Southern California was shaken Wednesday by the second moderate but widely felt earthquake in less than 11 hours, but no harm was reported.
A magnitude-4.5 quake centered in the same area struck late Tuesday night. Both temblors were followed by numerous aftershocks that were mostly too small to be felt.
Quakes of such magnitude are unlikely to cause damage in cities built to modern standards but can rattle nerves.
The Orange County Fire Authority did not receive any 911 calls about the latest quake, said Capt. Marc Stone.
"It was a decent sized shake and it's a reminder for everyone to have a plan for the Big One," said Stone. "How would you and your family survive for 72 hours with no water, no food and no amenities? Think about it. It's a reminder to go home and say, 'What if?' and make that plan."
Seismologist Kate Hutton of the California Institute of Technology characterized the quakes as a swarm.
The location is near the Whittier Fault, but the quakes could be occurring on an unmapped fault, she said.
"This is likely normal California earthquake activity," Hutton said.
The staff of the Richard Nixon Presidential Library & Museum in Yorba Linda was still talking about Tuesday night's quake when Wednesday's struck, said Jonathan Movroydis, director of communications.
"It did shake us pretty well," Movroydis said, but the jolt was so short no one ducked under their desks.
Meanwhile, newly acquired Los Angeles Dodgers outfielder Shane Victorino tweeted: "Why is the hotel shaking????? ... Welcome to LA!"