A Morning That Shook the World: The Seismology of 9/11

Seismologist Won-Young Kim heard the first reports of the World Trade Center attacks while driving to work. Soon, he would be enmeshed in helping figure out exactly what happened, and when.

Kevin Krajick
September 08, 2021

A version of this story was first published in September 2016 to mark 15 years since the Sept. 11 attacks. We have updated it here to mark the 20th anniversary.

As the World Trade Center was being attacked on Sept. 11, 2001, I was wheeling our year-old daughter from a routine medical exam. We were near St. Luke’s Hospital, next to the Columbia University campus on Manhattan’s Upper West Side. When we got home, about 10 blocks south, the phone rang. It was my mother. “Two planes just hit the World Trade Center,” she said. “Turn on your television.” I did, and I saw the burning buildings. I immediately feared for my wife, Ruby, at work just blocks from the site. I dialed her, but the phone system had collapsed. In succeeding hours, I watched on live TV, gasping as the towers came down, and deadly-looking clouds of debris roiled over lower Manhattan. All I could hope was that Ruby was hiding somewhere where the air was breathable, or had run and escaped.

As I learned later, she was in her glassed-in office at a federal court when the planes hit. At around a quarter to nine, her building trembled. Another construction-crane accident, she figured. Just after nine, a much larger impact traveled through the floor and up into the bones of her feet. Her big windows overlooking Duane Street and toward the Twin Towers bowed far in and then far back out, but did not break. “Like they were made of Jello,” she later said.

She crawled under her desk. When it seemed safe, she looked up into the blue, cloudless sky of that day. Smoke and flames were raging from a black hole in the North Tower. Countless papers and shiny bits of metal rained through the air. Looking down into the narrow street, she saw a wave of humans filling it side to side, everyone fleeing in the same direction.

Presently, with thick black smoke continuing to stream from the Trade Center, federal marshals told everyone to evacuate the building. “Walk north,” they said. No one was really sure what was going on, but there were plenty of signs of panic. A few blocks on, a man told Ruby that he had seen a big plane fly into one of the buildings; he demonstrated by running hard into a mailbox, almost knocking himself down. A woman screamed out to no one in particular that she had seen people jumping from high windows. A lady with a tiny baby in her arms stumbled and fell hard onto the pavement, baby first. The crowd scrambled to pick them up.

Somewhere around Broadway and Canal, Ruby looked back and saw the black-smoke pillar of fire from the South Tower suddenly turn white, and surge out fiercely; she thought it must have been around 10am. She was witnessing the collapse of the building, the first to fall. A fighter jet roared overhead, and people cowered in doorways; was it one of ours, or one of theirs? At Times Square, a stranger lent her his cell phone (then not so common) and she phoned me to tell me she was alive. The tight dress shoes she had worn to the office were too painful for hiking; she took them off and continued barefoot, reaching our home on West 104th Street some time in the afternoon. She had walked about 7 miles.

The following morning, we left our daughter with a babysitter and walked back up to St. Luke’s Hospital, holding hands. Our plan was to donate blood for the survivors we knew must be flooding in. But there were no survivors. The hospital was dead quiet; a nurse thanked us, and told us no blood was needed.

On the morning of Sept. 11, 2001, two jets streaked through the clear blue sky over lower Manhattan into the towers of the World Trade Center. This photo was taken near the site on the morning of Aug. 11, 2016. At right, the new Freedom Tower. (Kevin
Former site of the World Trade Center, photographed from the street, 9 am Aug. 11, 2016. At right, the new Freedom Tower. (All photos: Kevin Krajick)

Miles away, seismologist Won-Young Kim heard the first reports of the attack in his car as he drove to Columbia University’s Lamont-Doherty Earth Observatory. Lamont is located in Palisades, N.Y., on the west bank of the Hudson River, 21 miles north of lower Manhattan. From here, part of Kim’s job was to run a network of dozens of seismometers designed to monitor the U.S. Northeast for earthquakes. They also sometimes capture man-made seismic waves generated by explosions or other sources.

When he got to work, everyone was glued to the radio. But in a short time, it became difficult to listen to the news; the seismology department was being inundated by calls from reporters and all kinds of government officials. In the initial chaos, it was unclear exactly what was going on, what kinds of explosions had occurred, and the exact sequence of events. Callers were desperate for information from any possible source. Had the seismic network picked up anything?

“Up to then, all I knew was, something terrible happened,” said Kim. Like many others, his first impulse, and that of colleagues, had been: Should we go down there and help rescue people? Bring supplies? The question soon became moot; bridges and tunnels to Manhattan were shut down. With all the queries coming in, Kim started thinking that maybe in fact he and the other staff could be of some use. “I thought, yes, maybe there is a seismic signal,” said Kim. “Maybe we have data to contribute. Maybe we can assist and help pin things down.”

Kim received his first query, from a New York Times reporter, shortly after the collapse of the South Tower. There was no operating seismometer in New York City at that time; the closest was in a basement on the Lamont campus itself. Kim rushed over to the big rotating drum that continuously records its signals on paper. Sure enough, the needle had jumped. With signals coming in electronically from 33 other seismometers scattered across the region, Kim headed to his office to try and work out the exact time and magnitude of the collapse. While he was doing this, the second tower came down.

