The 5.2 magnitude earthquake that hit the desert southeast of Los Angeles on Friday was centered along the San Jacinto fault, which is one of the region's most active and potentially dangerous fault lines.
The fault runs through populated areas such as San Bernardino, Colton, Moreno Valley, Redlands, Loma Linda, Hemet and San Jacinto, as well as near Riverside, Rialto and Fontana. The epicenter of Friday's quake was in a more isolated area near Borrego Springs in eastern San Diego County.
The quake, which struck at 1:04 a.m., triggered more than 450 aftershocks but caused no major damage.
Experts have been warning for some time, however, that the San Jacinto fault — while less well known than the San Andreas — poses a major threat to the region.
"Because the San Jacinto fault cuts into the middle of the Inland Empire — instead of the edge of the desert — it cuts through a lot more people," Julian Lozos, a Cal State Northridge professor of geophysics, told The Times in March. "There's just more people directly living on this fault."
One big concern is San Bernardino — a city of 215,000 people that fell into bankruptcy in 2012 and suffers from one of the highest poverty rates among the nation's largest urban areas. San Bernardino has one of the largest concentrations of earthquake-vulnerable brick buildings in a location with a high risk of intense shaking.
The San Jacinto fault is also remarkably long, spanning 130 miles, which may explain why Friday's temblor reportedly was felt from Santa Barbara to the Mexican border, said Egill Hauksson, a research professor of geophysics at Caltech.
A Downey woman told KNX-AM (1070) that the quake struck with a "soft roll," then later "hit again stronger." A caller from Costa Mesa described a "back and forth" sensation.
Ron and Teri Walker were inside their hotel room in Palm Springs when the room began to shake.
"We held each other, not sure what the next step was," Teri Walker told KNBC-TV Channel 4.
The San Jacinto fault is characterized by less compression between its plates compared with the San Andreas fault, which means when slippage occurs, the ensuing quake is less severe, Hauksson said.
But the fault, which runs from the Cajon Pass in San Bernardino County southeast toward the Mexican border, is one of the region's most active. The 1987 Superstition Hills earthquakes, which hit about 90 miles east of San Diego, topped out at magnitudes 6.5 and 6.7, and caused $3 million in damage in Imperial County.
"Most of the time when you have an earthquake in Southern California, it's on the San Jacinto fault," Hauksson said.
In an unnerving scenario, both the San Jacinto and San Andreas faults could rupture together in a 7.5-magnitude earthquake, according to one study.
The study, which Lozos wrote as a researcher at Stanford University and at the U.S. Geological Survey, looked at whether a massive 1812 quake in Southern California was the result of shaking on both fault lines.
12:38 p.m.: This article was updated with more details about the fault and San Bernardino's vulnerabilities.