tremor reported on July 16 off the coast of Florida was not an earthquake, but a Naval test explosion.
The U.S. Geological Survey now lists the event on its earthquake hazards page as an "experimental explosion by the U.S. Navy." According to DefenseNews.com, the 10,000-pound explosion was set off to test the resilience of a combat ship, the USS Jackson. Navy officials have told multiple news outlets that they plan to release a statement about the testing.
USGS instruments measured the blast as a magnitude-3.7 earthquake, which would have been a rare seismic event in the tectonically quiet region. Florida sits on the passive margin of the North American continental plate, meaning that temblors are rare. The active margin of the plate is on the West Coast, where the North American plate is slipping under the Pacific plate in a process called subduction. Subduction is why California and the Pacific Northwest experience earthquakes on a relatively regular basis.