The PTWC initially said "hazardous tsunami waves" were possible for coasts located within 300km (186 miles) of the earthquake's epicentre.
This included parts of Belize, Cuba, Honduras, Mexico, the Cayman Islands and Jamaica, it said. But in an update at around 20:45 GMT, the PTWC said the "tsunami threat had now largely passed".
Shaking was reported in the Cuban capital, Havana, in Kingston, Jamaica and Miami - some some 708 km from the epicentre.
Pictures from Miami showed office workers gathered in car parks and thronging the streets after the quake struck at around 14:10 Miami time (19:10 GMT). There were no injuries or road closures, Miami police said.
USGS geologist Ryan Gold told the Miami Herald it was "completely reasonable" for a quake of that magnitude to be felt in Florida.
"It's a very large earthquake which can produce a lot of seismic energy," he said.
"I felt the house trembling and realised that it was a quake," Kingston resident Jawara Rawjers told AFP news agency.
Machel Emanuel, a doctor in the same city, added: "I was on the second floor of a building and there was a sustained shaking of the building. I felt dizzy. The door was slamming consistently for a while."
In the Cayman Islands, a warning to "move away from coastal areas" was issued by the government but stood down a few hours later.
On Twitter, Cayman authorities said there were "only a few reports of structural damage to buildings" and shelters were being opened as a precaution.
Mikhail Campbell, a police media relations officer in the Cayman Islands, told Reuters news agency he was not immediately aware of any reports of serious damage.
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