How Do Earthquakes Cause Tsunamis?

Chris - So James, tell us about what actually happened in Japan. There was this huge tsunami that surged in, tell us a bit about tsunamis and how they tend to happen.

James - The sort of tsunami we're talking about follow big earthquakes and the tsunami that followed the big Japan earthquake was much bigger than people anticipated. This was extremely interesting to us. The Japan earthquake itself was huge, it was one of the 4 or 5 biggest earthquakes over the last 100 years. But even the biggest earthquakes, we think we can anticipate the size of the tsunami which goes with them. But in this particular case, it was much bigger. That's what caused the trouble, it drowned lots of people because the tsunami was a bigger wave than we thought it was going to be and it also of course caused the damage to the nuclear power station at Fukushima.

So, trying to understand the connection between the earthquakes and the tsunamis is very important and trying to understand what happened to make that one so special is also something we need to understand in order to plan for it in the future.

Chris - What did happen?

James - Well, you have to understand what happens when earthquakes happen.  Earthquakes happen when rocks break and they slide past each other. As they slide past each other these rocks on a surface, you can imagine a knife cut in the Earth and the rock slide past each other - rather like rubbing two bricks together. The vibrations make sound, the sound travels through the Earth, the sound shakes buildings. That's what normally causes the damage. But what you also do is move the rocks past each other and if you move them past each other on the seabed, the sea has to get out of the way. You can't just change the shape of the sea bottom without the water going somewhere and that's what makes a wave at the surface. The wave at the surface in this case was very big because the seabed moved a lot more than we thought it ever could.

Chris - How much?

James - In this case in Japan, the seabed itself moved up to 50 meters. To put that in perspective, 50 meters is twice the length of a big swimming pool. That's a huge amount to move in one go. We didn't think it was possible.

The way earthquakes happen is that although the rocks will slide to pass each other on this surface, the surface is held together by friction. So, as you push harder and harder, the rocks bend like a spring until eventually they can't bend anymore and they go twang and they move. But you try bending rocks 50 metres. It's a lot to bend a rock. That was the surprise. We've seen earthquakes which have moved 10, even maybe 20 metres before in one go, but 50 was a lot more than anyone thought was possible. And so, once we realised and there were clever ways of measuring how much had moved, that was another special feature about Japan, that the Japanese are very, very good at observing what happens. We had fantastic information and it was quite clear that this movement was as much as 50 meters. At that point, it was clear that there was something going on which we didn't understand. It wasn't the only earthquake which has done this. There have been at least half a dozen earthquakes in the last 50 years which were also puzzles in this sense. They made...

Chris - How did you try and find out what it caused it?

James - Something had happened to all of these earthquakes which had made the tsunamis bigger than they should've been. And so, what we had to do to find out what caused it was to find out why the seabed had move so much. You can't really store 50 meters in rocks and it was clear something else had happened.

What had actually happened was that although the rocks had moved, they moved in a particular way. The easiest way for you to imagine this is if you imagine a triangular wedge of wood on the floor and if you tread on it, it shoots forward a lot. So, you don't have to tread on it much and then it will shoot forward a great deal. That's the extra effect which happened in Japan. So, we discovered that, by looking very closely at what the seabed look like afterwards.

Chris - Got a question here and we'll be taking questions from you in a second. So, if you have any questions about earthquakes in general or specifically Japan, get ready to put your hands up. Got a question here from John Berkit by email - chris@thenakedscientists.com, James and he says, "How has the earthquake in Japan affected the rest of the world?" Because there were some claims going round that it sort of knocked the Earth a bit off-kilter in the way it rotates and that kind of thing? Is that true?

James - Yes, the length of the fault, the break which moves was about 400 km and moved up to 50 metres, that's moving around a lot of rock. It actually does change the shape of the Earth. It affects the orbits of satellites and we can measure all these things and the vibrations as Arwen will probably tell you, if you hit the Earth with something that big, it vibrates like a gong for something like 6 months afterwards and we can measure that too.