From then on, all he did for the next several days was stare at data, make calculations, confer with colleagues and try to get a few hours of sleep. He does not recall ever seeing visual images of the plane impacts or the falling buildings until the following night, when he finally went home and saw on television what most other people had already seen a hundred times.

Over succeeding weeks, Kim and his colleagues worked up precise seismic signatures of both the jet impacts and the building collapses. Their first official product, coauthored by 12 Lamont seismologists and grad students, was a November 2001 paper published by the American Geophysical Union,  It described the waves generated by the attacks, their potential effects, and the precise timing of each event. Working with federal investigators, the New York Fire Department, the Port Authority and others, they distributed seismic data, which was later combined with photos, videos and eyewitness reports to form a comprehensive account of the attacks.

Visitors to the underground September 11 Museum view the massive retaining wall that once surrounded the towers' foundations. Seismic waves from the collapse first passing through this wall were detected more than 250 miles off. (Kevin Krajick)
Visitors to the underground September 11 Museum view part of the massive retaining wall that surrounded the towers’ foundations. Seismic waves from the plane impacts and tower collapses first passed through this wall and into the earth’s crust.

The seismologists determined that the planes had hit the towers at 8:46:26 and 9:02:54, give or take a few seconds—in the case of the first strike, about 2 minutes earlier than first reported by media. To make the calculation, in part they had to account for the 17-some seconds it would have taken for the waves to travel from the shocked towers’ upper floors down through their foundations and an adjoining concrete retainer wall. Then outward through complexes of shallow crustal rocks. First, the hard schist and amphibolite of lower Manhattan; then the gneisses of upper Manhattan; on to the sandstones and shales underlying the Hudson River; and finally up through the great sill of volcanic rock that forms the towering Palisades on the river’s west bank, where Lamont sits. Much of the energy also traveled through the river itself.

The jet strikes generated seismic waves comparable to small earthquakes of magnitudes 0.9 and 0.7 respectively—probably only a small part of the total energy generated when the planes hit the buildings. Kim believes most of that energy was released in fireballs and airborne shock waves. This helped explain my wife’s testimony: She felt the first impact only weakly, because intervening buildings largely blocked the waves (and her view). For the second, she was in a powerful direct line. The shape of the seismic waves looked quite unlike those of natural earthquakes, which originate under the surface, said Kim. These had started from above. “More like ringing a bell,” he said.

Seismograms of the plane impacts and subsequent collapses of the World Trade Center towers, recorded 21 miles away at Lamont-Doherty Earth Observatory. The records became part of the official record, helping pinpoint the exact time of each event. (Ki
Seismograms of the jet impacts and subsequent collapses of the World Trade Center towers, as recorded 21 miles away at Lamont-Doherty Earth Observatory. The records helped pinpoint the exact time of each event. (Kim et al., Eos Transactions, 2001)

According to the seismic data, the collapse of the South Tower came at 9:59:04. The North Tower came down at 10:28:31. Some federal investigators put the times about 10 seconds earlier, but they apparently measured from when the buildings began pancaking from the top; the seismologists pinpointed when they hit bottom. The South Tower collapse generated seismic waves comparable to a magnitude 2.1 earthquake. The fall of the North Tower, a half-hour later, generated the most powerful wave of the day, corresponding to a magnitude 2.3  earthquake. That event was recorded by 13 seismic stations in five states—including one at Lisbon, N.H., 266 miles away. Again, Kim calculated that most of the energy did not reach the ground; it was mainly used up converting steel, concrete—and human beings—to dust. He observed that the event greatly resembled the shape of seismic waves released by a pyroclastic flow, a lethal explosion of hot gases and debris running down the slopes of an exploding volcano.

Crash of the hijacked United Air 93 near Shanksville, Penn. Signals of this impact were less clear, pointing to the importance of other kinds of data. (Kim, Baum, 2004)
Seismic signals from the crash of the hijacked United Air 93 near Shanksville, Penn. (Kim, Baum, 2004)

The nearby 7 World Trade Center came down at 5:20:33 pm, and the instruments picked that up, too. “It was more gradual than the big towers,” said Kim. There was some speculation that this building and others nearby were fatally damaged by the earlier strong ground shaking, but the seismic analyses suggested otherwise. Modern New York structures are built to withstand much stronger natural quakes, up to magnitude 4 or 4.5. The seismologists advised that the weakening of adjacent structures was more likely caused by the sudden air pressure of the volcano-like debris flow, not shaking of the ground. These findings helped engineers more confidently judge which other still-standing nearby buildings were weakened, and which were safe.

