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.
The fault lies at one of the convergence areas where the Pacific tectonic plate is sliding under the North American plate.
Geologist Donald Argus of UCLA reported last week that the two plates are converging at a rate of about 0.2 inches every year, squeezing the Puente Hills fault like the two halves of a massively powerful vise.
As the two plates continue to scrape against each other, "The Puente Hills fault is part of what gives," Field said.
Last year, geologist James Dolan of USC reported that four major earthquakes with magnitudes between 7.2 and 7.5 have occurred on the fault over the last 11,000 years. His team has not been able to determine when the last one occurred.
"It was before the historic era and less than 3,000 years ago," he said Wednesday.
"But earthquakes don't occur at regular intervals like clockwork," he added, so that we don't know when the next one will occur.
A magnitude 7.5 quake on the fault would unleash about 15 times as much energy as the magnitude 6.7 Northridge quake of 1994, which killed 57 people and produced $40 billion in damage.
Northridge was the largest earthquake in the Los Angeles Basin in historic times.
Soil conditions for that quake, however, tended to shunt most of the energy northward, away from the city.
Energy from the Puente Hills fault, in contrast, would be focused toward the urban core of the city.
Although the fault runs under the downtown area, the high-rises there are probably among the safest locations in the area, said structural engineer Farzad Naeim, editor of the journal Earthquake Spectra, which published the report.
"The engineering of tall buildings is an order of magnitude more sophisticated than that of smaller structures," he said. "They are among the safest buildings we have."
Most of the damage, in fact, would occur among concrete block and brick buildings for commercial and industrial uses, particularly those that were built before 1975, when building codes were substantially upgraded to protect against earthquakes.
Buildings that have been retrofitted since 1975 have been made substantially safer, but they are still only about 75% as strong as those built after 1975, said seismologist Lucy Jones, the scientist in charge of the U.S. Geological Survey's Southern California office.