It wasn’t an earthquake. It was 10,000 pounds of explosives

A 3.7 magnitude earthquake hit off the east coast of Florida on Saturday afternoon.
A 3.7 magnitude earthquake hit off the east coast of Florida on Saturday afternoon. U.S. Geological Survey

It turns out the 3.7 magnitude Florida earthquake recorded on Saturday was actually something else: a 10,000-pound explosive charge the U.S. Navy set to test their own ship.

Dale Eng, a public affairs specialist for Naval Sea Systems Command, said the explosion was the third in a series of stress tests for the USS Jackson, a littoral combat ship. This explosion was the “most forceful” of the tests, Eng said, including the first, on June 10, and the second, on June 25.

The Navy won’t reveal the exact location of the test for classified reasons, but the “earthquake” registered 105 miles off Florida’s east coast, near Daytona Beach, around 4 p.m. Eng said a warning to boaters was released before the test telling them to stay away.

Don Blakeman, a geophysicist with the National Earthquake Information Center, said researchers can easily tell the difference between the two main events they see — earthquakes and mining accidents. But the difference between natural quakes and induced quakes (like those caused by fracking) is difficult to spot. This explosion had the seismic signature of an induced quake.

“There was no way to indicate it was a blast,” he said. “Without any other evidence we had to go with the assumption it was a natural event.”

Once a Navy spokesman reached out with a report confirming the exact time and location of the blast the information center revised its categorization.

“It was very suspect from the very beginning,” Blakeman said. “We don’t have much from Florida. It isn’t a seismically active area.”

The last Navy stress test was in 2008 for the USS Mesa Verde, and the next will be for the USS Milwaukee later this summer.

The Navy has come under fire from environmental activist groups over its use of high-intensity long-range sonar underwater, which produces sound waves around 140 decibels. That intense sound disturbs the breeding, nursing and foraging of endangered marine mammals. Protecting marine mammal habitat from Navy sonar is “of paramount importance” under the law, a judge ruled recently.

As part of a required environmental mitigation strategy, the Navy told the National Oceanic and Atmospheric Administration’s Southeast Marine Mammal Stranding Network Coordinator about the testing in advance. Teams of observers were present for the testing to make sure no protected species, like sea turtles or marine mammals, were injured, wrote NOAA spokeswoman Jennie Lyons in an email.

“There haven't been any protected species (marine mammal or sea turtle) mortalities observed in prior ship shock trials conducted by the U.S. Navy,” she wrote. “We are also not aware of any animals observed harmed so far in this round of exercises.”

Still, it can be hard to determine the full impact of that kind of explosion on marine life, said Zak Smith, a senior attorney for the Natural Resources Defense Council. Anything with a high-intensity, high-decibel sound can harm marine mammals, which might not be visible from the surface.

“They depend upon sound to interact with their environment in the same way we rely on our eyes,” he said.

Alex Harris: 305-376-5005, @harrisalexc