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Pattern of Destruction : New Lessons From Quake in Mexico

September 26, 1986|ROBERT A. JONES | Times Staff Writer

One year after the Mexico City earthquake struck the nation's capital and left its core in ruins, seismic experts are beginning to unravel several troubling mysteries surrounding the event. Ultimately, the scientists say, their studies will reshape their understanding of the way earthquakes inflict their damage on cities.

Erupting from the floor of the Pacific Ocean, the Mexican earthquake surged into the capital with crippling force after traveling 250 miles through several mountain ranges. Nearly a thousand buildings collapsed in the city, and more than 10,000 persons died. Since modern records have been kept, seismic waves had never wreaked so much damage from so great a distance.

In contrast, coastal cities much nearer the earthquake suffered only moderate damage. Lazaro Cardenas, a small city closest to the rupture, lost only 10 buildings.

Pattern of Destruction

Adding to the mystery was the pattern of destruction within the capital. Older brick or stone buildings--widely regarded as the most dangerous structures in an earthquake--withstood the rigors of the shaking while more modern buildings failed. The great majority of buildings that collapsed were constructed of reinforced concrete.

Now, after some of the most extensive study in seismic history, scientists believe that they have found the answers to the puzzles. Those answers, they add, may contain important lessons for cities in the United States and elsewhere that are in seismic zones.

"This was an historic earthquake," said Vitelmo V. Bertero, a UC Berkeley engineering professor who specializes in seismic analysis. "No one expected the intensities of motion that were recorded in Mexico City. No one had designed for it, and that is why so many buildings failed."

Instruments showed that the acceleration of seismic waves hitting buildings in parts of the city were up to five times greater than waves outside the city. The intensity was so strong that one of the instruments designed to measure the force broke under the strain.

Effect of Soft Soil

Mete A. Sozen, an engineering professor at the University of Illinois who visited the disaster area as a member of the National Academy of Sciences assessment team, recalled the collapse of a major hospital where hundreds died. "It was designed to withstand a lateral (sideways) force equal to 6% of its weight, and it probably met those standards," Sozen said. "The forces exerted on it were much greater than that, and it came down. Only some accidental feature that added strength would have saved it."

Instrument recordings have shown that the extreme vibrations were produced by soils beneath the city. For many years scientists have known that Mexico City is on top of soft, elastic sediments that tend to amplify some frequencies of shock waves from earthquakes. What they did not know was just how extreme the magnification would be.

"The statistical data on earthquakes is still very small," Bertero said. "Since the beginning of scientific record-keeping, we have had just a few great earthquakes striking large cities. This one taught us that soft soils can magnify motion to a degree never thought possible."

Seismologists agree that Mexico City's location is more dangerous for earthquakes than any found in the United States. The soft soils under the city are part of an ancient lake bed that was filled thousands of years ago with wet clay deposits. The lake bed is surrounded by mountains of bedrock so that the whole thing, in the words of one engineer, reacts to shaking "like a bowl of Jello."

Scientists have reconstructed this scenario for the disaster:

On Sept. 19, 1985, shock waves from the magnitude 8.1 earthquake began shooting across western Mexico toward the capital. The waves were grouped into two pulses produced by twin ruptures in the Orozco fracture zone off the Mexican coast.

Initially the waves were complex, containing many different frequencies. But over long distances the shorter frequencies were filtered out, leaving only long, smooth waves to strike the lake bed at Mexico City. Waves are measured by their "period," or the length of time that it takes to complete a cycle. These waves had cycles of 1.5 to 2 seconds, very long by earthquake standards.

Within the lake bed, a transformation began. The elastic soil, saturated with water, amplified the motion four to five times its previous level. The waves struck the surface with undulating regularity that continued for the unusually long time of a minute.

Like Battering Rams

In the city, some buildings began to sway in time with the waves. A few pounded against each other, acting like battering rams. Each sway became worse until the buildings tore themselves apart, the floors falling onto one another in a phenomenon known as "pancaking." At last count, 954 buildings collapsed inside the city, 1,100 others eventually will be demolished, and thousands of others suffered damage.

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