The odds of a catastrophic earthquake that could kill thousands of people in Southern California are higher than once believed, and the San Francisco Bay area faces a similar threat, a federal report says.

Recent concern about the so-called "big one" on the San Andreas Fault has centered on the greater Los Angeles-San Bernardino area because so much strain on the fault's north end was released by San Francisco's great quake of 1906, which measured an estimated 8.3 on the Richter scale.But a report issued this week by the U.S. Geological Survey and a working group of its National Earthquake Prediction Evaluation Council said the San Francisco Bay area is nearly as likely to suffer a major quake.

"Unless we aggressively pursue earthquake reduction measures, there is the potential for many people to die, perhaps more people than ever died in a single day from any (natural) disaster in the United States," said Richard Andrews, deputy director of California's Office of Emergency Services.

A 1983 state emergency plan said a great quake near metropolitan Los Angeles would be America's worst disaster since the Civil War.

Two separate studies, one published Friday in the journal Science, say that quakes along the San Andreas in the Mojave region near Los Angeles tend historically to occur in bursts of two or three within a relatively short time, followed by 200- to 300-year periods without large temblors.

By demonstrating such clustering, both studies suggest the Mojave segment may be in a quiet period, with another major quake unlikely for 70 years or more.

The new Geological Survey report, however, said there is a 50 percent chance in 30 years and a 20 percent chance within the next decade that a major quake measuring 7.0 on the Richter scale will rupture the northern San Andreas or Hayward faults, which border the west and east sides of San Francisco Bay.

In Southern California, the report said, there is a 60 percent probability within 30 years and a 20 percent chance within 10 years that a great quake measuring 8.0 or a major quake registering 7.5 will rupture two or three segments of the San Andreas Fault.

Many scientists said previously that Southern California faces a 50 percent chance of a great quake in 30 to 50 years.

The odds of a 7.5-magnitude quake in Southern California rise to 70 percent within 30 years if only one of the three segments breaks, but scientists don't expect that kind of fault behavior, the federal report said.

Chances are less than 10 percent in 30 years for an exact repeat of San Francisco's 8.3-magnitude 1906 disaster, which recent studies show killed at least 3,000 people.

A 1980 Federal Emergency Management Agency report said a repeat of the 1906 quake could kill 3,000 to 11,000 people, hospitalize 12,000 to 44,000, and cause $38 billion in damage. It said a 7.4 jolt on the Hayward fault could kill 3,000 to 7,000 people, hospitalize 13,000 to 27,000, and cause $44 billion in damage.

The disaster agency also said an 8.3-magnitude quake in Southern California could kill 3,000 to 14,000 people, hospitalize 12,000 to 55,000, and cause $17 billion in damage.

The Geological Survey report also listed chances of 50 percent in 30 years for quakes measuring 6.5 on the Imperial Fault and 6.5 to 7.0 on the San Jacinto Fault, which runs near the burgeoning San Bernardino-Riverside area.

The Richter scale is a gauge of energy released by an earthquake, as measured by the ground motion recorded on a seismograph. Every increase of one number, say from 5.5 to 6.5 magnitude, means that the ground motion is 10 times greater.

A magnitude of 8 is considered a "great" quake, capable of tremendous damage.