In the aftermath of the earthquakes that shook Mexico City last week, California is already beginning to learn many lessons that will benefit this earthquake-prone state. Important issues such as earthquake preparedness, new rescue technologies, and construction standards are again being reviewed. But a most important new lesson from Mexico City is this: When the earth starts to shake, the soil underneath a building is as important as the material it is built with.
Earthquake readiness in California has centered strongly on engineering safe buildings. In fact, within days of the Mexican quakes, the Los Angeles City Council was weighing a tough new ordinance for strengthening or weeding out quake-prone buildings.
Reports coming out of Mexico City have held that many of the buildings that collapsed were not constructed according to safety standards.
But many of the buildings that collapsed in Mexico City were not of the weak or flawed construction that engineers would have suspected, according to a University of California, Berkeley, engineer who just returned from Mexico City. Instead, the jelly-bowl geology of the city's ancient lake bed brought down tall structures made of concrete and steel frames alike.
To avoid an experience such as Mexico City's, Californians need to pay more attention to mapping the kind of soil structures that rendered the Mexican quake so destructive, says the engineer, Prof. Vitelmo Bertero.
``We have these kinds of soils in areas in California,'' he says, soils that amplify seismic shock waves.
In Mexico last week, the shock waves of an earthquake centered more than 200 miles west set a fairly small portion of central Mexico City jiggling like jelly in even, one- to two-second rolls. Buildings from six to 15 stories high ``tuned up'' to the waves and swayed violently. Some 200 collapsed and from 200 to 300 are damaged beyond repair.
Some of them, Dr. Bertero points out, were well-built steel-frame structures, considered the strongest type for earthquake stress.
Most of the buildings that collapsed, however, were reinforced concrete frame, according to Paul Jennings, an engineering professor at California Institute of Technology. These structures vary in quality, but generally the concrete-frame buildings in California are better built than those in Mexico, he says.
In California, the primary threat to life in a major earthquake is masonry buildings without reinforcement, according to L. Thomas Tobin, director of the state's Seismic Safety Commission. As a rule, they date from before the 1933 Long Beach quake, which put earthquake standards into California building codes.
Some 50,000 to 60,000 of these buildings still stand in California, mostly in older sections of large cities, says Mr. Tobin. To reinforce them properly would cost between $10 billion and $15 billion.
But some cities are aggressively pursuing reinforcement or condemnation of these dangerous buildings. Santa Rosa, north of San Francisco, has used redevelopment funds to replace or reinforce its old brick buildings after a 1969 quake wrecked 100 of them. Santa Ana, south of Los Angeles, is using the same approach.
Los Angeles city officials found 8,000 such buildings in 1981, when an ordinance was passed to bring them gradually up to standard. Since then, 255 of the worst have been repaired, and 651 others have had wall anchors installed as a stopgap safety measure.
The Mexican disaster, however, has spurred the L.A. City Council to consider a one-year deadline for finishing the rest of these buildings.
Directly east of Los Angeles, the San Andreas fault is locked up, and according to its geologic schedule of slipping and jerking, is ready to break loose and spring forward by some 10 yards. This is likely to produce a ``great'' earthquake -- one with magnitude 8 or greater on the Richter scale. (The first Mexican quake measured 7.8.)
There is now a 2 to 5 percent possibility of this temblor coming in any given year.
Southern California is riddled with smaller, less predictable faults, and some of them -- because they run through population centers -- could be more dangerous than a quake on the San Andreas.
So scientists and officials here are eager to learn from the Mexican quakes. Early this week, Caltech earthquake engineers in Pasadena were already studying accelerograph reports from Mexican seismologists.
A study by the US Federal Emergency Management Agency in 1981 determined that an 8.3-magnitude temblor on the San Andreas fault near either San Francisco or Los Angeles would be a disaster of unprecedented proportions. ``Indeed,'' the report said, ``the United States has not suffered any disaster of this magnitude on its own territory since the Civil War.'' The preventive work cities do now will determine the extent of damage after a quake.
The picture has improved considerably since then, according to federal officials, especially in planning government response to a major quake and in new rescue technologies.