Update: The U.S. Geological Survey now says the "earthquake" was actually an "experimental explosion by the U.S. Navy." Read more about the explosion.
An earthquake struck off the coast of Florida on Saturday (July 16), a rare event in a relatively tectonically peaceful region.
The 3.7-magnitude quake had an epicenter that was 104 miles (168 kilometers) east-northeast of Daytona Beach, according to the U.S. Geological Survey (USGS). It hit at about 4 p.m. local time and originated at a depth of about 3 miles (5 km). According to the USGS ShakeMap, some weak tremors were reported and picked up by scientific instruments on the mainland, but the quake was too weak to cause any damage.
Earthquakes are rare in Florida, and the reason for the relative peace has to do with Florida's position on the North American plate. The state sits on the passive margin of the plate, a transition from land to ocean that isn't seismically active. In contrast, the western end of the North American plate — the active margin — is slipping under the Pacific Plate, triggering the medium-to-large earthquakes that are commonly experienced in California. [The 10 Biggest Earthquakes in History]
According to the state's Department of Environmental Protection (DEP), there have only been about 24 "seismic events" reported since 1727. A 1997 review suggested that only five were actual earthquakes.
The Florida Geological Survey rounded up these events in a 1991 report that illustrates the challenge of identifying past "earthquakes" in the state. Many of the reported temblors have come from newspaper reports or other unofficial sources. For example, a "severe quake" was reported to have hit St. Augustine in 1727, but all original reports of the event have been lost. At some point in 1930, there were reports of shaking across Central Florida — tremors that some attributed to an explosion of some sort and others blamed on an earthquake. In January 1945, the windows at the De Land courthouse in Volusia County shook – the only evidence that anything might have happened. Rattling doors and windows were also reported on Captiva Island in 1948 and northwest of Tallahassee in 1952.
Other reports of temblors are easily linked to large quakes that occurred outside of Florida, but they were strong enough to be felt in the state. For example, in 1886, a large earthquake struck Charleston, South Carolina, and the shaking spread across northern Florida. Floridians also felt shaking from several aftershocks of the South Carolina quake. More recently, a magnitude-5.8 quake centered in the Gulf of Mexico was felt in Florida.
There has never been a recorded earthquake with an epicenter under Florida, according to the DEP.
Original article on Live Science.