A 3.9 magnitude seismic event, identified as an 'experimental explosion,' was detected by the U.S. Geological Survey (USGS) off the coast of Florida on Tuesday. The event occurred east of Flagler Beach at a depth of zero feet, indicating the source was at or very near the surface. This characteristic is more consistent with an explosion than a typical tectonic earthquake.
The location of the event is approximately 20 miles from where a similar seismic reading was registered on July 16, east of Ponce Inlet. This area is known for U.S. Navy Full Ship Shock Trials (FSST), a process where the Navy conducts controlled underwater detonations to assess the resilience of new or upgraded warships against battle-like explosions, such as those from mines or torpedoes.
Following the July 16 event, a Navy spokesperson confirmed that the future USS Ted Stevens (DDG 128) had completed the first of two planned FSST as part of the certification process for the Navy's DDG 51 Flight III destroyer program. The Navy has utilized FSST in this region previously, including a test explosion near the aircraft carrier USS Gerald R. Ford in 2021.
The USGS records such events because its sensitive seismographs can detect the ground vibrations produced by human-made blasts. Seismologists then analyze these signals to differentiate them from natural earthquakes. These events are commonly linked to controlled military exercises, scientific seismic research, and industrial mining operations. The Navy's shock trials have historically involved large explosive charges, sometimes in the tens of thousands of pounds, to simulate combat conditions.
While Tuesday's event is believed to be a planned test, public awareness was heightened by its appearance on the USGS website. Social media users noted the similarity in magnitude and location to the July 16 event, with some questioning the lack of immediate explanation on the USGS listing.
These Navy tests are permitted events designed to confirm that vessels can withstand the shock of combat. Historical data indicates the Navy has conducted FSST for decades on newly built warships, with past trials involving a range of vessels including littoral combat ships, an amphibious transport dock, an amphibious assault ship, and a guided missile cruiser. The last aircraft carrier to undergo such a trial was the USS Theodore Roosevelt in 1987.