Living, and Learning, on the Fault Line : Earthquakes: The Bay Area largely survived a 7.1 shaker. That tells us little about its ability to get through an 8.3.

Every now and then, like an irritated school teacher pushed too far, nature raises its voice and forces us to listen.

Last month, Hurricane Hugo captured the undivided attention of South Carolinians, illustrating the folly of building homes on hurricane-prone beaches. Last week it was the force and effect of powerful seismic waves marching north like a destructive army from ravaged Santa Cruz to San Francisco Bay that captured the undivided attention and sympathy of a nation. But while the sound and fury of the event have filled our newcasts and newspapers, the profound lessons cloaked within have been quieter and less obvious--and in danger of going unheard.

Yes, the Bay Area survived. But what did it survive? Certainly not a great quake or momentous seismic upheaval, In fact, the magnitude of the quake that caused all the damage from Santa Cruz to the Nimitz Freeway in Oakland was somewhere between one-tenth to one-sixtieth of the so-called Big One that still lurks in the locked section of the San Andreas Fault in the vicinity of Daly City. Last week’s quake has been rated at a magnitude of 7.1. The 1906 quake, by comparison--the previous Big One for San Francisco--was an estimated 8.3 magnitude upheaval that unleashed more than 16 times the shaking and 100 times the energy.

The two events can be compared in roughly the same proportion as a 13-pound baby compares to a muscular 200-pound man. When you look at the immense damage and loss of life that occurred at the hands of that 13-pound baby, you begin to see the scope and the breadth of the lesson: The fact that the Bay Area largely survived a magnitude 7.1 earthquake originating far to the south proves absolutely nothing about its vulnerability in the face of an 8.3 magnitude “great” earthquake.

Life goes on, of course, aided by the human tendency to put losses behind us as quickly as possible. In the case of an earthquake disaster such as this, we bury our dead and we go on living, trying not to dwell on the recent past. That’s normal. That’s human. But if we indulge in that tendency in this case, it will also be deadly.

Certainly the people of the San Francisco area were much more aware of and prepared for earthquakes than residents of other U.S. cities (with the exception of Los Angeles), and there is no doubt that Bay Area government leaders have shown more political courage than most in trying to get rid of seismic-bomb buildings (those brittle masonry structures guaranteed to fall down in major earthquakes). But the events of Oct. 17 have demonstrated all too clearly how flawed and incomplete those preparation efforts have been. After all, there are highways and bridges and concrete structures laced through the life of the Bay Area--the very sinew of the civilization. If a 7.1 quake can flatten just one such structure, then the possibility of an 8.3-magnitude monster finding a wide variety of additional “surprise” weakness at the expense of thousands of lives is high.

Consider Candlestick Park, full of World Series fans. Was it the strength of the stadium or the weakness of the quake that saved thousands of lives from the nightmare of structural collapse? Until we ask the right questions and do the proper research, no one can say with assurance.

We have failed to go back and apply the latest knowledge about earthquake shaking and seismic engineering to older, existing structures. The fact that the Nimitz Freeway’s concrete double-decked structure met existing standards and codes when it was built more than 30 years ago may provide a nice legal defense, but that legality protected no one when the quake arrived. The accident investigation into the freeway collapse may show that when measured against today’s standards and codes, the structure was clearly unsafe. That raises a disturbing, haunting question for federal, state, and local leaders: What other structures are out there and in use that are clearly unsafe by today’s measures? And what if we crank in the very real possibility of a magnitude 8.3? If Transportation Secretary Samuel K. Skinner’s recent comments on the rickety state of American highway bridges is any measure, the question extends not just to California, but to each of the 39 states subject to large earthquakes.

One-hundred-thousand feet below San Francisco, there are no snags holding the two great tectonic plates together. The rocks are hot and plastic there, and the Pacific Plate on the west slides steadily northwestward against the North American Plate on the east, moving along a boundary line that cuts the San Francisco suburb of Daly City roughly in half. That’s what the San Andreas does: move, inexorably, day by day and hour by hour miles below, each centimeter of slippage building more pressure on the two plates where they’re locked together and temporarily immovable within 10 miles of the surface.

In Daly City there has been no appreciable slippage on the surface along the fault since 1906. But that can’t continue forever. Eventually the snag will break and the western side will rumble northwestward to balance the accounts and make up what seismologists refer to as a “slip-deficit.”

When that happens, as eventually it must, San Francisco will have its Big One, and the Bay Area earthquake of 1989 will seem very minor indeed. Whether the area will be truly ready depends on learning the lessons and taking the appropriate actions now.