Testimony from the Deep

The death agony of a Russian submarine was heard by the world's seismograph network

Scientists half a world away from the Russian Arctic have brought us hard news about the sinking of the submarine Kursk in the Barents Sea on August 12, 2000. How did they find this out? They had their ears to the ground. Their specialty is a tiny subfield of earthquake science, a micro-discipline they call forensic seismology.

The solid Earth is a noisy place. That's because sound travels so well in it, and that's why seismographs need to be carefully placed and their data carefully analyzed. Natural earthquakes have competition from many other natural sources of earth noise: landslides, surf, atmospheric motions, meteorite impacts, trees rocking in the wind, waterfalls, and volcanic eruptions, for instance. Humans also make earth noise from mining, traffic, sonic booms, and explosions.

Noise from explosions has long been of interest to scientists. Nuclear testing is the most compelling reason, of course, but seismologists have gotten involved in other cases-plane crashes, pipeline bursts, and the Murrah Building bombing in Oklahoma City in 1995. More recently, the controlled demolition of a Seattle stadium was studied through its seismographic records.

Keith Koper, a University of Arizona scientist, spends part of his time analyzing unusual events recorded by seismographs. For instance, he wrote a paper about the seismic signals from the August 1998 truck-bomb blast at the U.S. embassy in Nairobi. So when the Russian submarine Kursk sank during naval exercises in the Arctic Ocean, he examined that day's data from seismic stations in the region. Some 16,000 seismic stations cover the globe, and many of them share their data openly, so it did not take long for Koper and his colleagues (Terry Wallace, Hans Hartse, and Steven Taylor) to collect the records.

The Russian authorities at first blamed the tragedy on a collision, hinting darkly of a foreign submarine that had been in the area. But Koper's team saw two events on the seismograms, a small one of magnitude 2.2 and, 135 seconds later, a much larger one of magnitude 4.2. They ran a spectral analysis of the records, a graph showing how strong the signals are at different frequencies of vibration.

These events had clear signs of an underwater explosion. One sign was a strong reflection of energy off the surface of the water, a kind of ringing that shows up clearly in a spectral analysis as a peak in the line around 9 hertz (cycles per second). Given the speed of sound in water, a sound wave would ring between sea surface and seafloor at 9 Hz if the water were about 100 meters deep. That jibes with the depths of the sea where the naval exercises were taking place.

The other sign of an underwater explosion is the so-called bubble pulse. When explosions create a huge bubble of hot gases, the surrounding water presses back on the bubble until it reaches a minimum size, then "bounces" back. This oscillation continues for several seconds, changing as the bubble rises to the surface. That shows in the spectra graph as a set of distinct, scalloped low-frequency peaks.

Spectra of the Kursk explosion. Frequency in hertz across the bottom, signal strength in arbitrary units up the left. Figure reproduced by kind permission of Terry Wallace, University of Arizona.

Koper's team compared the Kursk record to the seismograms from an explosion study that Israel conducted in 1999 in the Dead Sea. From that they concluded that the second event was an explosion equivalent to 3-7 tons of TNT, or about a half-dozen torpedo warheads.

The small first event, they think, was likely a single torpedo exploding very near the sub, perhaps even inside it. Because film of the sunken boat shows the periscope up, this must have occurred near the surface. And it was known that the sub had just radioed for permission to launch its weapons. The other evidence suggests that the larger explosion occurred on the seafloor, probably as the sub's impact set off the other torpedos. Presumably the two minutes in between was when the surviving submariners were sealing off their compartments, taking stock of what had happened, and scribbling the notes that divers would later find on their bodies.

Koper's team believes that forensic seismology will grow in importance. First, there are more seismometers than ever with data openly accessible on the Web, they say. And large-scale projects like USArray (here's my article about that), which will deploy hundreds of high-quality instruments in a huge portable network across North America, will surely detect many exotic events to test the skills of seismic detectives.

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