Engineering found reports of an earthquake centered near Le Sueur in the late 1800s, which is estimated at about 4.3 on the Richter scale.
One thing's for certain — Mankato likely won't experience an earthquake like this any time soon.
"The accumulated stress in a portion of the rock has now been released by the earthquake, so another earthquake in the Mankato area is less likely in the foreseeable future, even with continued blasting at the Jefferson Quarry," James Aiken of Barr Engineering wrote in the report.