Neither movie star was hurt in the quake - which measured 6.1 on the Richter scale - but many people on the Caribbean island were thrown from their beds when it struck.
George, 47, and Ewan, 37, were in Puerto Rico filming anti-war satire film 'Men Who Stare at Goats' at the time of the quake. Guests at the hotel where the two actors are staying overheard the pair discussing the incident the next day.
Saturday's earthquake was the most powerful to strike the island for 20 years. So far there have been no reports of injuries or damage.
George and Ewan returned to the movie set soon after the incident passed.
The quake came on the 90th anniversary of the most major earthquake ever recorded on the island, which measured 7.1 on the Richter scale and killed between 90 and 116 people.
'Men Who Stare at Goats' is set during the early day of the US invasion of Iraq and looks at a paranormal unit of the US army called the First Earth Battalion, which reportedly believed soldiers could use psychic powers to adopt a cloak of invisibility, pass through walls and kill goats simply by staring at them.
George plays Lyn Cassady, a US operative sent on a secret mission into the warzone, while Ewan portrays news reporter Bob Wilton.
George has been spotted sporting a very unusual look on the set of the film, including a brown moustache and a long haired brown wig.
'Men Who Stare at Goats' also stars Jeff Bridges and Kevin Spacey and is set for release in 2010.
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