SPECTRUM: Could you discuss the nature of the global network of seismology stations used to detect explosions, such as the North Korean test?

RICHARDS: Broadly speaking, there are three different types of network that matter here. There’s the type of network that’s operated by national technical means unilaterally. For example, the United States has agencies with responsibility for explosion monitoring, and they do the work mainly with their own network. The international community associated with monitoring the Comprehensive Test Ban Treaty of 1996 works from a headquarters in Vienna that is currently building up a global network of about 321 stations. They use four basic monitoring technologies: seismology, hydro-acoustics, infrasound, and radionuclides. So those are two broad, organized networks of sensors.

But there’s a very important third network, namely the loose aggregation of thousands of seismometers and other types of instrumentation operated for general purposes of research around the world. Whenever some interesting seismic source is recognized, one can go and search for a station that was not established for any monitoring purpose but was simply in the vicinity. And in the present case of October 9, there was a station that was operated for general purposes of seismological research that provided excellent signals because it was at a distance of only about 350 kilometers to the north.

SPECTRUM: How are recordings from various seismology stations correlated into the needed data to determine whether an event such as the explosion in North Korea on 9 October satisfied a conclusion that a nuclear device had been detonated?

RICHARDS: Working from the highest quality recordings from the morning of October 9, my colleague Won-Young Kim at my institution, was one of many seismologists who was able to get, over the Internet, the signals recorded at that station 350 kilometers to the north; but then he was able to find examples of a small earthquake recorded at the same station and at about the same distance of 350 kilometers; and it’s a comparison of the two signals recorded at the same station that gives us great confidence that the event on October 9 was explosive.

I’ve already mentioned one particular seismic wave we could see from that October 9 event that indicated that the seismic source was very shallow, unlike an earthquake. There are two other characteristics of the seismograms in the case of both an earthquake and an explosion. Both these sources put out compressional waves, that’s like sound waves or P waves where the motion is longitudinal in the same direction that the wave is traveling; and they both put out shear waves in which the particle motion is transverse to the direction in which the wave travels. Sometimes we call them P waves for longitudinal and S waves for the shear waves. The latter are much more efficiently excited by earthquakes, and compressional waves are more efficiently excited by explosions. The P to S ratio of those two different wave types is much larger for the explosion than for the earthquake.

SPECTRUM: What's the relationship between seismic readings and explosive yield of a nuclear detonation?

RICHARDS: Essentially, you look at the seismic magnitude of the signals. There are a number of different scales that are used for that purpose. One of the common ones is to look at the size of the P waves recorded at great distance and then, in practice, look at the largest amplitude of those P waves and work out what the ground motion at that distant station must have been, taking the logarithm of measured ground motion and making a correction for the distance. That is traditionally how the Richter body wave seismic magnitude is calculated from an observation, and it’s long been known that there’s a linear relationship between the magnitude and the logarithm of the yield in kilotons.

For example, for explosions conducted in hard rock the yield is a linear relationship for the magnitude against the logarithm of the yield. The relationship is slightly different for soft rock. There’s some scatter in the data points about a straight line fit, but that’s the type of relationship that’s long been used to get an approximation of the yield from a seismic signal.

SPECTRUM: Was your team able to make a conclusion about the physical explanation of the 9 October explosion and, if so, when?

RICHARDS: Well, we didn’t detect it with our own instruments, but we simply used data that other organizations had gathered, first of all to get information on the source of the event, and the U.S. Geological Survey did that work. But just a few hours after the event happened, we did our own work getting the data from the station operating in southeast China and drawing our own conclusions to identify the signals as indeed having come from an explosion. So we did that work ourselves, although it wasn’t done with our own instruments.

SPECTRUM: As a scientist, what are your thoughts on the North Korean nuclear test?

RICHARDS: This is a very dangerous development, a step back from the non-proliferation of nuclear weapons, which has been the subject of a treaty that went into effect in 1970. I’ll go on to express the hope that ways will be developed to restore confidence in that treaty, and that we won’t just to see North Korea and many other countries stepping away from it.