Though terrorists triggered the collapse of the World Trade Center's Twin Towers, much of the energy -- calculated by a physicist to have been at least 2 percent that of the atomic bomb dropped on Hiroshima -- was supplied by Nature's most ubiquitous force: gravity.

The force that keeps our feet on the ground and holds planets in orbit swiftly fueled a runaway momentum that created thundering impacts so great they were detected by equipment designed to monitor earthquakes.

Meanwhile, some scientists have suggested that even without the searing fire, the structural integrity of the buildings might have been compromised enough by the planes' impacts to cause the towers' ultimate collapse anyway.

When the crumbling towers thudded to the ground, they created ground tremors equal to earthquakes of magnitude 2.1 and 2.3, as measured by a seismic station in Palisades, New York, 21 miles (34 kilometers) north of Manhattan.

These would be small numbers for earthquakes, not likely to be felt by anyone, since an earthquake is typically centered miles below the surface, its energy radiating out and dissipating over great distances. But the energy from the collapse of the towers was focused in a very small area, making it remarkable that the events were detected at all outside lower Manhattan.

Much of this energy went into destroying the buildings themselves and generating the cloud of debris.

More surprising, even the impacts of the airliners shook the ground, registering with the equivalent of earthquakes with magnitudes of 0.7 and 0.9.

Magnusson, the structural engineer, said that if fire had not brought the buildings down, a strong wind might have eventually toppled them.

"If there was no fire, in my opinion the buildings would be standing today," Magnusson said in a telephone interview. "And they would stand until there was a significant wind storm."

The Twin Towers, like most skyscrapers, were designed to handle hurricane-force winds. But all of this wind resistance is built into the exterior of the buildings, Magnusson explained. The holes carved into the side of the buildings by the impacts "would have been a problem for the wind resisting system," he said.

Mir Ali, a professor of architecture at the University of Illinois, disagreed. He said that while the gaping holes would have caused the buildings to sway more in the wind, they would likely have withstood the pressure. In any event, the holes could have been beefed up, he said, assuming no big storms arrived before builders had time.

Another scientist suggested that the building might have had other structural damage, something that might never be determined from the rubble of the disaster.

The impact of the hijacked jets could have created a vibration, or oscillation, in the buildings, said Arthur Lerner-Lam, Associate Director for geology and geophysics at Columbia University's Lamont-Doherty Earth Observatory, which reported the seismic observations.

All tall buildings can oscillate in the wind, a factor taken into account by architects.

Lerner-Lam said that the vibrations set in motion early in the morning on Sept. 11 would likely have been at a higher frequency than those caused by wind. He said that it's possible, though speculative to say so given scant evidence, that the higher frequency might have damaged the building to the extent that it would have ultimately toppled.

Some eyewitness reports told of what appeared to be buckling in walls below the level of the fire, Lerner-Lam said.

He described the potential vibration as akin to a ringing bell. The initial ringing would have damped out quickly. But if large enough, it could have left permanent damage. In such a scenario, the steel support beams would behave like plastic, Lerner-Lam explained: Bend them a little, they spring back. Bend them further, they stay bent. Bend them enough, they break.

Both Magnusson and Ali said they doubted this scenario.

"Oscillations can damage a building," Magnusson said. "But it would have revealed itself in the first few minutes."

The south tower collapsed 56 minutes after being struck. The north tower collapsed 1 hour and 40 minutes after impact.

Still, several engineers say the construction of the buildings, in particular their combustibility, contributed to their demise.

Each tower was supported primarily by a series of steel columns built into the exteriors, 61 on each side. These pillars support their own weight and half the weight of the floors, Magnusson said. A cluster of columns at the center did heavier lifting, supporting themselves, half the weight of the floors, and all of the elevators and other mechanical systems.

Steel trusses tied the two sets of columns together, providing reinforcement.

Engineers aren't sure if the central columns were compromised in the fires. But regardless, an important question has emerged: Can future buildings be designed to survive similar attacks?

Ali, the University of Illinois architecture professor, said improvements in design can be made. Using less combustible materials on floors and walls would slow the spread of fires, allowing for more effective evacuations.

Structurally, concrete is less vulnerable to fire than glass and steel, Ali said, and designers will have to consider thicker external structures.

When the World Trade Center was built, in the 1970s, concrete was too heavy to be a practical material for a 110-story building. More recently, stronger types of concrete have been developed, and steel-reinforced concrete is increasingly the material of choice for skyscrapers.

But future design changes will be limited by cost. To make a building truly terrorist-resistant, it would have to be built like a nuclear plant, Ali said, so that an airliner could not penetrate the exterior and a fire could not spread.

"Huge concrete walls" would be required, Ali said. "Cost-wise, it is impractical."