Photo: Lamont-Doherty Earth Observatory

Paul G. Richards is the Mellon Professor of the Natural Sciences at Lamont-Doherty Earth Observatory of Columbia University, in Palisades, N.Y. For the last twenty-plus years, his work has focused on the use of seismological methods to study nuclear weapon test explosions and their implications in both the scientific and political arenas. He has served as an advisor on nuclear arms control at the U.S. Department of State, a visiting scientist at the Lawrence Livermore and the Los Alamos national laboratories, a technical expert to the Comprehensive Nuclear-Test-Ban Treaty Organization, and a scientific panelist for the National Academy of Sciences' report "Technical Issues Related to the Comprehensive Nuclear Test-Ban Treaty," among many other assignments in the field of nuclear testing detection and measurement.

From 2000 to 2003, he led an applied research project to improve the accuracy with which treaty monitoring organizations routinely locate seismic events, particularly in Eastern Asia. He describes the work of event location as the "last corner of seismology that is still dominated by methods developed during the era of analog recording." He says that traditional methodology has been greatly enhanced by the practice of measuring comparable seismic events in the same locale simultaneously and "locating each one relative to its neighbors, preferably using waveform cross-correlation to measure relative arrival times."

He spoke with Spectrum Online soon after the North Korean government announced, on 9 October, that it had detonated its first nuclear device underground, on the science of detecting and measuring such events.

SPECTRUM: How do scientists use seismology to distinguish powerful explosions from earthquakes?

RICHARDS: The detection of the two is more or less the same on the question of actually detecting the signal, but then the way we make the identification from the seismic end is based on the fact that each of these different types of seismic source puts out a mix of seismic waves and the mix is very different for earthquakes than it is for explosions. For example, an earthquake will almost always be greater than 5-kilometers depth in the earth, whereas explosions typically would always be much shallower than that. There are some seismic waves that, uniquely, are observed just for very shallow sources, so that would be one example.

SPECTRUM: How sensitive can seismology stations be in detecting events such as a small-scale nuclear detonation?

RICHARDS: Well, you hear my hesitation. There’s such a huge variety of signals and the answer depends on how near the station is, or out to what great distance, or on the frequency range, and so on. So it’s a little difficult to give a direct, simple response. In the present case of interest, an event in North Korea of approximately magnitude 4, that is something that can be detected at seismographic stations all around the world. Except to say that when you get to very great distances, say on the other side of the globe, you can only see that type of signal if you’re working with a very quiet station.

But that’s just the detection. In order to be very confident in making the actual identification, typically those very distant stations aren’t a whole lot of use for a small event. You really need to get high-quality recordings, preferably from distances not more than a few hundred kilometers. And again in the present case, there are a number of stations in South Korea, in Japan, and in southeastern China that captured very good signals at a relatively short distance on October 9, which enabled the work of identification to be done with high confidence.

SPECTRUM: Seismology is well-established science. What new developments are helping to refine our analysis of seismic events?

RICHARDS: For one,I think the ability to record signals at higher frequencies than we have usually cared about with earthquakes. So, for example, with earthquakes, almost all of the scientific work is done to study earth’s structure and to do earthquake location with signals at frequencies of not more than about 10 hertz, and a lot of the work is done even with signals of not more than about 2 hertz. For many years, seismographic instrumentation didn’t perhaps pay as much attention as it should have done to the recording of signals at much higher frequencies, which are more efficiently excited by explosions—where one needs to go up to, at least, 20 hertz to do a good job of discrimination. This is because an explosion generates typically higher frequencies than an earthquake of comparable overall amplitude of seismic signals.