Seismic Waves: The Ultimate Black Box

Eyewitness reports of plane crashes are notoriously unreliable: When United Airlines Flight 93 crashed in Pennsylvania on September 11, some locals saw a fighter jet in the area, others heard an explosion before the plane went down. Terry Wallace has contrary testimony from Earth itself. Wallace, a seismologist at the University of Arizona, has studied records of ground vibrations triggered by the crash to reconstruct how the event unfolded.

Based on the amount of seismic energy, Wallace could estimate how the plane came down: "The UA flight produced a significant signal, consistent with a fully-loaded jet that was intact, or nearly intact, on impact." That finding disputes rumors that the hijacked jet was shot down, he says, because a missile or other explosion would have broken the craft into smaller pieces that would have caused less seismic disturbance. The Pan Am crash over Lockerbie, Scotland, which blew apart in midair, produced only a faint signal, even though the crash occurred close to an array of ground-motion sensors. David McCormack, a seismologist at Natural Resources Canada who studied the Lockerbie crash, agrees with Wallace's interpretation. "To detect a signal even marginally, the aircraft would have to be intact," he says.

Wallace is refining his analyses to help unravel future airplane accidents. He also envisions constructing urban arrays that could pick up the "seismic fingerprint" of trespassers or identify the weight of a fleeing criminal.