Shuttle’s Rattle and Roll--Does It Rock the Ground? : Seismology: Sonic booms may have nudged 400 downtown high-rises, moving the earth as far away as Pasadena.

Can the space shuttle cause the earth to “quake” when it glides in for a landing?

Of course not, any seismologist would tell you.

At least, that’s what they would have said before scientists discovered something really strange about the landing of the space shuttle Columbia last Aug. 13: The high-rises of Los Angeles, reverberating from the shuttle’s sonic booms, apparently caused the earth to shake as far away as Pasadena.

Unlike most landings, when the Columbia returned from a secret military mission it passed directly over the Los Angeles Basin on its way to Edwards Air Force Base. As a result, the twin sonic booms that are characteristic of the shuttle’s delta wing configuration were heard by millions of residents.

Seismographs scattered throughout the basin routinely record the shock waves from sonic booms, but they are easily distinguished from earthquakes. But when scientists were looking at the data from an especially sensitive seismometer in Pasadena they discovered something they described as “very odd.”

The seismograph recorded a “long period” pulse 12 seconds before it recorded the sonic boom.

That left the scientists “very puzzled,” said Hiroo Kanamori of Caltech.

A long period pulse is caused by a pressure wave of very long wavelength, and it is characteristic of certain types of earthquakes. After puzzling over the data for some time, scientists came up with an answer that they presented Monday during the fall meeting of the American Geophysical Union.

“We could tell that the pulse came from the southwest at a distance of approximately nine miles,” Kanamori said. “We looked at a map and realized that downtown Los Angeles is approximately nine miles southwest of Pasadena.”

U.S. Geological Survey seismologists Jim Mori, Tom Heaton and Lucy Jones and Caltech geophysicist Don L. Anderson joined Kanamori on the study and “after some thought we came up with a solution,” Kanamori said.

“We believe that the sonic boom pushed almost simultaneously against the 400 high-rise buildings in downtown Los Angeles and the Wilshire District,” he said. “The high-rises, in turn, pushed against the relatively soft sediment of the L.A. Basin, and it’s this ground motion we recorded in Pasadena.”

So the sonic boom shook the buildings, which shook the soil that caused the wave that shook the seismograph.

The downtown pulse was recorded 12 seconds before the sonic boom because seismic waves travel faster through the ground than sonics travel through the air.

The discovery, however, is of more than passing interest. If swaying buildings can cause the basin to move as though there had been an earthquake, that might mean the basin is particularly vulnerable to certain types of seismic waves.

The pulse recorded by the Pasadena seismograph had a period of between two and three seconds. That tells the seismologists that the Los Angeles Basin “preferentially transmits” pressure waves of that period.

In addition, the time it takes a high-rise to sway back and forth--called its “resonant period”--depends on how tall it is. Buildings of 20 to 30 stories have a resonant period of two to three seconds.

That suggests that the basin and buildings of that size may be a bad match, some seismologists believe. The basin could amplify the shock waves that hold the greatest potential for damage to 20- and 30-story buildings.

Caltech civil engineer James L. Beck said the discovery calls for further study, but he doubts that the waves would be amplified to the extent that caused many buildings to collapse during the Mexico City earthquake in 1985.

“I don’t believe there’s any cause for alarm,” Beck said.

The Los Angeles situation is quite different from that of Mexico City, which has been studied extensively because damage there was so much greater than had been expected. Mexico City sits on top of a dry lake bed that “amplified ground motion” so much that seismic waves with periods around two seconds were up to 10 times stronger than they would have been on solid rock, Beck said.

“That situation isn’t operating to anywhere near the same extent in the Los Angeles Basin,” he added.

So Beck would not expect the same degree of amplification here as in Mexico City, but he plans to take another look at it.

Shock waves from the Columbia were recorded at many of the 250 seismographic stations in Southern California. The sonic booms are heard at different times by people on the ground as the shuttle passes overhead, so seismologists were able to actually plot the course and speed of the Columbia as it glided toward its desert landing.

It was traveling about 2,000 miles per hour as it passed over the San Andreas Fault, Caltech said.