USGS seismologist Lucy Jones says the Puente Hills thrust fault runs from the edge of Orange County all the way to downtown L.A., before coming to an end in Hollywood.
"It's potentially one of our most devastating earthquakes that we can imagine," said Jones. "It's a fault very much like the one that produced the Northridge earthquake but located down under downtown Los Angeles instead of up in the hills of the northern San Fernando Valley."
The Puente Hills fault was responsible for Friday's 5.1 magnitude earthquake in La Habra, but unlike other faults that run vertically, the Puente Hills fault runs horizontally, which, coupled with its location, could make for a far more destructive quake than the so-called "big one."
"This fault actually produces a worse earthquake than the San Andreas can. It won't be quite as big magnitude, but because we have so many people on top of it...it will be worse," said Jones.
Few people we spoke to had ever heard of the Puente Hills thrust fault, but a lot of people became well acquainted with its potential on Friday night. Andrew Rivera recently moved from New York to downtown Los Angeles. He says Friday's quake has him rethinking his choice.
"I thought everything was concrete and solid, but everything moved here. I'm having second thoughts," said Rivera.
Aftershocks continue to shake the ground, with the latest being a 2.7-magnitude that struck at 11:17 a.m. Monday. About 150 aftershocks rumbled the area over the weekend.
The cleanup continues in cities across Orange County, including Fullerton and Brea, following the earthquake that knocked items off store shelves, cracked buildings and even crumbled walls.
The Fullerton Fire Department said seven homes have been red-tagged due to the damage. In Brea, one residential building and one commercial building were red-tagged.
The Brea City Hall remained closed until about 1 p.m. to give employees time to clean up the damage, and classes at Fanning Elementary School in Brea were canceled Monday to give officials more time to assess the safety of the campus.
"We had a hazmat team in here over the weekend and it was determined that there was some asbestos found in some of the dust that had settled after the earthquake, so we're still awaiting more details," said Fanning Elementary Principal Susan Metcalf.
The school principal says some students will return Tuesday to parts of the school deemed safe. But 75 percent of students will have to relocate to another school while they await additional air quality tests and cleanup.
Carbon Canyon Road remains closed on Monday in the Chino Hills area. A landslide knocked over a car and blocked the road shortly after the earthquake.
Caltrans crews worked to reinforce the hillside before the road can be reopened. They removed any sand, debris and rocks that might be loose and cause a slide, and they also tried to put up a mesh blanket.
Police escorted school buses through the closure, but all other traffic was forced to find an alternate route.
Carbon Canyon Road is expected to reopen around 4 a.m. Tuesday, but there will be another temporary closure on Friday or Saturday to reinforce the hillside.
Meantime, the U.S. Geological Survey is warning residents about a fake letter circulating online about an impending earthquake in Southern California. Officials say that it is a hoax.
The letter has a USGS logo and claims to be from the agency. Jones said the agency is aware of the letter and that it is not from them. She says they do not predict earthquakes.