Ripples Spread Wide from Ground Zero

Even as personal and political shock waves began emanating from the Sept. 11 attacks on the World Trade Center, the magnitude of the violence literally made the ground shake, as well. Seismic vibrations induced by several events that day were captured by seismometers scattered across the Northeast.

Detailed analysis of the data these instruments captured suggests that buildings surrounding the fallen twin towers probably didn’t sustain structural damage from collapse-induced seismic vibrations. Nevertheless, scientists and engineers argue that inspections of the devastation in the surrounding area could lead to updated building codes that would render structures safer for their occupants.

Thousands died in terrorist attacks that transformed a sunny autumn day into one of the darkest in recent times. The 110-story towers of the World Trade Center–which together contained more office space than any other building in the world–collapsed after they were struck by hijacked, fuel-laden aircraft.

Seismometers operated by the Lamont-Doherty Earth Observatory of Columbia University in Palisades, N.Y., headquartered just 34 kilometers north of lower Manhattan, registered many of the day’s catastrophic events. At least 13 instruments in the observatory’s five-state network, including one 428 km away in Lisbon, N.H., detected the collapse of the north tower, says Won-Young Kim, a seismologist in Palisades. He and his colleagues describe their observations in the Nov. 20 Eos, the weekly newspaper of the American Geophysical Union in Washington, D.C.

The seismometers show that the first plane hit the north tower at 8:46 a.m. and triggered vibrations in Earth’s crust just as if the planet were a bell struck with a hammer. The 12-second shock was the equivalent of a magnitude 0.9 earthquake, says Kim. When the second plane hit the south tower just before 9:03 a.m., it generated ground motions like those from a magnitude 0.7 temblor.

The collapse of each tower generated much larger vibrations. The implosion of the south tower, just after 9:59 a.m., lasted about 10 seconds and sent out ripples equivalent to those of a magnitude 2.1 quake. The north tower triggered ground motions like those produced by a magnitude 2.3 earthquake.

Tremors of this size, which aren’t rare, are barely felt by people nearby, says Klaus H. Jacob, a geophysicist at the observatory and a coauthor of the report. In fact, two earthquakes of similar magnitude have struck Manhattan this year.

Each of the twin towers was more than 415 meters tall, and each of the buildings’ floors enclosed more than 1 acre of office space. The buildings, which for a while in the 1970s were the world’s tallest, didn’t have the traditional skyscraper’s skeleton. They essentially were hollow tubes made up of 47-centimeter-wide, aluminum-clad steel columns spaced about 56 cm apart.

Floor panels stretched from this strong outer skin to the building’s inner core of steel girders, which housed the elevators.

Each of the towers was designed to survive the impact of a Boeing 707–the largest commercial aircraft aloft when the buildings were designed –flying at about 290 km per hour, a speed typical of a plane about to land. However, the aircraft that struck the towers on Sept. 11 were more massive Boeing 767s flying at a cruise speed of about 800 km/h. Each of these planes held about 75,700 liters of jet fuel, said Leslie E. Robertson, the lead structural engineer for the World Trade Center. He described the devastation of the buildings surrounding the complex in a Nov. 9 symposium at the National Building Museum in Washington, D.C.

The aircraft that slammed into the north tower severed about two-thirds of the steel columns on the building’s north side. Even though the towers absorbed much more of an impact than they were designed to sustain, they eventually succumbed when the remaining steel supports softened in the extremely high temperatures of the subsequent fuel-fed conflagration. Nevertheless, the buildings held together long enough for an estimated 25,000 people to escape.

Only about one-ten-thousandth of the energy of each collapse, which the Lamont-Doherty researchers estimate equaled the explosion of 23.5 tons of TNT, was converted into seismic vibrations. Much of the total energy went into twisting the buildings’ steel framework and crumbling other construction materials into rubble and dust (SN: 9/29/01, p. 200: Dust, the Thermostat).

Most of the damage to structures within a few blocks of the towers probably came from falling debris or from fist-size projectiles carried by the wind created at the base of the towers as they collapsed, says Jacob.

Because there aren’t any seismometers in lower Manhattan, scientists can’t be sure how large the ground motions were at the surface there. However, according to distant readings and information gleaned from interviews with people in the area, Jacobs contends, the ground vibrations near the collapsing towers were too small to cause significant structural damage. Buildings in the eastern United States typically aren’t damaged during earthquakes smaller than magnitude 4.5, he notes.

Jacob and his colleagues have sought to inspect buildings surrounding Ground Zero but haven’t gained access because rescue and clean-up efforts have taken priority. Information about how nearby structures held up to the events of Sept. 11 could help engineers develop building codes that would give people inside a building more time to escape after the structure sustains massive damage, says Jacob.