The Puente Hills fault, which scientists believe could be responsible for Friday's 5.1 earthquake in La Habra, is considered very dangerous.
Here are some basic questions about the fault.
Q: What would be the difference in shaking between a 5.1 quake and a truly huge quake?
Friday night's earthquake was caused by the underground fault slipping for half a second, said U.S. Geological Survey seismologist Lucy Jones, prompting about 10 seconds of shaking at the surface.
But a 7.5 quake on the Puente Hills fault could cause the fault to slip for 20 seconds — and the shaking could last far longer.
Scientists say that quake would be more destructive than the so-called Big One on the San Andreas fault.
Q: Why are scientists so worried about the fault?
The Puente Hills thrust fault is so dangerous because of its location, running from the suburbs of northern Orange County, through the San Gabriel Valley and under the skyscrapers of downtown Los Angeles before ending in Hollywood.
The Puente Hills fault could be especially hazardous over a larger area because of its shape. Other local faults, like the Newport-Inglewood and Hollywood, are a collection of vertical cracks, with the most intense shaking occurring near where the fault reaches the surface. The Puente Hills is a horizontal fault, with intense shaking likely to be felt over a much larger area, roughly 25 by 15 miles.
Video simulations of a rupture on the Puente Hills fault system show how energy from a quake could erupt and be funneled toward L.A.'s densest neighborhoods, with the strongest waves rippling to the west and south across the Los Angeles Basin.
Q: What is the worst-case scenario?
According to estimates by the USGS and Southern California Earthquake Center, a massive quake on the Puente Hills fault could kill from 3,000 to 18,000 people and cause up to $250 billion in damage. Under this worst-case scenario, people in as many as three-quarters of a million households would be left homeless.
One reason for the dire forecast is that both downtown L.A. and Hollywood are packed with old, vulnerable buildings, including those made of concrete, Jones said.
The violent motion would be amplified by the soft soil underneath the Los Angeles Basin and the valleys, which produces a jello effect as shaking waves wobble off the basin.
By contrast, a magnitude 8 "Big One" on the San Andreas fault — more than 30 miles from downtown L.A., on the other side of the San Gabriel Mountains — would cause up to 1,800 deaths, according to estimates.
Q: How often does a huge quake erupt on the Puente Hills fault?
Scientists believe the Puente Hills fault has a major quake roughly every 2,500 years but don't know when the last one was.
Q: Has there been larger quakes on the fault before?
In 1987, another quake on that fault killed eight people and caused more than $350 million in damage. The magnitude 5.9 Whittier Narrows quake left old brick buildings in Whittier's downtown area battered and also damaged some freeway bridges. More than 100 single-family homes and more than 1,000 apartment units were destroyed.