Magnitude Can Be Misleading

The Mexico City quake registered 8.1 on the Richter scale; anything above 6 is considered serious, with each one-point rise meaning a 10-fold increase in strength. "But magnitude alone can be misleading," Bielak says. Seismologists measure the number 100 kilometers from the earthquake's epicenter, assuming the seismic waves from the quake travel only through rock and nothing else gets in their way. "So for design purposes, magnitude doesn't tell the full story," Bielak says. "Mexico City was an eye opener from this point of view."

Since the 1950s, the building code in Mexico City recognized that the city had three distinct types of soil. Downtown is built primarily on a lakebed basin containing soft clays, surrounded by dense sands and rock, and engineers used this geological information to design buildings. Soil in each region vibrates at a different characteristic frequency when seismic waves disturb it, and the same is true with structures. So one important design criterion is making sure a building's vibrational frequency doesn't harmonize too closely with that of the soil it's built on -- usually accomplished by stiffening the structure's steel or concrete frame. Otherwise, the building and ground may vibrate in unison -- called "double resonance" -- and eventually the structure sways violently. This phenomenon occurs on park swings every day when you pump your legs in time with the swing's oscillations to go higher and higher.

Although designers had addressed resonance concerns in Mexico City's three soil regions, they weren't precise enough, and three unexpected events occurred. First, the seismic waves resonated with the basin's soft clay, meaning the earthquake's strongest part lasted 40 seconds, instead of the normal five to 10 seconds, and the entire quake lasted about 200 seconds. As a result, there was ample time for the basin's clay to begin quivering like a bowl of jello being swished back and forth. This phenomenon, which wouldn't have been nearly as pronounced during a shorter quake, created an unequal distribution of ground motion and unfortunately, some sections of the basin resonated with structures above, causing 99 percent of the quake's overall damage.

"So the questions came up, 'Do we know enough about ground motion? Do we need to know more? Should we subdivide the cities into smaller regions if a certain part is going to respond much more strongly than another one?" Bielak says. "The work that we are performing here is geared to answer those questions."

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