The collapse of the World Trade Centre towers created shock waves which registered on seismographs at New York's Columbia University. But the aftershocks of the catastrophe will be deeply psychological. What happened challenges our capabilities of comprehension.
How does one regain a sense of equilibrium when the community in which one lived and worked is destroyed in a hellish day of flames, dust, soot, ash and glass? For the twin towers were not just office blocks. They were the symbols of the world's largest economy, the first sight of New York for passengers arriving on ships passing the Statue of Liberty. They towered over a world beneath in which tens of thousands of people worked, shopped, dined, had their shoes shined, bought newspapers, drank coffee, hurried for subway trains.
It is now a heap of rubble and tangled metal in which thousands of people are entombed and which has been renamed "ground zero" by the rescue workers. And surrounding it, much of southern Manhattan, from Chambers Street and City Hall to the river, the location of the greatest financial institutions of the world, including the New York Stock Exchange, has been reduced to a dust bowl.
Burned-out cars, buildings, grassy parks, canopies, scaffolding, all have a coating of two inches of dust and millions of torn bits of paper, some of which floated on the smoke-choked westerly breeze across the East River to Brooklyn.
Electricity and gas have been cut off in southern Manhattan to prevent more fire and explosions - which meant that the riverside Tribeca Point building, where The Irish Times has its 42nd-storey office four blocks to the north-west, had to be evacuated late on Tuesday as its 1,000 inhabitants joined the exodus to uptown hotels.
How long ago it seems now since I sat on the bench outside on Sunday looking over the financial district and telling the building manager, Mr Rosie Rosenstein, how this was such a wonderful place to live.
Walking away from the building through the darkening streets on Tuesday evening, past exhausted firefighters who had watched helplessly all day, grieving for 300 dead comrades, we encountered people standing in groups, too shocked to talk.
They stared south to where the twin towers had stood in all their magnificence at the start of a wonderful sunny day, stunned by the enormity of watching two skyscrapers collapse, imploding in on themselves, exploding floor by floor and slipping down out of sight as if swallowed by the earth.
The towers, they had been told, had been built to withstand hurricanes, earthquakes and ordinary fires, but they could not cope with the impact of a 707 and a 767 passenger airliner, gorged with aviation fuel which burned at 2,000 degrees and warped and weakened the 61 steel support columns in each tower so that they could no longer stand the weight of the concrete floor slabs, the office furniture and banks of computer terminals.
The Trade Centre was in fact seven buildings, the two 110-storey towers, the Marriott Hotel, the US Customs House and three office buildings. Buildings five and seven also collapsed late on Tuesday.
For those who lived and worked there, life changed forever on Tuesday morning, as it did for the witnesses whose psyche will be scarred forever at the sight of falling bodies, among them children in three schools on Chambers Street. Many financial workers escaped the holocaust, covered in blood and dust, but it was almost too much for them to bear to think of the people who would not be evacuated alive, the employees in 350 banks, insurance office, law firms and investment houses trapped inside the towers.
Had a generation of financial workers, the best and brightest, those who occupied the prestigious top floors, been wiped out? Morgan Stanley Dean Witter was the largest tenant at the centre with 3,500 employees in one million square feet of office space.
Two thousand of them worked at mid level in No 2 Tower. Marsh & McLennan had 1,700 executives and secretaries on floors 93 to 100 of No 1 Tower. The Aon Corporation, the world's second-largest insurance brokerage, had 1,100 employees on floors 92 and 98 to 105. All the workers in Sidney, Austin, Brown & Wood escaped and the 200 employees of Bridge Information Systems were able to walk down from the 58th floor of No 1 tower, but there was an ominous silence from other companies, like Cantor Fitzgerald, the leading brokerage company in Treasury bonds high in No 1 Tower, whose screens went blank at the moment the plane hit.
And what of all the workers in the sprawling malls beneath the buildings encountered walking across the busy pedestrian walkway, as wide as a street, over West Side Highway from the World Financial Centre - a smaller set of office blocks housing Merrill Lynch and the Winter Gardens.
The Financial Centre escaped damage but the glass-encased bridge, with a large banner advertising a Quebec circus act this weekend, lay crushed across the roadway, brought down by falling masses of concrete. The performance would have been held this Saturday on Tobin Plaza, the area between the towers where open-air concerts are held throughout the summer and visitors gaze in awe at the startling sight of two of the world's tallest man-made structures soaring above. It was closed in winter to protect people from falling ice which formed on the 1,350-ft-high towers.
Emerging from the pedestrian walkway people would also crowd onto escalators to go beneath the towers to get elevators to work or to post a letter in the marble-floored hallway, or shop in the malls, or take a subway uptown, or queue for last-minute theatre tickets at one of the only two such box offices in Manhattan, or to bring a visitor to the Windows on the World Restaurant at the top of Tower 2, in where Irish America magazine recently celebrated its annual Irish on Wall Street event.
It was also an air-conditioned short-cut in hot or rainy weather to Wall Street a few blocks on the south-eastern side or to the Amish Market store where Trade Centre workers liked to have wholesome buffet lunches, or to Borders Bookstore, where it was always impossible to get a seat in the popular coffee bar. How many of the people who we encountered every day were doomed to die there on Tuesday, September 11th, a day which for all Americans will live in infamy, as Franklin D. Roosevelt described the day of the Japanese attack on Pearl Harbour 60 years ago?
How many of these tens of thousands of people will be able to ever resume normal life again? Many financial workers liked to end the day by going to Chelsea Piers sports complex on the Hudson to hit a few balls at the golf driving range.
Yesterday, Chelsea Piers was turned into a morgue and some were brought back there, in body bags.