A strong earthquake with a preliminary magnitude of 5.4 jolted large parts of Southern California late this morning, shaking a wide swath from Ventura County to San Diego and causing minor damage and a few injuries. The quake rattled buildings in downtown Los Angeles and was felt as far east as Palm Springs. It was centered near Chino Hills, about 30 miles east of Los Angeles, the U.S. Geological Survey said. * An interactive earthquake primer An interactive earthquake primer * 5.4 quake rocks Los Angeles area Photos: 5.4 quake rocks Los Angeles... * Video: Strong earthquake hits SoCal * Post-quake callers overload phone systems, both wireless and wired * What's closed, delayed after quake "It felt like something had exploded underneath us," said Vanessa Rojas, 21, a hair salon employee at Blondie's Clip Shop in nearby Chino. "The ground lifted, then it began to shake. It was a big ripple." There were minor injuries and structural damage at the St. Jude Centers for Rehabilitation and Wellness in nearby Brea, an outpatient medical facility, according to a representative. TV helicopter footage of the Chino Hills area showed people being evacuated from schools and some buildings, but no major damage. Pomona City Hall was closed as workers surveyed the damage, which was mostly broken glass. On the northern side of the building's third floor, a worker used a shovel to remove what little glass remained in a large window frame. Earthquake maps Map Larger Map | KML » USGS shake map | KML » How strong did it feel? » Aftershock forecast » Nearby fault lines Resources » Quake details from USGS » Major historical SoCal quakes | Statewide » Graphic: How to quake-proof your home More news » Latest reports from L.A. Now » Quake occurred about eight miles under Chino Hills » 'Really, really scary' in Chino Hills quake epicenter » L.A. Unified finds no damage at schools » SoCal Theme parks, rides remain open » Quake has everyone a-Twitter | Live search By midafternoon, some normalcy was returning. Nerves were still frayed in Chino Hills, but lives otherwise had largely returned to pre-quake status. At the Vons supermarket on Chino Hills Parkway, for example, all the aisles had been cleaned up two hours after the quake toppled glass jars off shelves and sent plastic containers bouncing to the floor. The magnitude of the quake, which struck at 11:42 a.m., was originally set at 5.8. Caltech officials later downgraded it to 5.4 and said they doubted the temblor caused major structural damage. Kate Hutton, a staff seismologist at Caltech, said that by 12:30 p.m., 27 aftershocks had been recorded at various sites, with the largest measuring 3.8. Today's temblor "was somewhat similar to the [1987] Whittier Narrows [quake]," Hutton said. "Most everyone in the L.A. Basin felt it. Things will have fallen off shelves. I'd be surprised to see some structural damage. There could be some cases of cracked plaster and maybe broken windows, but not structural damage, which is when a building is compromised." She called the quake an "oblique flip on a thrust fault," which means one fault line slides over another one. The quake occurred seven miles below the Earth's surface, which she said was fairly shallow by global standards. A much deeper one would not be felt so strongly but, overall, would be felt over a larger area. "The most interesting thing to us is that this is the first one we've had in a populated area for a long time," Hutton said. "People have forgotten what an earthquake feels like. We should look at this as an earthquake drill for the Big One that will come one day." She said today's quake was significantly smaller than the Whittier Narrows quake. Gov. Arnold Schwarzenegger activated the Office of Emergency Services and in a statement urged people throughout Southern California to cut back on telephone use because the system was being "maxed out." Raja Mitwasi, chief deputy director of the California Department of Transportation office in Los Angeles, said traffic on the county's highway system was not disrupted by the earthquake. He said Caltrans has been monitoring traffic from its traffic management center. The agency is also inspecting highway bridges and pavement. "So far, there is nothing visible. We don't even see minor damage," Mitwasi said. "Traffic appears to be moving normally. We don't see any delay." Irvine police closed the Bake Parkway connector to Interstate 5, fearing that the overpass might have been damaged. Glenda Dobbins, a Caltrans representative, said that engineers found a small separation in the expansion joints of the truck overpass. She said the joints were intended to absorb earthquakes and swaying roads. Dobbins said engineers had also inspected a reported problem at the eastbound 91 Freeway west of the 241 Freeway, but found no damage. "Our engineers are out there checking all of our freeways, and at this point we have nothing to report as far as damage," Dobbins said. The biggest initial concern was in San Bernardino County, which is particularly susceptible to damage because its high water table makes the ground unstable during shaking. But there were no reports of injury to people or of structural damage, said Jodi Miller, a spokeswoman for the San Bernardino County Sheriff's Department. Phones in the Chino Hills Sheriff's Station have been working intermittently, but the emergency dispatch center has had no disruptions, she added.

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