Science Sunday: Tsunamis
I have been spending the last 10 days in Japan (and yes, it’s awesome). So I thought that I would use that as a rather thin excuse to talk about tsunamis, because it’s an interesting phenomenon. A tsunami is a wave, or multiple waves, that are in many cases bigger than your everyday waves. They are usually caused by some form of shock, like an underwater earthquake or glacier calving. Tsunamis are often termed natural disasters, because of the damage that the more noticeable ones tend to do.
The word tsunami is Japanese, as you might have guessed, and literally means “harbour wave”, which you might have not. They have also been referred to as tidal waves. Neither of these are completely accurate, since they don’t always hit harbours and have nothing to do with tides. With that little linguistic nitpick aside, let’s look more into these harbour waves. Generally, a tsunami can be divided in to three phases, namely the generation, the moving and the impact.
Tsunamis can be created in many ways. Some of those that we know of are landslides, volcanic eruptions and underwater explosions. The common factor between all of these is the displacement of a large amount of water. In the case with a landslide or a glacier calving, the water will be displaced by ice or mud. It is roughly the same for underwater earthquakes, where sliding tectonic plates pushes the water upwards, and by that transfers enough energy to cause a tsunami. Still, a lot of the science behind the generation of tsunamis is not very well understood. We know that some tsunamis are caused by underwater earthquakes, but not all underwater earthquakes cause tsunamis, even when they are powerful.
The part where the tsunami moves outward from the point of origin is quite interesting, because it is actually kind of hard to detect. No tsunami will start out as a monstrous wave of several tens of metres in height, but be more in the range of, oh, say 30 cm to 1 m. This isn’t very different from your everyday wave, but the difference lies more in the wavelength of the wave. The wavelength is the distance between two crests, and in normal wind waves this length is usually around 100 metres. In a tsunami, the wavelength will be able to reach several hundred kilometres in deep sea. In other words, it will displace an amount of water that is several orders of magnitude bigger than a normal wave.
The last phase of a tsunami, the impact, is the part that has been given most attention by the media. Not undeserved attention either, as it is the part that usually affects people the most. A classic tsunami increases substantially in height as it nears the coast, and this is caused by the reduction in ocean depth. As the water grows shallow, the waves become compressed, meaning that the wavelength becomes much smaller. When this happens the water has only one way to go, and that’s up, resulting in a truly enormous wave. Many tsunamis can also come in multiple waves with several hours in between, which makes them very unpredictable.
Another, smaller part of the last phase is the drawback. A drawback often happens when the tsunami is really big, and can cause a big part of the seabed to be exposed. They happen as the tsunami is being compressed, and are caused by the wave drawing back the water in front of the crest. This might make it seem as if the tide is going out very fast, hence the name “tidal wave”.
Even though the end result of a tsunami is a massive wave, its origin is rather small. Therefore, few tsunami warning systems are yet very reliable, and predicting tsunamis far into the future is on even more shaky ground than predicting volcano eruption. So, tsunamis are great demonstrations of the force of nature, but hardly one that you’ll want – or probably ever have the opportunity – to see up close.