Tuesday, 12 February 2013

Other stuff our research can be good for - or how we know that North Korea blew something up

So the North Koreans apparently conducted a nuclear test. How do we know this? Because of seismology! The CTBTO (Comprehensive nuclear Test-Ban Treaty Organization, http://www.ctbto.org/) has seismometers all around the world to detect potential nuclear explosions. But seismometers, aren't those the thingies that record earthquakes? That's right. But an earthquake and an explosion are very similar: Both make the ground shake, quite a lot if you're close by or it's a really big earthquake/explosion, and only a little if it's a smaller event. A seismometer has a mass with a needle with ink attached to it (ok, this is how seismometers USED to work back in the day, now it's all fancy electronics, but the principle remains the same). When the ground shakes even just a little bit the mass wants to stay where it is (inertia! Ever experienced that? It's that feeling when you're watching a movie and you feel like eating some chips, but you don't want to get off the couch to get them out of the kitchen cupboard, because you need energy to get up. Isaac Newton was pretty big on inertia, I bet he always wanted to sit on his couch and watch movies - or whatever the equivalent was back in the day. You might also know what I'm talking about from driving your car: If you slam on the brakes because you're trying to not run a red light you feel your body moving forwards - it wants to maintain its current state, which is moving forward. Anyways, back to seismometers.). So the mass wants to stay put, but the ground moves under it, so the needle attached to the mass draws wiggles on the piece of paper that's attached to the ground - boom, you have a seismogram! That's also how the volcano generated wiggles are recorded that I was talking about last time.
Because in an explosion the ground shakes too, we can see it on our seismograms. People have worked very hard to figure out what type of ground movement causes different types of wiggles, so now we can use that knowledge to figure out the type of ground movement by looking at the wiggles. That's also how scientists know whether an earthquake happened e.g. on a subduction zone (Indonesia, Japan) or on a so called strike-slip fault (San Andreas Fault!). 
Ok, now we have some wiggles on paper and think they look like they might come from an explosion. But where did this happen? This is why we need several seismometers in different places! If you drop a pebble in a pond it generates waves, and they grow bigger and bigger around the place where you initially dropped your pebble. The same happens with seismic (ground shaking) waves. If we look at a few different stations and the exact time at which this explosion wave arrived we can backtrack where it came from. Once we know that, just be sure, we can look at whether there have been earthquakes recorded in the same area before. Here's a map of earthquakes around North Korea since 2004 (data courtesy of Google Earth and the GEOFON program, http://geofon.gfz-potsdam.de/).
Circles are past earthquakes since 2004 around North Korea. The two red dots are the nuclear tests in 2009 and 2013.
As you can see North Korea doesn't usually get many earthquakes, and the location of the event yesterday is suspiciously close to the location of the 2009 nuclear test. And this is what the seismograms look like:
Nuclear tests conducted by North Korea in 2006, 2009 and 2013. Image courtesy of NORSAR.

So this is how we know that there was indeed a nuclear test conducted by North Korea. Without science, seismometers might have never been invented and we might not be able to know whether anything happened at all. Granted, without science we might also not have the technology to have nuclear explosions in the first place, but that again is a whole different story... Still, isn't it cool how we can sit somewhere peaceful and awesome and yet find out within minutes of an event (North Korea is some 70º from Vancouver, which means the waves took just over 10 minutes to get here) that it has happened thousands of kilometers away?