Update: Seismologists at Caltech have downgraded the main shock at Devore from magnitude 4.1 to magnitude 3.8.
A magnitude 3.8 earthquake struck the Inland Empire at 8:07 a.m. today, with an epicenter in Cajon Pass near the San Andreas Fault at Devore.
Slight damage was reported by homeowners in Rancho Cucamonga, just west of the epicenter. One viewer told KCAL-TV that he was jolted out of bed.
A 2.0 aftershock hit about a half mile away about two minutes after the biggest shake, at 8:09, according to preliminary reports from automated USGS sensors. A 1.8 shock followed at 8:14.
The epicenter of the largest quake was two miles northwest of Devore, in San Bernardino County. That location is approximately where the San Andreas Fault crosses the San Gabriel and San Bernardino mountain ranges.
and was centered about one mile west of San Juan Capistrano. Experts say the temblor was on a thrust fault under the San Joaquin Hills. Known as the “ it was discovered 13 years ago and runs along the coast from Newport Beach and Costa Mesa to San Juan Capistrano.
USGS seismologist Lucy Jones said the Inland Empire quake was near, but not on, the big San Andreas Fault.
"It's very clearly not the San Andreas Fault," Jones told KCAL. "It was on the juncture of very many major faults."
"It was a solid boom and everything shook," viewer Tarcie Thompson told KCAL. "But it was very sudden, not a rolling action. It didn't roll or anything like any other earthquake, it was very sudden and very hard."
A police dispatcher in Victorville said 10-20 residents called in to ask if there was an earthquake, but none reported damage.
The quake was felt as a small tremor, and muffled thunder-like sound, at the City News Service offices in West Los Angeles.
The California Highway Patrol reported no unusual incidents on Interstate 15 or Interstate 215, which merge in the Cajon Pass near the epicenter.
Los Angeles County fire dispatchers reported nearly all of its on-duty battalion chiefs did not feel the quake, and no damage.
The quake was 49 miles east-northeast of the Los Angeles Civic Center, and 23 miles north-northwest of downtown Riverside.