When the submarine Kursk sank on 12 August 2000 during maneuvers in the Russian Arctic, for a long time there was no hard news—except seismic evidence. While the Russian authorities were denying the tragedy, vibrations from the event were picked up by geophones, and half a world away some specialists in unusual events took notice. These people occupy a tiny subfield of earthquake science they call forensic seismology.
The Noisy Earth
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. Besides earthquakes, there are many other natural earth noises: landslides, surf, atmospheric motions, meteorite impacts, trees rocking in the wind, waterfalls, and volcanic eruptions, for instance.
Human noise comes from mining, traffic, sonic booms, and various 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. And the bombing and collapse of New York's World Trade Center in 2001 appeared on seismograms, too.
Usually seismologists take a known stimulus—an explosive charge or an earthquake—and deduce the Earth's properties from the seismographic record. Forensic seismology uses the record to deduce something about the stimulus instead.
Keith Koper, now at St. Louis University, spends part of his time on forensic cases. 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 Kursk sank, 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 to collect the records.
Spectral Signs of Underwater Explosion
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 took the records and ran a spectral analysis: a graph showing how strong the signals are at all the different frequencies of vibration. (See the illustration.)
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. Earthquakes have much lower frequencies—they ring the whole crust, sometimes the whole planet (as in Sumatra in 2004). 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 sea depths where the Kursk was.
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 larger again. A back-and-forth 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.
The Kursk Scenario
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 concluded, 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, a huge moveable swarm of high-end seismographs that will tour North America for ten years, will surely detect many exotic events to test the skills of seismic detectives.
PS: The 1995 Oklahoma City bombing was also recorded on nearby seismographs, and a wave of wild and ugly speculation based on the recorded traces was the first great eruption of the Web's dark side. Science had the last word, though, demonstrating the true character of the event and discouraging conspiracy theories.