Abstract

Seismologists sometimes do their work of data acquisition and analysis against a tragic background. Usually the context is fieldwork far from home, in an area subjected to the natural but sometimes devastating effects of an earthquake. As the appalling events of September 11, 2001 unfolded, we found that we had recorded numerous seismic signals of two plane impacts and building collapses, often at times different than those being reported elsewhere. Collapses of the two World Trade Center (WTC) towers generated the largest seismic waves, observed in five states and up to 428 km away. The North Tower collapse was the largest seismic source and had local magnitude ML 2.3. From this we infer that ground shaking of the WTC towers was not a major contributor to the collapse or damage to surrounding buildings. The time of plane impact at the Pentagon on September 11, 2001 had often been reported with large scatter. We analyzed seismic records from five stations in the northeastern United States, ranging from 63 to 350 km from the Pentagon to examine whether we could obtain an accurate time of the Pentagon attack based upon our seismic network. Despite detailed analysis of the data, we could not find a clear seismic signal. Even the closest station (∆= 62.8 km) at Soldier's Delight, Baltimore County, Maryland (SDMD) did not record the impact. However, we positively identified seismic signals associated with United Airlines Flight 93 that crashed near Shanksville, Somerset County, Pennsylvania. The time of the plane crash was 10:06:05+/-5 (EDT). We recognized that information on accurate timing of earthquakes and other events is very desirable for emergency management agencies and government authorities handling mitigation efforts as well as for general public, and that the modern seismographic stations with accurate clocks can provide such reference time as long as there are discernable ground motion associate with such sources.