You can’t blame Floridians for being skeptical when they heard there was an earthquake off the east coast last week.
Life on Florida's Space Coast means you’re used to sonic booms, loud rumblings that travel more than 40 miles when big rockets lift off and trailing flames streaking across the sky. As commercial launch traffic picks up at Cape Canaveral, there’s more such activity than ever before.
There’s also no fault line along Florida’s Atlantic coast and very little earthquake activity. It's one of the few natural disasters the state doesn't have to cope with on a regular basis.
In every previous instance where the U.S. Geological Survey lists an "earthquake" off Florida's Atlantic coast, dating back to 1900, all but one previous event was a massive explosion set by the U.S. Navy to test the seaworthiness of its new ships against underwater mines. The Navy also detonates big practice bombs at its Pinecastle Bombing Range complex in the nearby Ocala National Forest, occasionally rattling doors and dishes for miles away.
So forgive Floridians for questioning what really happened at 10:48 p.m. on Wednesday night. But the USGS says it was an earthquake, and other seismologists agree.
"The signature of the seismic waves is consistent with an earthquake," said Oliver Boyd, a research geophysicist with the USGS. The most likely cause of the earthquake is stress on fractures in the Earth's crust. "Just about anywhere you are on the planet, the plates are experiencing stress."
Expert seismologists were skeptical when word spread Thursday that a magnitude 4 earthquake had been detected about 100 miles due east of Kennedy Space Center. An earthquake? Off Florida?
Even the seismologist who directs the USGS National Earthquake Information Center, Paul Earle, admitted he was skeptical at first. When he saw the report pop up, he thought "In Florida?" He wrote to ask the analysts "Are you sure?"
“It is highly unusual to have a seismic event in Florida or close to Florida,” said Jochen Braunmiller, a research assistant professor at the University of South Florida’s school of geological sciences. “So I was very surprised when I heard about it.”
The USGS records dating back to 1900 show only one real earthquake offshore in the Atlantic within 400 miles of the state's coast, a magnitude 3.2 in the Bahamas – about 75 miles east of Florida – in 1992.
USGS data shows more than a dozen earthquakes have been recorded farther out in the Atlantic, 400-1,000 Miles east of Florida.
Inland, a couple of earthquakes of at least 1.5 magnitude have been recorded on the peninsula, near Jacksonville and between Gainesville and Ocala, and three at the northwest corner of the Panhandle at the Florida/Alabama line, according to USGS data. At least four have been recorded off the continental shelf in the Gulf of Mexico.
Earle said it's possible that seismic activity below a magnitude of 2.5 could have been missed in earlier years.
Braunmiller happened to be with a group of seismologists on the coast in Melbourne and the earthquake was a hot topic. “We were all very intrigued by it and suspicious and wondering what was going on,” Braunmiller told USA TODAY. “Is it an earthquake? Is it an explosion? Is it a landslide?”
By Thursday afternoon, the seismologists Braunmiller was talking to, including colleagues at USF, agreed it was an earthquake. They compared the seismic data from a 2021 ship shock trial with the signal from Wednesday night's event. "They look very different," he said. “We are fairly sure it was an earthquake.”
Wave signals on seismometers show slight differences between earthquakes and explosions, he said. "We have some comparison to some explosions the Navy did three years ago and the signal looks different."
If there's an explosion in the water, a lot of energy gets trapped in the ocean and propagates slowly toward land, he said. So the sound wave goes up and down in the water column and comes in much later than the other seismic signal that travels faster.
Ray Russo, an associate professor at the University of Florida's geological sciences department, agreed this recorded motion was more consistent with an earthquake than an explosion.
"The waveforms look like an earthquake - the S (for shear) waves ... are much larger than the P waves (for primary)," Russo told USA TODAY. "Explosions typically do not produce large S waves, although an explosion in the water could conceivably do so."
The survey estimated Wednesday's activity took place at a depth of six miles, plus or minus five miles.
"I've learned to never say 100% certain but I’m very certain this is a tectonic earthquake," Earle said.
Explore:Find earthquakes near you
Floridians and seismologists wondered if the earthquake was the Navy offshore with explosives again. They've conducted ship shock trials on vessels at least nine times in 23 years.
The trials detonate the equivalent of several tons of TNT near a ship that is new for its class, to ensure the ship is battle ready and can withstand blasts from underwater mines. One of the explosions in 2016 registered 3.7 on the Richter scale and triggered a response on seismographs as far away as Venezuela and Minnesota.
The Navy said it’s not to blame this time.
"We're not tracking any sort of large operation or shock trials or anything that would be seismic-related," said Lt. Commander Andrew Bertucci, a Navy public affairs officer.
Live marine vessel tracking maps did show a couple of Navy and Coast Guard vessels out in the Atlantic Thursday, as well as several tankers, cargo ships and an unidentified tug or special craft.
These are some of the previous trials off Florida's east coast.
The National Weather Service office in Melbourne posted on Facebook, asking if people had felt the shaking and the responses revealed just how much the region’s residents are accustomed to such activity.
“Thought it was a rocket,” multiple people responded.
Some said they ran outside to try to catch a last-minute glimpse of a launch. They knew a SpaceX rocket was set to launch NASA’s new PACE satellite into orbit. But that didn’t happen until nearly three hours later, at 1:33 a.m., from the Cape Canaveral Space Force Station’s launch complex. Others thought it was thunder, not unusual in a state that gets the most lightning strikes per square mile.
Again on Friday morning, coastal residents turned to social media to ask about a rumbling sound and learned the Dragon spacecraft had returned to Earth carrying three astronauts from the International Space Station in a joint Axiom and SpaceX mission.
SpaceX launches NASA PACE spacecraft from Cape Canaveral Space Force Station in Florida
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