Report: Earthquake, not blast, caused shaking in Mankato last April

MANKATO, Minn. (KMSP) - A recently released report claims to solve the shaky mystery that captivated Mankato, Minnesota, in April: what caused the shaking felt in the city and by instruments 250 miles away?

The report blames an earthquake—not a controlled blast at the silica-sand quarry owned by Jordan Sands. The report, commissioned by Jordan Sands, concludes “an earthquake was likely to occur in the area regardless of the surface blast at the Quarry.”

The report says the earthquake was centered “1.2 miles to the south” of the quarry and “at a depth of 2-3 miles below the surface.” According to the report, the earthquake happened seven seconds after the blast, but “there is no evidence of a causal connection between the two events.”

Before the April 25 quake, it had been more than a century since an earthquake hit Mankato. The report says an earthquake “was likely to occur” regardless of the blast.

The report posed the question of how the blast and quake could be unrelated despite the two events occurring within seven seconds. The report’s answer: “The two events are correlated in time, but a relatively low amount of energy released by a quarry blast would be nearly unmeasurably small by the time it reached more than a mile below the ground surface…Timing does not provide a mechanism that would show causation…”

In addition to the quarry-commissioned report, the City of Mankato also investigated to determine whether there were any laws violated in the blast and concluded there “was no evidence” showing a “negligent blast had occurred,” according to Jeff Bengston, deputy director of public safety.

Though blasting was suspended while the city investigated, Jordan Sands has been allowed to resume blasting.

While the report blames the shaking on an earthquake, the USGS still classifies the incident as a "quarry blast." A representative said the agency is still reviewing reports, and it's possible it could change the classification.