In the days following the attacks, Kim and many others discovered something else: Among more than 8 million New Yorkers, there are few degrees of separation. Nearly everyone knew someone, or knew someone who knew someone who was missing. For our part, Ruby and I had rung in the millennial New Year, Jan. 1, 2000, at the Brooklyn home of our close friends Bob and Sally. At this wonderful party, we met two of their other good friends, Brooklyn firefighter Dave Fontana and his wife, Marian. We liked them a lot, and talked about all kinds of things, from sculpture to baby names. (For a girl, we all agreed on Willow, though none of us later ended up using that name.) Shortly after the attacks, Bob called to tell me Dave Fontana had been in the South Tower when it came down. He never had a chance to name another baby. Bob and I wept together at his empty-coffin funeral.

For Kim’s part, “Everyone on my little street, they had a cousin, or a friend,” he later told me. “It was very painful.” Among them: several former Lamont geophysics students who worked in or near the Trade Center. Highly skilled at manipulating computers and analyzing complex data, they had figured out they could make a lot more money crunching stock prices for Wall Street than plotting earthquakes.

One of them was Kim’s former student, a Chinese immigrant named Jing-Hua Shi. Lamont staff tried frantically to reach her, but could not. Finally, after three days, she returned a message to say she was OK.

Another was 41-year-old Weibin Wang, who had immigrated from China and studied earthquake mechanics at Lamont. In 1999, instead of pursuing seismology, he bagged a good-paying job at the financial firm of Cantor Fitzgerald. He and his wife, Wen Shi, moved with their three kids into a suburban house not far from Kim with his wife. On weekends, Weibin Wang cooked for the family and took the kids to piano lessons. In March 2001, he became an American citizen. On Sept. 11, he was at work for Cantor Fitzgerald on the 103rd floor of the North Tower. He was never heard from again.

Some time later, Kim and Gerald Baum of the Maryland Geological Survey tried to help pinpoint the time when the hijacked United Airlines flight 93 slammed into a field near Shanksville, Penn., killing everyone on board. They came up with 10:06 am, but the signals were mixed with too much noise, and they turned out to be unreliable; using radio transmissions and other data, the 9-11 Commission put the crash three minutes earlier. And Kim could not come up with any signal at all from the crash of a Boeing 757 into the Pentagon at about 9:37 am. He speculates that the marshy sediments on which much of Washington is built had absorbed the impact. Or, additionally, that the Pentagon is built over a massive, largely secret labyrinth of reinforced underground spaces that scattered the energy.

Seismologist Won-Young Kim pulls out the original paper seismogram showing the jet impacts, and subsequent collapses, of the World Trade Center towers. "It was a very painful day," he said. (Kevin Krajick)
August 22, 2016: Seismologist Won-Young Kim pulls out the original paper seismogram showing the jet impacts and subsequent collapses of the World Trade Center towers (marked by white labels). “It was a very painful day,” he said.

Later, when Kim looked back over the New York City data, something popped out that dumbfounded him. Interspersed around the plane impacts and the fall of the towers were five or six smaller seismic signals. At first he thought they were incremental collapses. But he tracked their locations to a line of sites along a formation of hard rock in northern New Jersey. He had seen these before many times: dynamite blasts at rock quarries. “I was very surprised, they were still doing it that day,” he said. “I thought they would have stopped, but they continued.”

For the seismologists, the events highlighted the fact that New York City did not have a single operating seismic station. This, although Manhattan is cut by several known faults, has a history of small to moderate earthquakes, and could be subject to much larger ones. In 2002, Kim and his colleagues filled some of the gap by installing stations in Central Park and at Fordham University in the Bronx.

Their research fed into the final narrative of the U.S. 9-11 Commission, published in 2004. An image of one of their seismograms is enshrined at Ground Zero in the September 11 Museum.

At the time of the attacks, Kim and his family were all still citizens of South Korea, where Kim grew up. Sept. 11 changed this. “It was very hard for me to see my neighbors,” he said. “I realized, I don’t feel pain when I read in the news about something bad happening in Korea. It’s too far away. This was very close to me. I knew, this is where I live now.” A few months later, he and his family all applied for U.S. citizenship. They were sworn in together as citizens in July 2002.

As for us. Ruby was ordered to return to work just three days after the attacks, and had to walk every day near the towering, smoking pile of twisted metal, rubble and dust, where workers were pulling out red-hot steel beams, and conducting a hopeless search for life. The streets were littered with aircraft parts, pieces of Fire Department trucks and a lot of unidentifiable debris. For months, a rancid, chemical stench hung in the air, both outside and in her workplace. She felt it clung to her clothes and body everywhere, all the time. She still works nearby.

Five years ago, approaching the 15th anniversary of the attacks, I visited the 9/11 Memorial, and took some of the photographs you see here. I was alone; Ruby has refused to ever set foot in the place.

The September 11 Memorial. Just as this photo was taken, a leaf drifted from the sky to the inscription for Brooklyn fire fighter David J. Fontana. Fontana and 14 other from his squad perished when the south tower collapsed after they had climbed up
The September 11 Memorial. As this photo was taken, a leaf fell by the name of Brooklyn fire fighter David Fontana. He and 11 others in his squad perished after racing up 45 stories inside the South Tower, trying to save people. It was his and his wife’s eighth wedding anniversary. He had insisted on the date because he wanted it to be nine-one-one, the number people call for help.