Following a string of big, deadly temblors in Mexico, Turkey and Taiwan, this is the kind of news that gets people's attention here: The U.S. Geological Survey today announced a 70 percent chance that a large earthquake will strike the San Francisco Bay Area sometime in the next three decades.

The "probability report" by the USGS, based on ever more accurate measurements of the stresses and migrations of the region's many--and some newly uncovered--faults, estimates that a magnitude 6.7 earthquake is likely to rattle the Bay Area between today and 2030--meaning, in essence, that a Big One, or another Pretty Big One, is likely to shake Bay Area residents during their lifetimes.

This event would be the same magnitude as the 1994 Northridge earthquake that killed 57 people, left 20,000 homeless and caused $20 billion in damage in Los Angeles.

And of course, the quake could be bigger. Or even worse, it could be located beneath the most densely populated portions of the Bay Area.

Indeed, the new estimates warn that the Bay Area is becoming more vulnerable with each passing year, as population grows and the region becomes more intensely developed. "Since much of this broad region is becoming heavily urbanized and many of these faults run right through urban areas, future earthquakes have the potential to cause much more damage than the 1989 Loma Prieta earthquake," said David Schwartz, the USGS geologist who heads the San Francisco Bay Area Earthquake Hazards Group.

Loma Prieta was a 6.9-magnitude temblor, and even though its epicenter was in the mountains near Santa Cruz, about 50 miles from San Francisco, it still killed 67 people and caused $6 billion in damage as a World Series game began that night in October 1989. Experts say that if a quake of Loma Prieta's intensity would have occurred in downtown San Francisco, the disaster could have been magnified many times over.

While many Californians look upon the possibility and reality of earthquakes with a fatalistic shrug, the big earthquakes that recently have killed thousands and caused billions of dollars in damage, particularly in Turkey and Taiwan, have set nerves a bit on edge--and are motivating state and local governments to continue pushing ahead with retrofitting homes and offices, and making sure everyone has one of those earthquake "survival kits" at home and in their vehicles. Overall, California officials say the state is far better buttressed against big quakes than other regions of the world. The building codes are strict, and getting stricter, and they are mostly enforced.

But there also is an evolving understanding among scientists that in addition to the big faults, such as the San Andreas and the Hayward, that many smaller, but potentially dangerous, faults lie directly below the most densely populated centers.

A new fault, for example, was found to be running under downtown Los Angeles. After reviewing previously secret oil company data, a pair of geologists announced in a paper in March the presence of the so-called Puente Hills fault below the City of Angels. It is major and active.

In a recent issue of the Los Angeles Business Journal, the newspaper asked the engineering risk and assessment firm EQE International to run computer models on what might happen if a 6.7 quake was centered underneath downtown during the middle of the day. The firm's conclusions? As many as 5,000 dead, 140,000 injuries, and 450,000 people displaced from their homes.

A team with the Southern California Earthquake Center did a probability study in 1995 for the southern region similar to the one released today for the Bay Area. It concluded that there was an 80 to 90 percent chance of a 7.0 earthquake or greater in the region in the next three decades.

"Together, we're talking about a very likely occurrence of a large earthquake in either northern or southern California over the next few decades," said Tom Henyey, director of the Southern California Earthquake Center.

The experts point out that their ability to estimate probabilities is still an evolving science. The 70 percent figure for today's Bay Area quake probability comes with a fudge factor of plus or minus 10 percent. Also, the USGS report describes the Bay Area as stretching from Santa Rosa down to Monterey Bay. The new report puts the chance of a large quake slightly greater than a similar study a decade ago, though it increases the size of the area studied and makes the cutoff for a large quake a 6.7 magnitude vs. a 7.0 magnitude from the 1990 study.

But the experts are getting much more information these days. They are employing Global Positioning Satellite data to measure movement of large land masses that move only millimeters a year, and they are digging trenches down across the fault lines to measure their stretches and strains.