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New insights from Mexico City quake

Teams of US scientists have begun returning from Mexico City, armed with reams of earthquake data that will take months to fully analyze. But they already have uncovered new information on how earthquakes can affect buildings. Indeed, researchers say that Mexico City's unusually violent earthquake will provide invaluable guidance to engineers trying to design quake-resistant buildings and to seismologists who study earthquakes and, eventually, hope to be able to predict where and when they will occur.

They add that the saga of Mexico City is a cautionary tale to localities that have not incorporated seismic safety precautions into their building codes.

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Buildings in western regions considered likely to experience earthquakes often meet stringent quake-resistant construction guidelines. But many structures in other areas considered to be at some risk -- such as the areas around Memphis, St. Louis, and Boston -- would collapse under the strain of tremors those areas have experienced in earlier times.

``In human terms, Mexico City was a disaster of the first magnitude, but in a scientific sense it has been enormously helpful,'' says Dr. Richard N. Wright, director of the Center for Building Technology at the US Commerce Department's National Bureau of Standards (NBS). Dr. Wright testified at Senate hearings last Thursday on the Mexican earthquake and is supervising analysis of some of the data. ``Some structures did survive, while there was extensive damage in other areas. That means the lesson of Me xico City is that you can design [buildings] so that these catastrophes do not happen.''

While there have been many other serious earthquakes in recent decades, a few even rivaling the intensity of the Mexican quake, none have ever been so thoroughly studied.

Only a few instruments were in place to capture the details of the colossal Alaskan earthquake of 1964. Important data from a sizeable quake that hit Caracas, Venezuala, in 1967 was lost because someone forgot to load a strategically placed seismograph with recording paper.

But scientists were prepared for Mexico City, although no one predicted the Sept. 19 earthquake. For example, researchers from California's Earthquake Emergency Research Institute and the Universidad Nacional Aut'onoma de Mexico recently installed, near potential earthquake troublespots, a device called a Guerro accelerograph array, that measures the three-dimensional ground motions of earthquakes. NBS structural engineer Dr. William Stone, a member of a joint US Geological Survey and NBS team sent to M exico City, reports the instrument caught the shifting and shaking of the Mexico earthquake with unparalleled detail.

With such data-collecting capabilities, Mexican and foreign researchers gleaned several unexpected yet significant facts about the earthquake.

For instance, in most quakes, it takes about a half second for the ground to complete one cycle, or period, of shifting back and forth. Some areas of Mexico City experienced tremors with periods of one to two seconds.

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That was what spelled trouble for an area of the downtown populated with many buildings between eight and 20 stories. Buildings, like all structures, have a natural resonant frequency. That frequency for many buildings is determined by dividing the number of stories by 10. If a building's resonant frequency matches the tremor's frequency, that building will vibrate with particular intensity.

In some cases where a building's resonant frequency matched that of the earthquake, the walls tumbled. An adjacent building with a different resonant frequency, however, might emerge relatively unscathed. US experts say Mexican officials are entertaining thoughts of a ban on the construction of buildings between eight and 20 stories in certain parts of Mexico City.

The decision whether to ban will depend partly on the makeup of the ground. The worst damage in Mexico City occured throughout a two-mile swath covering the old lake of Texcoco. The sediments, which may extend to a depth of more than a mile, amplify tremors and increase their periods, thus causing the most damage.

The only way to thoroughly determine the makeup of ground is by taking extensive core samples in a process known as microzonation. Mexican scientists are said to be undertaking this process now. Ultimately, US experts say, the process should be carried out in many unanalyzed areas of this country to highlight regions particularly vulnerable to quake damage.

Data collected from the earthquake also showed that many of Mexico City's seismic building standards, said to be comparable to those found in California cities thought to be of high risk, were simply not strong enough. Even if they had been, however, American inspectors reported suspicions that the codes had sometimes not been followed -- a suspicion shared by some of their Mexican colleagues.