The quake struck at 2:09 a.m. about 8.1 miles northeast of Trabuco Canyon and 14.9 miles south-southwest of Riverside.
People reported feeling the shake all over the Southland, including San Diego County, parts of the Inland Empire and sections of the San Fernando Valley.
ABC7 Quake Cam captures 4.0 earthquake near Trabuco Canyon
Those close to the epicenter described feeling different sensations.
"Kind of like a roller coaster, you know? You rode it. It wasn't like a serious jolt - the initial was - but then it kind of rolled," described Lake Forest resident Chase Bryant.
The U.S. Geological Survey reported several small aftershocks, including a 2.2 at 2:50 a.m. and at least two others with a magnitude of 1.1. All were northeast of Trabuco Canyon.
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There were no reports of any injuries or damage.
Watch our live ABC7 Quake Cam here: http://abc7.com/2440166/
Foothill Ranch resident Kristen Bailey said the jolt made her think of an even bigger one.
"I think just because we haven't had any, it just kind of shook me a little bit, and then I worried about that. I was like, 'OK, when is the big one coming?'" she said.
Seismologist Dr. Lucy Jones tweeted about the shake, saying, "This morning's M4.0 near Trabuco Canyon is near Elsinore fault but not on it. Mechanism shows that it moved on an east-striking thrust fault. Many small quakes are on small faults, too small to be mapped."
Many people said the temblor was a good reminder that a much more damaging earthquake could strike at any time.
"It was a little wake up call for me because I think I need to prepare a little bit," said Lake Forest resident Debbie Williams.
The earthquake comes just two days after a 7.9-magnitude quake off the coast of Alaska triggered tsunami watches all over the West Coast.
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