A hoax letter circulating on the Internet claims that a "sizeable earthquake" is going to strike Southern California, U.S. Geological Survey officials said.
The agency said the letter used an unofficial USGS logo and was sent to residents.
"USGS had no part in this letter or any alleged alert," the agency said on its Facebook page. "USGS does not predict earthquakes."
The letter made the rounds after a magnitude-5.1 earthquake struck La Habra on Friday, and before April Fool's Day.
The USGS advised residents to check its website for timely, scientific information after a temblor.
"The message of being prepared is always valuable," the USGS said.