Oklahoma City Bombing Recorded on Seismogram

Seismic waves generated by the April 19, 1995, federal building bombing in Oklahoma City were recorded by at least two seismometers in the area. One seismograph, the Oklahoma Geological Survey's, was located sixteen miles from the blast site near Norman, Oklahoma. The other seismograph was located four miles from the blast site in the Omniplex Science Museum in Oklahoma City.

Both seismographs showed two or three events (ground motion) happening around the time the bomb is supposed to have gone off. Many people have speculated that the later events are the result of a second bomb, but scientists have generally disagreed.

As reported by Ambrose Evans-Pritchard and Hugh Davies in the April 23, 1995, Sunday Telegraph:

The Sunday Telegraph has received copies of a seismogram from the Geological Survey at the University of Oklahoma showing that there may have been two bomb blasts, 10 seconds apart. The FBI says that there was only one blast.

But the director of the institute, Charles Mankin, says that "this is clearly not a refraction. The two surface waves had the same amplitude, wave form, and shape. It's very puzzling."

The U.S. Geological Survey together with the Oklahoma Geological Survey conducted experiments to investigate the issue. The USGS/OGS recorded the May 23, 1995, demolition of the federal building's remains, comparing them to the recording of the bombing.

The scientists differed in the interpretation. The USGS issued a June 1, 1995, press release, reporting that they believe the two seismogram events were from a single bomb. They also said the events were about twice as long as would be expected from an ammonium nitrate bomb. They did not publish a report for peer review.

A Ph.D geophysicist and assistant professor of geology and geophysics at the University of Oklahoma updated the Libernet mailing list several times regarding the progress of the investigation. In posts to Libernet on April 26, May 18, May 23, May 29, August 17 and September 14, 1995, he described his research into the matter.

The record does show three distinct arrivals, timed at 9:02:02, 9:02:13, and 9:02:23 am, CST. The first arrival appears to be distinctly different from the other two, and is similar to micro-earthquake activity which is very common on other sections of the record.

... The demolition [of the Federal Building's remains] was recorded on the same seismometer, but, according to Dr. Charles Mankin, director of the Oklahoma Geological Survey, looks "nothing like" the original blast event(s). I have not seen the record myself. The preliminary interpretation would argue against an interpretation of the second event on the April 19 record as "the building falling down".

... I have also spoken with several people in the immediate vicinity of the blast site. They unanimously report hearing only one blast.

The results discredit the two-explosion hypothesis, because "if the two arrivals at the FNO station 16.23 miles from the blast site were separated by 10 seconds, then the 2 arrivals at the Omniplex seismograph, 4.21 miles from the blast site, should have been separated by 10 seconds or less - yet they were separated by 16 seconds."

The unexplained factor in the seismograms is how one truck bomb could produce the same length of disturbance (approximately ten seconds) as the demolition, when the demolition consisted of a series of staggered explosions plus the building falling down.