The keyword phrase "earthquake sept 11" functions as a noun phrase. The core component is the noun "earthquake," which is modified by "Sept 11." In this context, "Sept 11" acts as a noun adjunct or a proper adjective, specifying a unique, non-geological event rather than a naturally occurring tectonic phenomenon. This grammatical structure is critical as it defines the subject not as a general earthquake that occurred on a particular date, but as the specific seismic signatures generated by and identified with the events of September 11, 2001.
Seismographs, notably those at Columbia University's Lamont-Doherty Earth Observatory located 34 km away, recorded distinct ground vibrations corresponding to the attacks on the World Trade Center. The data includes small seismic signals from the two aircraft impacts (magnitudes 0.9 and 0.7) and much larger signals from the collapses of the South and North Towers. The collapse of the South Tower at 9:59 AM (EDT) registered as a magnitude 2.1 event, while the collapse of the North Tower at 10:28 AM (EDT) generated the largest signal, registering as a magnitude 2.3 event. Analysis of the waveforms confirmed they were short-period surface waves, consistent with an above-ground energy source (the collapses) rather than the shear-wave motion of a typical tectonic earthquake.
The analysis of this seismic data provides a precise, independent timeline of the WTC collapses and serves as a key case study in forensic seismology. It demonstrates the capacity of seismological networks to detect and analyze significant anthropogenic events, distinguishing them from natural seismic activity. The classification of the event with the noun "earthquake" is metaphorical, borrowing a term for a natural disaster to describe the magnitude of a man-made one, while the modifier "Sept 11" anchors it to a specific historical moment, highlighting its unique origin and significance.
