Newly developed computer models applied for the first time to the Puente Hills fault beneath downtown Los Angeles suggest a 7.5 magnitude quake could cause as much as a quarter of a trillion dollars in damage and kill as many as 18,000 people.
Scientists have known for the last two years that the fault is the major quake threat to urban Los Angeles, but the new projections released Wednesday provide the first rough picture of the potential loss of life and property.
The computer models also projected that up 268,000 people could be injured and as many as 735,000 families forced from their homes, according to the computer analysis.
Scientists said that such a quake might not occur for 3,000 years.
The projections are based on two computer models -- one developed at the Southern California Earthquake Center that estimates the shaking associated with quakes of various sizes, and a second developed by the Federal Emergency Management Agency that translates the shaking into economic and human damage.
The modeling was done to give structural engineers and emergency planners a better picture of the possible effects of a Puente Hills quake.
There is a great deal of uncertainty in the projections. Damage could be as low as $82 billion, with 3,000 deaths.
If the quake occurred at night when most people are home, the number of deaths would be between 1,000 and 6,000, according to the analysis.
The worst-case scenario assumes a quake occurring on a weekday afternoon, when the downtown population is at its height.
The vast majority of the damage would be in Los Angeles County, with smaller amounts occurring in northern Orange County and eastern San Bernardino County.
"It's one of the worst disaster scenarios you could imagine for the United States," said geologist Edward Field of the earthquake center. "It would be on a par with Kobe."
The 1995 quake in Kobe, Japan, killed 6,400 people and caused $100 billion in damage.
All this from a fault that scientists didn't even know existed a few years ago.
But Field cautioned: "We can't predict when it will happen, just what its effects will be."
The Puente Hills fault was identified after the 1987 magnitude 5.9 Whittier Narrows quake, which occurred along a small stretch of the fault.
It is about 25 miles long and 15 miles wide, snaking from the northern edge of Orange County to Beverly Hills at a depth of two to four miles below the surface.
