Bay Area Quake Estimate Raised : Disasters: Federal experts now put the likelihood of a magnitude 7 temblor within 30 years at 67%.

The U.S. Geological Survey on Thursday sharply increased its estimate of the chance that a major life-threatening earthquake could occur during the next 30 years in the San Francisco Bay Area.

In a study spawned by last October’s 7.1-magnitude earthquake, the U.S. Geological Survey said there is a 67% probability that a temblor of magnitude 7 or greater will occur in the San Francisco area within the next 30 years.

Put another way, a quake the size of the one last October is “twice as likely to occur as to not occur” in the Bay Area between now and the year 2020, said Peter Ward, a geophysicist for the survey.

Previously, earthquake experts thought that a quake as large as the Oct. 17, 1989, Loma Prieta shaker had a 50-50 chance of occurring over the next 30 years.

William Ellsworth, a USGS geophysicist who worked on the study, emphasized that even the new estimate is “conservative,” and suggested the probability of a major quake is even greater than 67%.

He also repeated geologists’ belief that the Bay Area has entered a renewed era of heavy seismic activity. The Loma Prieta earthquake was the first major quake (7 or greater) since 1911 in the San Francisco area. Such temblors occurred at a rate of one every 10 years during the 19th Century.

Ward said a “manpower” shortage has prevented the U.S. Geological Survey from performing a similar analysis in the seismically active areas of Southern California.

But the report pointed out that given the same type of detailed analysis, the probability of a major quake in, for example, San Bernardino, most likely would increase as well.

Geologists revised their estimate of the quake threat in the Bay Area after concluding that the land masses on each side of two major faults are moving faster than previously thought. One of the faults is the Hayward, which runs under East Bay cities. The other fault is the section of the San Andreas that runs up the San Francisco Peninsula.

Another reason for the revision is that geologists took a new look at the Rodgers Creek Fault, which runs from San Pablo Bay to Santa Rosa, and concluded that it has generated earthquakes far stronger than previously thought.

A final factor is the belief that last year’s quake on the section of the San Andreas Fault in the Santa Cruz Mountains area known as Loma Prieta increased the stress on the San Andreas closer to the Bay Area population center.

Earthquake experts renewed their plea that more money be spent on earthquake safety. USGS officials said they plan to distribute more than 2 million copies of a magazine next month in the Bay Area detailing how people can prepare for the next quake.

“What we do in the next several months and years is going to change the outcome of the next event,” Richard K. Eisner, of the state Office of Emergency Services, said at the news conference.

“If we ignore the threat, if we don’t take it seriously and we don’t commit the resources to strengthening the older buildings, we’re essentially saying, ‘Yes, people will die unnecessarily.’ ”

Without major preparation, the geologists believe a quake on the Hayward, Rodgers or peninsula section of the San Andreas could result in far more destruction than was wrought by the Loma Prieta quake. The Loma Prieta quake hit in a relatively isolated spot 60 miles south of San Francisco and Oakland, yet resulted in 62 deaths and $6 billion in damage.

The new analysis contained no new estimate of the loss of life from a slip along one of the major faults. But a 1987 study by the state estimated that 1,500 to 4,500 people would die should a quake of magnitude 7.5 strike on the Hayward Fault.

Ward said an earthquake of 7 magnitude on the Hayward Fault would result in shaking 12 times stronger in Oakland than was caused by the Loma Prieta quake.

Oakland, where several downtown buildings remain unsafe because of quake damage, was the site of the greatest loss of life in the Oct. 17 quake. The quake caused a section of the Nimitz Freeway to collapse, killing 41 people.

A quake on the San Francisco Peninsula section of the San Andreas would result in shaking at least five times stronger in the city than resulted from the Loma Prieta quake. The effects on areas such as the Marina District, where unstable ground experienced liquefaction, would be even worse.

In addition to estimating that there is a 67% probability of a magnitude 7 or greater quake in the next 30 years, the analysis noted that there is a 50% chance of such a quake during the next 20 years, and a 33% chance of one in the next 10 years.

The new analysis is a revision to an estimate issued in 1988. Then, the USGS estimated there was a 50% chance that a major quake would hit within 30 years, a 30% chance of one in 20 years, and a 20% of one within 10 years.