Friday, October 13, 2006

 

Guest blog: Teachable Moment: North Korea Nuclear Test

A friend of mine in Japan had a great post abot the North Korean nuclear test at his blog. It's reproduced, with his permission, below:

North Korea announced that they performed an underground nuclear test this week. This is obviously bad news no matter what, but how can we verify that they are telling the truth? That's where seismology can help!

Scientists here at the Earthquake Research Institute just sent out information about the North Korea nuclear test, as recorded by seismometers here in Japan. Take a peak at the seismograms here.

Read more for a quick seismology lesson that explains what they mean.

Here is a brief seismology lesson. Let's begin by looking at a regular, natural earthquake: Click here to view some example seismograms. Earthquakes are sudden releases of energy, a lot like explosions. The seismic waves they release travel through the earth and can be recorded by seismometers literally on the other side of the world. However, it takes time for the waves to travel this distance and some waves travel faster than others. The example link above shows recordings of ground shaking from a number of different recording stations around the globe. Time starts at the bottom in these images and goes up. The line starts out straight at the bottom and then there there is a sudden increase in energy (p-wave) represented by wiggles on the seismogram. Almost all the lines then show a second, larger, burst of energy a few minutes later (further up on the plot). This comes from slower traveling s-waves. It's a lot like a running race where everyone starts at the sound of the gun, but they arrive at the finish line over a certain period of time because some of them travel faster than others.

What are these waves and why are some slow and others fast? Different directions of motion generate different types of waves. Imagine that I am standing behind you with my hands out in front of me and resting on your shoulders while we do a conga line. I can move my arms such that you rock side-to-side, or I can push-and-pull you forward and back in the direction that we are walking. Earthquakes mostly involve "shear motion" caused by sliding along faults (side-to-side motion). Some of you may have seen me use my hands to demonstrate earthquake motion by having one hand slide past the other. Explosions, however, have very little shear energy and produce mostly forward and back blast-like motion. It's easy to imagine getting thrown backwards by the energy of a blast (and fun to demonstrate an explosion with your hands -- they fly apart rather than slide past one another). In an earthquake, you'd more-likely get thrown to the side (relative to the direction of where the earthquake is coming) because earthquakes involve more side-to-side motion. It turns out that the different directions of motion travel through rock at different speeds. I won't go into the reasons why. But this is why there are two main bursts of energy in earthquakes -- the faster forward-and-back motion called P-waves and the slower side-to-side motion called S-waves.

It's important to remember that even though earthquakes have more energy in side-to-side motion, they release energy in BOTH both p and s waves. Why? Even though earthquakes are caused by side-to-side motion along faults, ruptures have ends and there is pushing or pulling near these endpoints that generates some back-and-forth wave motion.

Let's take another look at the nuclear test seismograms. In these, time starts on the left and marches to the right. Each horizontal line is the recording at one location in Japan. Here, there is a lot of background noise before the blast energy arrives, so the line is not perfectly straight on the left side. Most of this background noise is caused by storms and ocean waves and is not actually related to earthquakes or explosions. All of the recordings show a single big increase when the first, fastest waves arrive. Which horizontal line is closest to the blast? The top one because the waves arrive at it sooner. While shaking continues for some time in these seismograms, there is NO second burst. That's because this was an explosion that had very little side-to-side energy, and therefore no s-waves.

Other aspects of the waveform can tell us where the nuclear test site was (the precise location in North Korea) and using some fancy math we can use the amplitude of the waves to calculate the yield of the explosive device. Now you are a trained seismologist and can recognize the difference between a natural earthquake and a nuclear blast. Let's hope you don't have to use this information any more...

UPDATE: Posts at WG about the North Korea test

Radioactive material detected from North Korea

Guestblog: Teachable Moment: North Korea nuclear test

More recordings of nuclear blasts

North Korea nuclear test

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