IN the year since the earth moved here, California has reinforced its position as the nation's Eagle Scout when it comes to earthquake preparedness: It has the toughest building codes and probably the most detailed emergency plans of any state. Just about everyone here now carries a flashlight and portable radio when moving about, protections against a blackout such as occurred when the 7.1 temblor struck last Oct. 17. A special newspaper supplement on preparedness put out by the United States Geological Survey (USGS) has become a virtual best seller.
Despite the vigilance, however, experts say California and the San Francisco Bay Area are still not ready for the ``Big One'' that scientists are increasingly confident is going to strike somewhere in the fault-veined state within the next 30 years.
``Our capacity to deal with another major earthquake is vastly improved,'' says San Francisco Mayor Art Agnos. ``But we have no illusions about the future. We know the clock is ticking.''
In a study released in July, a panel of earthquake experts predicted that there is now a 67 percent chance that a quake of magnitude 7 or greater will strike the Bay Area by the year 2020, up from a prediction of 50 percent two years ago. The Bay Area and southern California - now on about the same odds - are the two populated areas facing the greatest threat of a major quake.
While some scientists have disputed the findings, those involved with the study of four Bay Area faults believe there is no doubt that the earth here will one day do the rumba again.
``Is the 67 percent very uncertain? Yes,'' says Allan Lindh, a seismic expert with the USGS in Menlo Park, Calif. ``Is it likely the probability is much lower? No. The weight of the evidence suggests 67 percent is too low.''
TO gird for another quake, the state is putting in a satellite communications system to be used by state and local governments and emergency response teams.
It also is training ``strike'' teams around the state to help with search-and-rescue operations. And lawmakers passed landmark legislation this session mandating earthquake insurance for California homeowners.
Readiness has become a watchword at the local level, too. San Francisco is organizing a series of neighborhood response teams. The city has beefed up its firefighting capacity, installed an emergency communications system, and begun work on a crisis command center set to open in 1991.
But money for such efforts is tight. At a press conference last week, San Francisco Mayor Agnos acknowledged that fiscal woes at all levels of government are having an impact - but he feistily defended the progress the city has made on preparedness.
The Loma Prieta quake also underscored the vulnerability of structures, particularly unreinforced masonry and old concrete buildings. Officials estimate there are 25,000 unreinforced masonry buildings in California.
While some progress is being made in retrofitting them, Fred Turner, an engineer with the state Seismic Safety Commission, says not enough is being done.
More work needs to be done as well on shoring up bridges and roadways. Even before last October's quake, the state had identified 25,000 bridges and highway spans that needed upgrading.
This will be costly. A recent engineering study says needed reinforcements to the approaches and span of the Golden Gate bridge will cost $100 million.
``If the earthquake comes in 10 years, I think we will have enough time to change the outcome,'' says Richard Eisner, director of the Bay Area Regional Earthquake Preparedness Project. ``If it comes in two years, it will be a devastating event.''
WHILE engineers probe the tensile strength of civilization above ground, seismologists are trying to fathom what is going on beneath the surface. The Loma Prieta quake provided both some new clues and maddening mysteries in the quest for the seismological Holy Grail: better earthquake prediction.
The quake showed that scientists are getting better at determining long-term probabilities, over 20 to 30 years. It occurred on a segment of the infamous San Andreas fault recognized as having the greatest chance of any fault zone in the area of producing a 6.5 to 7 magnitude quake.
``It confirmed that the approach we are using to long-term forecasting in California is correct,'' says the USGS's Dr. Lindh.
Short-term prediction remains problematic. There were no immediate seismic or strain precurors to warn of the Loma Prieta quake. Lindh thinks there is a possibility that two magnitude 5 quakes that occurred in the area within 16 months of Loma Prieta may have been foreshocks - and thus advance warnings.
Tantalizing but a bit bizarre is a change in low-frequency radio waves recorded by a Stanford University electrical engineer just before the quake. If it was caused by changes in the earth, it could revolutionize earthquake prediction. But it may have been simply a malfunction of his instruments. Scientists don't know.
Some unusual characteristics displayed by the quake mean the San Andreas - though probably the most studied fault on earth - remains an enigma.
``It has made people rethink the complexity of the San Andreas fault,'' says Bruce Bolt, professor emeritus at the University of California Berkeley. Second of two articles. The first appeared Oct. 16.