A mysterious blast off the Florida coast has sparked confusion after being recorded as an "experimental explosion".
A seismic reading of 3.9 magnitude appeared on the US Geological Survey's monitoring system on Thursday afternoon, located east of Ponce Inlet near the state's Daytona Beach.
The agency categorised the event as an "experimental explosion" rather than a natural earthquake.
Recorded at a depth of zero feet, the surface-level origin suggested an explosion rather than tectonic activity.
CBS Miami meteorologist Nic Merianos said he had "never seen this before" after the blast.
And the tremor's unusual classification sparked concern and speculation on social media.
"What type of experimental explosion generates a 3.9 earthquake?" one person questioned.
Others suggested bomb testing was underway off the Florida coast.
A seismic reading of 3.9 magnitude appeared on the US Geological Survey's monitoring system on Thursday afternoon
|USGS
A spokesman for the USGS offered an explanation for the mystery motion.
He told the Mail: "The recorded ground motions from this event are more typical of an explosion than a naturally occurring earthquake.
"The Navy has conducted Full Ship Shock Trials (FSST) in this region in the past."
These controlled underwater detonations are periodically carried out by the US Navy to assess how warships perform under battle-like conditions.
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The USGS suggested Full Ship Shock Trials may have played a part in the blast
|US NAVY
A similar "earthquake" happened in the same waters five years ago while the USS Gerald R Ford aircraft carrier was being tested.
That earlier event involved deliberate explosions designed to evaluate the vessel's capacity to withstand nearby blasts.
Military testing remains the most probable explanation for Thursday's seismic reading.
Full Ship Shock Trials employ substantial explosive charges, sometimes reaching tens of thousands of pounds, to replicate combat scenarios involving mines or torpedoes.
PICTURED: The USS Gerald R Ford undergoes a Full Ship Shock Trial
|US NAVY
The tests are planned out well in advance, and include safeguards for marine wildlife, and checks that no damage or injuries have been reported.
The Navy began conducting these trials after discovering that nearby detonations could disable critical ship systems even without causing major structural damage.
The USS Gerald R Ford is not the first American vessel to undergo the trials, however.
Previous vessels subjected to shock trials include USS Jackson and USS Milwaukee in 2016, USS Mesa Verde in 2008, and USS Theodore Roosevelt in 1987.