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Thursday, 20 September, 2001, 14:47 GMT 15:47 UK
Collapsing towers caused seismic shock
Graphic BBC
By BBC News Online science editor Dr David Whitehouse

The devastating impacts on the twin towers of the World Trade Center and their subsequent collapse shook the ground with the force of a small earthquake.

Scientists have released seismic recordings made at several monitoring stations situated in northeast America.

The seismic signals generated by the collapsing north and south towers were much stronger than those from the two airliner impacts.

Many smaller signals were registered at the Palisades monitoring station - a short distance from Manhattan - that may have originated from the further collapse of the Twin Towers and the fall of walls and other debris in the surrounding area.

The seismic disturbance caused by the first airliner impact
The seismographic monitoring stations are situated in southern New York, northern New Jersey, western Connecticut, and Pennsylvania. The closest station, at Palisades, New York, is located just 34 kilometres (21 miles) north of lower Manhattan in Rockland County.

The Palisades recordings of the twin tower collapses were comparable in size to the signals from a small earthquake of magnitude 2.4 that was felt on the east side of Manhattan and in the western parts of Queens earlier this year, on 17 January.

However, the seismic signals from the five events on 11 September differed from a small earthquake in significant ways.

They were richer in low-frequency energy and poorer in high-frequency energy. The differences can be attributed to the short-time duration of the fault rupture responsible for the earthquake as compared with the long and complex collapse of the buildings.

The south tower goes just before 1430 GMT
See also:

18 Sep 01 | Sci/Tech
The destructive forces unleashed
13 Sep 01 | Americas
How the World Trade Center fell
11 Sep 01 | Americas
Eyewitness: The twin towers fall
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