Damage in Mexico: a double quake.

Damage in Mexico: A double quake

The big question nagging seismologists and engineers since the Sept. 19 earthquake devastated sections of Mexico City and killed almost 10,000 people (SN:9/28/85, p. 196) has been why this quake caused so much damage. One factor is that the ground-shaking was amplified -- scientists now say by about five times -- because Mexico City lies on an old lake bed that resonates with the seismic waves. The thickness of the bed is such that the seismic waves that are the most amplified are the low-frequency signals, which can do the most damage to taller buildings. Other notable quakes may not have been as devastating, scientists say, because there were not as many a tall buildings in the past.

But recently, seismologists have come to think that the deadliness of the Sept 19 quake was also due in part to its long duration. In fact, the mainshock of the magnitude 8.1 quake was really a double event, consisting of two 16-second tremors spaced about 26 seconds apart. The rupture started in the northwest part of the Michoacan gap -- a segment of the Mexican subduction zone (where the Cocos oceanic plate is plunging beneath the North American plate) that had been tagged as a likely place for an earthquake. The second tremor was triggered by the first, 90 kilometers to the southeast. According to James Beck at Caltech in Pasadena, Calif., the unusually long period of shaking allowed more time for the lake bed to amplify ground motion and made some buildings more flexible so that they resonated more easily with the low-frequency seismic waves. Beck notes that the building codes in Mexico City had taken into account the effect of low-frequency waves, but Jorge Prince of the National Autonomous University of Mexico adds that none of these conds had specified the number of cycles of shaking that a building should withstand.

Seismologists say the Sept. 19 earthquake is one of the best-documented quakes in history. "This is the first time we can make a good comparison between the source region of such a big earthquake and the energy radiated out to large distances," observes James Brune at Caltech. With strong motion instruments, Brune and his co-workers found that ground motion near the fault was actually quite low, about 15 percent of the acceleration due to gravity (g). In comparison, the ground motion near California faults can reach values six to seven times that. Brune says the acceleration, which had dropped to about 4 percent g outside of Mexico City, was boosted to about 20 percent g inside the city. While the scientists had known that the lake bed would resonate, they had little idea of what the amplification would be, he adds.

Because the Mexico earthquake had little or no precursory signals, Burne thinks seismologists should concentrate on ground-motion monitoring so that engineers designing buildings know what to expect when an earthquake does come. And now that the Michoacan gap has ruptured, all eyes are turned to the Guerrero gap to the south, which has been ominously quiet since 1911.