A seismometer tucked away in a Baghdad office has just delivered an action audio-replay of explosive violence at a US military base miles away.
The instrument, picking up data 100 times a second, recorded every tremor from a series of explosions that followed a mortar shell or rocket impact in an ammunition store on 10 October, 2006.
What followed was what the US infantry called a “cook-off.” The heat from the first impact at Forward Operating Base Falcon, outside Baghdad, caused a series of explosions, the heat from which then triggered more and more blasts. The noise and violence and the mushroom-shaped cloud of smoke was so great that observers at first wondered if a nuclear device has been deployed.
The story is told by Ghassan Aleqabi and Michael Wysession of the Washington University in St Louis and others, in a study for the Bulletin of the Seismological Society of America. Seismic waves – tremors in the Earth’s surface after a blast or an earthquake – travel at anything from two to eight kilometres a second, and the speed, amplitude and pattern of the signal recorded on a seismometer can within seconds answer questions about potential devastation from an earthquake.
The seismometer has very rapidly become a tool for geophysical research: the data from a network of instruments can answer questions not just about the nature of the bedrock but even about the planet’s core and mantle, far below the surface.
To make sure that they interpret the data correctly, seismologists like to test the signals recorded against events that are well documented. In 2012, a network of US instruments tracked the impact of the ocean waves generated by Hurricane Sandy as they pounded the Atlantic coast of the US, all the way north to end in a catastrophic storm surge in New York. The readings were so sensitive that the researchers could distinguish between waves that hit the coast, and waves that smashed into each other.
And in 1992, British scientists reported that they had registered the rhythmic stomping at a concert in London of 30,000 people responding to a song by the band Madness: the ground tremors were enough to make nine-storey apartment blocks nearby sway in resonance.
So the US scientists who installed their instruments in Baghdad in 2005 and 2006, could hardly have missed the explosions at a forward operating base in Iraq during a period of bitter military action.
The seismic readings, transmitted entirely by tremors in the ground, provided both the signatures of the explosions and the sequence.
“It was an accident that we got such a rich recording. But sometimes science works that way, you get lucky,” said Wysession.
The instruments picked up the “very specific” signature of mortar fire, and even the swooping near and away again of passing helicopters. “You can look at how much the frequency drops and over what length of time and determine how far away the helicopter is, and how fast it’s going, which is really fascinating,” he said.
The pattern of tremors from the events of 10 October allowed the researchers to reconstruct the action: at 7.22 am the instruments recorded mortar fire. At 7.31, a helicopter flew over. And explosion at 7.36 was followed by a small series of blasts that alerted the troops and at 7.40, there was a huge explosion.
The scientists see seismology as another potential weapon against terrorism. “A network of seismometers in an urban area can tell you a lot about a terror attack,” Wysession said. “I think we’ll hear more about forensic seismology as time goes on.”
