Resonance makes Earthquakes Rock ‘n’ Roll

Singing in an echo-y room can lead to a beautiful sound, if you hit the precise frequency at which the room itself resonates. But if an earthquake is doing the singing, being in tune with nearby soil and buildings can be disastrous.

As reported in the April 2003 issue of Discover, Mexican seismologist Cinna Lomnitz has a theory to explain why strong earthquakes sometimes do little damage, while weaker ones sometimes bring massive destruction: the natural frequency of some earthquake waves sometimes is the same as the natural resonating frequency of tall buildings.

January saw a powerful (roughly 8.0) quake rock Colima, Mexico, killing two dozen. But a similarly strong 1985 quake in Mexico City killed 10,000, having knocked down 400 high-rise buildings. Why the difference?

Prof. Lomnitz (National University of Mexico) points out that Mexico City is built on an ancient lake bed, whose muddy sediment has a natural "pitch" of one vibration every 2.5 seconds. That's the same natural resonating frequency of many buildings whose height is in the 7- to 18-story range. And unfortunately, that's also the frequency of a certain kind of shallow earthquake wave. Such earthquake waves are already big trouble for buildings in that height range, which would start to sway with increasing violence, but the coincidental effect of the soil would amplify the effect, leading to certain destruction.

Lomnitz points out that Los Angeles and Seattle are built on soils with similar characteristics and are vulnerable to the same phenomenon. He advocates modifying buildings to dampen their resonance -- akin to the shock absorbers used in automobiles.

Read Discover magazine (April 2003 issue not yet online, at the time of this writing)

Read Lomnitz' lecture at The Society for Earthquake &Civil Engineering Dynamics: "The Road to Total Earthquake Safety"