Monday morning's magnitude 4.7 earthquake in Riverside County was the largest temblor to hit the Los Angeles region in three years and has produced more than 100 aftershocks.
It caused no major damage, but it was felt over what seismologists said was an unusually large area.
The quake was initially recorded as three separate temblors because a foreshock tricked seismograms into recording multiple quakes of multiple sizes, said Susan Hough, a USGS seismologist.
Earthquakes of a 4.7 magnitude are typically felt only about 120 miles away from the epicenter, but Monday morning's quake traveled farther, shaking coffee cups as far as Los Angeles.
That's because the quake occurred in the San Jacinto Mountains, which are composed of hard granite that transmits energy more efficiently, Hough said.
The quake occurred along the San Jacinto fault zone, which runs through San Bernardino, Riverside, San Diego and Imperial County roughly parallel to the San Andreas fault. It's one of three fault zones that absorb friction from the motion of the North American continent and the Pacific plates rubbing against each other.
"It's capable of generating moderate to large earthquakes," said USGS seismologist Robert Graves. "Today's activity was not out of the ordinary. Actually, it's pretty typical of the area."
There is some evidence that the largest quake ever recorded in the fault zone, of magnitude 7, occurred sometime in the early 1800s, Graves said.
The fault zone has generated eight earthquakes of magnitude 6 or larger in the last century, Graves said. About five earthquakes of similar size have occurred within 20 kilometers of the area within the last 20 years, he said.
The most recent large earthquake in the fault zone occurred in 1968. The magnitude 6.5 Borrego Mountain earthquake severed power lines in San Diego County, cracked plaster in Los Angeles and rocked boats in Long Beach for five minutes, according to Caltech's website.
That quake struck just a few miles to the south of today's temblor.
"It's a good idea to take it to heart and make sure you're prepared," Graves said. "We live in Southern California, and we have lots of active faults, and every once in a while it's large enough to cause damage."
The quake knocked some items to the floor at a market in rural Anza. And it produced some anxious moments.
Minnesota transplant Shannon Haber said that even though she's lived here since 1996, she definitely has not gotten used to earthquakes.
Haber was working in Los Angeles Unified School District headquarters in Westlake when the earthquake shook at 9:56 a.m. more than 100 miles away. "I was just a little frightened," she said. "There was small shaking and it made me nervous because I'm 23 floors up."
The shaking was the hardest and longest she could remember. "It was a slow, swaying motion," she said. "It sort of felt like I was on a boat, a sort of wavy feeling that lasted 10 to 20 seconds. … No one else reacted around me. They're all veterans of earthquakes."
Holly Lawson was working in a campground kiosk at Anza-Borrego Desert State Park, about 60 miles south of the epicenter, when the windows in her tiny kiosk began to rattle.
The rolling rumble lasted about six seconds, she said, and she could see a man outside in his truck as it swayed back and forth.
"The truck was actually physically moving," Lawson said.
A San Diego native, she had already guessed the temblor's magnitude by the time the shaking stopped.
"I'm always concerned about these windows when we feel a quake," she said. "We're surrounded by them."
Lawson, who lives in Anza, got a call from her teenage son, who reported there had been a loud, sudden crack of sound before the shaking began. Their manufactured house had experienced small cracks after a similar earthquake about a year ago.
Christine Mai-Duc contributed to this report.