{"content":[{"text":"Seismic data recorded at various sites in New York State on September 11th, \n 2001, show two significant events, the first at about 9:59 a.m. (EDT) and the second, \n slightly more powerful, at about 10:28 a.m. (EDT). Traces of the seismic events may be \n viewed at www.ldeo.columbia.edu, and show a lot of “ringing” decay extending over 10 \n or more seconds. Thus, cursory inspection of the traces suggests that the WTC collapse \n events were indeed “about 10 seconds” in duration. However, since the correct \n interpretation of the seismic data is critically important to the analysis of the collapse of \n the two WTC towers, we will now consider the seismic data in more detail.","bbox":{"left":0.147227,"top":0.121497,"width":0.694588,"height":0.131914}},{"text":"The first event, occurring at about 9:59, was the collapse of WTC 2, also called \n the south tower, (N.B. This was not the first tower to be hit by an aircraft). The north \n tower, WTC 1, collapsed at about 10:28 and was responsible for the second seismic \n disturbance. The traces recorded at the Palisades station provide the best seismic data for \n the events of September 11th, 2001. Because the published traces begin at 9:59:07 (WTC \n 2) and 10:28:30 (WTC 1), these times are frequently quoted as the actual collapse times. \n This is erroneous for two reasons. First, it should be noted that the start of the major \n oscillations in the seismic signature of each collapse event corresponds to the ground \n impact of the main upper section of the towers. As TV coverage of the event shows, this \n impact occurred about 10 seconds after the start of the collapse of each tower. Second, \n the Palisades seismic data are delayed by about 17 seconds compared to the actual events \n in New York City because of the travel time for the 34 km distance between the towers \n and the Palisades seismic station.","bbox":{"left":0.147227,"top":0.266323,"width":0.7012240000000001,"height":0.21055600000000008}},{"text":"The CNN TV coverage of the collapse of the North Tower (WTC 1) provides a \n very useful time calibration of this event that may be compared with the Palisades \n seismic data. The CNN TV images show that WTC 1 starts to collapse at 10:28:23. The \n ground impact of the upper section follows about 10 seconds later at 10:28:33. This is \n consistent with the Palisades data if we allow 17 seconds for travel time of the seismic \n waves. Thus, if we treat the Palisades data as if it were recorded at the WTC site, the \n published seismic trace would now effectively begin at 10:28:13 and the ground impact \n responsible for the large oscillations of the trace would occur at 10:28:32. These values \n are in good agreement with the visual result derived from the CNN TV images.","bbox":{"left":0.147227,"top":0.489791,"width":0.6886190000000001,"height":0.14642300000000008}},{"text":"Having made these adjustments to the timelines of the 911 seismic data we are \n able to conclude that the small ripples in the traces of the WTC collapse events - ripples \n that precede the period of large oscillations - represent the first stage of collapse as \n defined more precisely below. The seismic signal for this first stage is small, as would be \n expected, since kinetic energy is being transmitted to the ground only through the steel \n support structure. Furthermore, a significant fraction of this kinetic energy is being \n absorbed as the energy needed to buckle and crush the structural elements of the \n buildings. The major seismic signal of each collapse is generated by the ground impact of \n falling debris, and constitutes what we will call a second stage of collapse. Given the \n above considerations and a careful evaluation of the seismic data, it is estimated that the \n first stage of collapse took 11.3  1.5 seconds for each WTC tower. We will show in the \n following Section that the second stage of collapse added 1 – 2 seconds to the total \n collapse times.","bbox":{"left":0.147227,"top":0.649126,"width":0.703724,"height":0.211696}}],"images_bbox":[],"images_bbox_not_text_overlap":[]}