Explosion off Florida coast registers as magnitude 3.9 earthquake

A magnitude 3.9 seismic event detected off Florida’s Atlantic coast was caused by an apparent explosion rather than a naturally occurring earthquake, according to the U.S. Geological Survey (USGS).

The event was recorded at about 3:04 p.m. Thursday, roughly 91 miles east-northeast of Ponce Inlet and 93 miles east-northeast of Daytona Beach. USGS data placed it at or near the surface.

“The recorded ground motions from this event are more typical of an explosion than a naturally occurring earthquake,” the agency said.

The USGS did not identify the source of the explosion but indicated that the U.S. Navy has previously conducted Full Ship Shock Trials in the region. Those trials involve detonating large underwater explosives near a warship to determine whether its systems and crew can withstand the shock of an attack.

Thursday’s event was the ninth classified as an experimental explosion in the region since 2016, according to USGS data.

Explosions can register as earthquakes because part of the blast’s energy travels through the ground or seabed as seismic waves. Seismometers detect those waves and calculate a magnitude based on their strength, although their patterns can differ from those produced when faults move.

A similar event was recorded on May 28, when the explosion of a New Glenn rocket operated by Blue Origin at Cape Canaveral registered as a magnitude 2.5 earthquake.