Seismographs picked up Trade Centre's collapse

The collapse of the World Trade Centre towers shook the ground with the strength of a minor earthquake.

Monitoring stations in throughout the north-east of America recorded the tremors.

The closest station, 21 miles north of lower Manhattan, was near enough to register the relatively smaller impacts of the two airliners as well.

Equipment also picked up the smaller signals generated by the collapse of other buildings and walls throughout the area.

All the monitoring stations are operated by the Lamont-Doherty Earth Observatory of Columbia University.

Seismographs collected from September 11 are published on its website.

It reports the twin tower collapses were similar in strength to a small earthquake of a magnitude 2.4 on the Richter scale.

Experts say the collapse of the buildings was longer and more complex than the fault shifts which cause small earthquakes.