In the movie Dr. Strangelove, Soviet ambassador de Sadesky warns that renegade U.S. Air Force general Ripper has put the whole world in peril. The reason, the ambassador explains, is because his countrymen have deployed a doomsday device—50 nuclear bombs spiked with “Cobalt-Thorium G.” These bombs were rigged to go off if the Soviet Union were to suffer a nuclear strike, thus serving as the ultimate deterrent. Unfortunately, the Soviets failed to announce the existence of this system, and as the Dr. Strangelove character scolds de Sadesky, “The whole point of the doomsday machine . . . is lost if you keep it a secret!”
North Korea’s underground test of a nuclear bomb yesterday wasn’t any secret. It wouldn’t serve that nation’s aims if it were. But it is nevertheless interesting to explore how such nuclear tests are detected from afar and whether North Korea could hide such activity if it wanted to.
Four distinct technical systems have been established to detect clandestine nuclear explosions: incorporating seismic, hydro-acoustic, infrasound, and radionuclide sensors. These systems were put in place to support the Comprehensive Nuclear Test Ban Treaty, which was adopted by the U.N. General Assembly in 1996 and which 159 nations have so far ratified (not yet including the United States).
The infrasound sensors detect the low-frequency pressure waves that would result from an atmospheric nuclear test, and the hydro-acoustic sensors detect similar pressure waves from explosions in the oceans, so neither of these systems would be a great value in this instance. The radionuclide-monitoring stations can be used to sniff out radioactive gases generated during a nuclear explosion, so they should prove useful in any event. But it takes some time for the wind to carry these telltale atoms to the monitoring stations, which are situated around the world.
Were we lacking an announcement from North Korea, the detection and characterization of its bomb test would probably hinge at this point entirely on seismic measurements. So it’s interesting to ponder how the seismologists can tell the difference between a nuclear bomb and a run-of-the-mill earthquake.
To understand that, you need to know a little bit about how earthquakes are routinely monitored. When a fault gives way, the earth moves very suddenly, producing seismic waves, which propagate out in all directions. Some waves go upward and reach the surface nearby; others dive downward and can travel a lot farther before they ultimately surface—sometimes on the other side of the world. So seismic-monitoring stations don’t have to be close to the epicenter to detect the action. Indeed, having measurements from many stations at many different distances and directions from the source is important because the motions they register allow seismologist to learn about how the causative fault moved.
Some earthquakes, like those on California’s famed San Andreas fault, or those along the ocean floor’s many transform faults, result from what geologists call strike-slip motion: One side of the fault shifts to the left, while the other side shifts to the right. Others occur on what are called normal faults, where the overhanging side of the fault drops downward—the mid-ocean ridges are rife with such faults. Yet other earthquakes, often some of the word’s biggest, result from thrust faults, where one side of the fault overrides the other. By examining the highly asymmetric pattern of first motions registered at different seismic stations, experts can infer what sorts of motions took place at the source, and that can be an excellent clue as to the nature of the event.
When you set off a big bomb underground, you would generate seismic waves. As with an earthquake, these waves would go zooming out all over the world if the energy released is large enough. The pattern of first motions is, however, quite different from what happens after an earthquake. It won’t show that one side of a fault went one way while the other side went the opposite. Rather, the set of seismic measurements would show that the initial motion was uniformly outward all around the source.
Seismic measurements can provide other clues as well. For example, explosions should, in theory, only produce compressional (“P”) waves within the body of the earth, whereas earthquakes also generate shear (“S”) waves, ones for which the displacement of the rock is transverse to the direction of wave propagation. Similarly, explosions aren’t normally able to create Love waves, a transverse seismic wave that travels along the earth’s surface, whereas earthquakes can.
Sounds simple to finger nuclear blasts this way, right? In principle it is, but in practice there are complications that can make discrimination tricky. One is that not all underground explosions are clandestine nuclear-bomb tests. In particular, buried conventional explosives are routinely used around the world in mining. But mining blasts are, in general, carefully orchestrated to take place, not a single big explosion, but as a rapid series—ripple firing as it is called—and those events produce a characteristic seismic signature.
Despite these complications, seismic measurements are pretty reliable for monitoring nuclear tests, for the simple reason that they can tell you where and when the energy was released. If the event took place tens of kilometers deep within the earth, or far out at sea, for example, chalk it up to an earthquake. If, on the other hand, if it was within a few kilometers of the surface and happens to be located on North Korea’s Punggye-ri Nuclear Test Facility . . . well, you won’t need an advanced degree in seismology to figure out what happened.
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