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Background Attack Aftermath Evidence Misinformation Analysis Memorial

Won-Young Kim

Notable Retractions

Seismographic stations Shanksville, PA
Station Code Station Name Distance (kms)
SDMD Soldier's Delight, MD 190.6
MVL Millersville, PA 217.8
SSPA Standing Stone, PA 107.6
MCWV Mont Chateau, WV 92.1

In the spring of 2002, earth scientists Won-Young Kim and G. R. Baum published Seismic Observations during September 11, 2001, Terrorist Attack in a report to the Maryland Department of Natural Resources. In that paper they established the time of the crash of Flight 93 at 10:06:05, plus or minus five seconds. Their conclusion was based on the analysis of the seismic records from the four seismic recording stations listed in the table on the right.

Kim and Baum quantified the signal-to-noise ratios (SNRs) of the relevant parts of the signals from the four stations. They found that whereas the SNRs from MCWV and SDMD signals were low, those from SSPA and MVL were about 2.5:1 and 2:1, respectively. The authors examined the three-component records from the SSPA station (vertical, North-South, and East-West), noting that "Short-period surface waves, Rg and Lg waves, are quite clear."

Pentagon crash seismic stations
This illustrations from Kim and Baum's paper shows the locations of the four seismic stations whose records they used to establish the time of Flight 93's crash.

The authors also note that the features of the signals are consistent with an airliner crash.

The three-component records at SSPA are dominated by strong Lg arrivals, whereas the Pg waves are difficult to discern and have amplitudes comparable to the noise level. This is typical for seismic waves generated by airplane impacts and crashes.

The authors state with confidence their conclusion that Flight 93 crashed at 10:06.

The seismic signals marked as Sg in Figure 5 propagated from the Shanksville crash site to the stations with approximately 3.5 km/s. Hence, we infer that the Flight 93 crashed around 14:06:055 (UTC) (10:06:05 EDT). The uncertainty is only due to seismic velocity at the uppermost crust near the surface in which the Lg waves propagated.

According to the 9/11 Commission Won-Young Kim later retracted his conclusion that Flight 93 crashed at 10:06. The Commission Report states that Flight 93 crashed at 10:03, and provides the following footnote.

168. Ibid., pp. 23-27.We also reviewed a report regarding seismic observations on September 11, 2001, whose authors conclude that the impact time of United 93 was 10:06:055 (EDT). Won-Young Kim and G. R. Baum, "Seismic Observations during September 11, 2001, Terrorist Attack," spring 2002 (report to the Maryland Department of Natural Resources). But the seismic data on which they based this estimate are far too weak in signal-to-noise ratio and far too speculative in terms of signal source to be used as a means of contradicting the impact time established by the very accurate combination of FDR, CVR, ATC, radar, and impact site data sets. These data sets constrain United 93's impact time to within 1 second, are airplane- and crash-site specific, and are based on time codes automatically recorded in the ATC audiotapes for the FAA centers and correlated with each data set in a process internationally accepted within the aviation accident investigation community. Furthermore, one of the study's principal authors now concedes that "seismic data is not definitive for the impact of UA 93." Email from Won-Young Kim to the Commission,"Re: UA Flight 93,"July 7, 2004; see also Won-Young Kim,"Seismic Observations for UA Flight 93 Crash near Shanksville, Pennsylvania during September 11, 2001," July 5, 2004.

All of the sources that the Report cites to support its claim of a crash time of 10:03 are apparently unavailable for public inspection, including Kim's 2004 paper, for which one will search in vain on the web. The " FDR, CVR, ATC, radar, and impact site data sets" cited by the Report all remain unavailable to the public. This contrasts with Kim and Baum's 2002 paper, which documents its case for the 10:06 crash time.

Early press reports consistently pegged the crash time at 10:06 or later. Only NORAD asserted that Flight 93 had crashed at 10:03. 1 2 3 4 5


1. What was the danger to city? Doomed United Flight 93 passed just south of Pittsburgh, post-gazette.com, 9/13/01
2. Investigators locate 'black box' from Flight 93; widen search area in Somerset crash, post-gazette.com, 9/13/01
3. A Bell Tolls In Shanksville, CBS News, 9/11/02
4. 'America is grateful' to Flight 93 heroes, CNN.com, 9/11/02
5. , September 11 News, Heroes - Flight 93

page last modified: 2007-04-05