Numerous residents of St. Johns, Flagler, and Volusia Counties reported feeling and hearing rumbles, at a little after 9 AM, on Friday morning. So, what force caused the commotion? No one knows. It was reported that an military aircraft broke the sound barrier, which caused the earth to rumble. This rumor turned out to be false. NAS Jax and the FAA both say military exercises were not in progress, and there was not even a single plane in the sky, near the affected region.
Could the rumble have been an earthquake? Possibly. The USGS did not record any seismic activity. But one has to remember that the nearest and only seismograph is on the west side of Orlando. This means it is possible that the earthquake recording device could have easily missed the earthquake. The USGS agrees with this plausible scenario.
Another scenario that exists is that a meteor entered earth's atmosphere and plunged into the Atlantic Ocean. Just like the earthquake scenario, we will never know whether the meteor hypothesis is what actually happened or not. A meteor can cause trembles to occur, just like the ones experienced in Florida. One shred of evidence that backs up the meteor theory is the NWS radar, from approx 9:15 AM.
Notice the little, green "speck" to the east of Florida. Could the doppler radar have picked up the meteor or is it simply a false return? (The air was too dry to have showers around.) No observations exist of meteors during the 9:00-9:30 AM time period. It would have been hard to see such an object with the rising the sun.
It is likley that we may never know the true answer to what caused the earth to rumble, in North Florida, on 3/18/2011.