The quake occurred at 2 a.m. and was centered just offshore, with an epicenter located about 15 miles southwest of Santa Monica, the USGS said.
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Its depth was measured at just over 7 miles, officials said, and was followed by multiple aftershocks, including one with a magnitude of 3.5.
ABC7 viewers said they felt rumblings in Whittier, Culver City, Chatsworth and Torrance.
Shannon Lewis of Cerritos said in a social media post: "Man, I was about to be knocked out sleep, now my adrenaline skyrocketed and I'm back at square one."
How to make your own earthquake kit
There were no immediate reports of injuries or major damage.
Santa Monica resident Kiley McGill said: "It sounded like a truck was running through my house, and some shaking, and woke up to it." She added that she was "born and raised here, so it's nothing new."
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The incident prompted the Los Angeles Fire Department to briefly go into "earthquake mode."
All 106 of the agency's neighborhood fire stations conducted "a strategic survey of their districts, examining all major areas of concern (transportation infrastructures, large places of assemblages, apartment buildings, power-lines, etc)," the LAFD said in a statement. The survey was conducted "from the ground, air and sea," and ended about 2:45 a.m.
Although the ShakeAlert system was activated by the earthquake, it "was too small to trigger the delivery of #ShakeAlert-powered alerts to cell phones," the USGS said on Twitter.
The National Oceanic and Atmospheric Administration said no tsunami was expected as a result of the quake.
In an interview later Wednesday morning, seismologist Dr. Lucy Jones told ABC7: "At this point it's also been several hours, right, so the most likely time to trigger a bigger earthquake is right away. So, as the time goes on, by the time you're half a day out, instead of a 5% chance it's more like a 2% chance."
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In a somewhat remarkable coincidence, Wednesday's earthquake comes exactly five years after a magnitude 4.0 earthquake struck Southern California -- also around 2 a.m.
The Jan. 25, 2018 quake was centered in the Trabuco Canyon area of Orange County. No injuries or major damage were reported in that quake either.