The first of a swarm of earthquakes hit the border of La Habra and Brea shortly after 8 p.m. with the 3.6 temblor. About an hour later, at 9:09 p.m., the 5.1 shock hit, followed by at least two more aftershocks in the magnitude-3 range in the next half hour. At least 20 aftershocks had been recorded by late Friday.
U.S. Geological Survey seismologist Lucy Jones said the 5.1 quake has a 5% chance of being a foreshock of a larger temblor.
"There could be even a larger earthquake in the next few hours or the next few days," Jones said during a media briefing at Caltech.
Experts said that based on preliminary data, the series of earthquakes appeared to have occurred on the Puente Hills thrust fault, which stretches from the San Gabriel Valley to downtown Los Angeles. The fault also caused the 1987 Whittier Narrows earthquake.
Friday night's 5.1 quake was relatively shallow, which "means the shaking is very concentrated in a small area," said Caltech seismologist Egill Hauksson. Hauksson said the quake sequence was unusual in that the 5.1 quake was preceded by the weaker foreshock.
Scientists are particularly concerned about the Puente Hills thrust fault because it goes directly underneath downtown Los Angeles. "This is the fault that could eat L.A.," seismologist Sue Hough told The Times in 2003.
On Friday, though, police and fire departments in Los Angeles reported no damage. "Tonight's earthquake is the second in two weeks, and reminds us to be prepared," Los Angeles Mayor Eric Garcetti said in a statement.
Although the shaking was felt throughout the region, it didn't rattle the professionalism at Walt Disney Concert Hall. Microphones above the stage swung back and forth and the hall shook, but the orchestra continued playing.
"The L.A. Philharmonic should get combat pay," said audience member Michael Healy of Studio City.