U.S. Raises Odds of a Major Quake in Southland to 60% Within 30 Yrs.

There is a 60% likelihood that a great or major earthquake will bring death and destruction to Southern California within 30 years, while the San Francisco Bay Area faces 50-50 odds of a deadly major temblor by the year 2018, the U.S. Geological Survey said today.

In each region, chances are one in five that a disastrous quake will hit within the next decade, said the government’s most definitive report yet on the odds California will be ravaged by great quakes with a magnitude of 8 or more and major quakes registering 7 to 8.

A previous federal report said such quakes could kill thousands of people, injure tens of thousands and cause multibillion-dollar property damage. A 1983 state emergency plan said a great quake near metropolitan Los Angeles would be America’s worst disaster since the Civil War.

Estimate May Be Low

The 12 scientists who wrote the new report said it deals only with the San Andreas, Hayward, San Jacinto and Imperial faults, but not other major faults such as the Newport-Inglewood in Los Angeles, so the likelihood of large earthquakes “may significantly exceed the probabilities we present.”

Many scientists have said previously that Southern California faces a 50% chance of a great quake in 30 to 50 years, so the new report boosts those odds somewhat. The report’s probability estimate for major quakes in the San Francisco area is apparently the first such official forecast for the region as a whole.

Requested by U.S.

The researchers prepared the report at the request of the USGS’s National Earthquake Prediction Evaluation Council. The cumulative risks they list for disastrous quakes in each region are based on the most likely estimates for big quakes on distinct segments of the four faults.

The USGS report said:

--There is a 60% probability within 30 years and a 20% chance within 10 years that a quake with a magnitude of 7.5 to 8 will rupture two or three segments of the San Andreas Fault in Southern California. The odds of a 7.5-magnitude quake may be as high as 70% within 30 years if only one segment breaks.

--There is a 50% chance in 30 years, and a 20% chance within the next decade, that a quake of magnitude 7.0 will rupture the northern San Andreas or Hayward faults, which border the west and east sides of San Francisco Bay.

--A magnitude 6.5 to 7.0 quake is 50% likely by the year 2018 and 20% likely by 1998 on the San Jacinto Fault, the north end of which is close to the fast-growing San Bernardino-Riverside area.

--A quake measuring 6.5 is 50% likely by 2018 on the Imperial Fault, which has produced two to five 6-magnitude quakes this century and runs through agricultural areas spanning the U.S.-Mexico border.

The new report doesn’t address possible casualties and damage. But a 1980 report by the Federal Emergency Management Agency said a magnitude-8.3 quake in Southern California could kill 3,000 to 14,000 people, hospitalize 12,000 to 55,000, and cause $17 billion in damage.