High-rises rock to shuttle shock.

High-rises rock to shuttle shock

When the space shuttle Columbia flew over the Los Angeles area en route to a landing at Edwards Air Force Base on Aug. 13, its sonic boom caused a minor stir among seismologists. Not surprisingly, sensitive seismometers in Pasadena recorded the shuttle's atmospheric shock wave. Yet they also picked up waves of ground vibrations a full 12.5 seconds before the shock wave hit. For two weeks, these early waves puzzled Hiroo Kanamori and his colleagues at the California Institute of Technology and the U.S. Geological Survey in Pasadena. How, they wondered, could shock waves in the air transfer energy into the earth to generate waves of ground motion? One clue: The waves appeared to originate in downtown Los Angeles, 9 miles southwest of Pasadena.

Kanamori and his co-workers now believe they have an answer. According to their scenario, as the atmospheric shock waves passed over the downtown area, they shook its 400-plus high-rise buildings all at the same time. The collective motion of the buildings vibrated sediments filling the Los Angeles basin, sending long-period waves speeding toward Pasadena. The seismometers recorded ground motion before the sonic booms because seismic waves travel faster than sound waves.

Kanamori says such a process would not damage the tall buildings. But it could help engineers understand how buildings may behave when a quake sends seismic waves rippling through the basin, he says. Sedimentary basins can greatly amplify ground motion, causing catastrophic damage during a quake -- a fact dramatically demonstrated during the 1985 Mexico City quake.