Chris - And you can see the waves bouncing around.

James - You can measure them not see them, but you can actually measure them with modern equipment, yeah.

Chris - Questions from audience. Put your hands up if you'd like to ask anything about earthquakes.

Ewan - I'm Ewan from St Neots. My question is about earthquakes and tsunamis. When an earthquake hits and a tsunami comes up, how far the distance of the tsunami wave can go from when it collapses on itself?

James - That's a good question. What happens is you make a wave because you move the seabed and that wave can keep its shape and it can travel all the way around the world. So, the wave which started off in Japan could cross the Pacific, it could enter the Atlantic Ocean and even reaches up here. By the time it gets here, it's smaller, but it still has the same shape. It takes about 30 hours to get here from Japan. It will travel about the speed of a jumbo jet and it's quite small. If you were in a middle of the ocean when this thing went past, it's only about a metre high and it's about 300 km in the length - the wavelength of this wave and you wouldn't notice it. If you were in the middle of the ocean on a flat calm day, you wouldn't see it. What happens though, what causes the problem is it slows up when it reaches the shallow water of the coastline and because it slows up, the water piles up and up, and up, and up. By the time it hits the coast, it may be 40 metres high.

Chris - Over here...

Fergal - Yes. My name is Fergal and I'm from Cambridge. I was wondering how does the depth of the source of the earthquake influence the size of the resulting tsunami?

James -  Only that the deeper it is, the less it will move the seabed. So, what you heard about on the news quite a bit at the time of the Japan earthquake is that the fault which moved, the break between the rocks came all the way to the seabed and upset it a lot. It moved the seabed much more than it might otherwise have done. If the fault doesn't come to the surface, it's rather like taking a telephone directory. If you move the top over the bottom, because of the binding, it will turn into a fold which is much less dramatic than if it just breaks altogether.

Mark - Hi, there. My name is Mark. I'm from St Neots. I was just wondering how much rock type affects to how this works?

James - That's an excellent question - a very large amount and one of the problems in Japan is, although the surface of the seabed moved 50 metres, it was mud. Mud has no strength. How can you possibly bend mud 50 metres? You can't do it. You know it. You can just about imagine that a really strong rock, you can bend a bit, but soft mud, you can't. And that's why we realised there must be some other process going on which we didn't know about and didn't understand.

Chris - Tehnuka, do you see volcanoes getting triggered by earthquakes?

Tehnuka - I think that's still an area that people aren't quite sure of. We do get earthquakes on volcanoes which are related either to the rocks fracturing as gas and magma moves up through the volcano or you might also get earthquakes that are from just the fluid moving itself. These earthquakes are usually a lot smaller.

Chris - Arwen, the waves that James is talking about, ricocheting around the planet, they must give scientists like you quite a good insight into what's inside the Earth because you can sort of use that data when something big happens?

Arwen - Yes, that's right. What you have and you need a big earthquake for it, like the one in Japan. The whole Earth will ring like a bell. It's a bit like a musical instrument. Say, you hit a drum then there's a certain tone coming off it. What we seismologists can do, we can listen to these tones. For the Earth, they go on for a long time. They can go on for weeks, but for a big earthquake like the one in Japan, they can go on for months. We listen to these different tones and if the Earth was very simple and didn't have things like plate tectonics and volcanoes then the Earth would be perfectly in tune. But what we find is the Earth sounds slightly out of tune and we can listen to that and that will tell us about the deeper parts of the Earth.

Brendan - Hi, there. I'm Brendan from Cambridge. Are you familiar with theory that a lot of very big earthquakes in the last decade are sort of linked and whilst not aftershocks of each other, are sort of caused by each other and might lead on from each other?

Chris - So, can one earthquake beget another earthquake?

James - Yes, a lot of people are thinking about this at the moment. If you look at the biggest earthquakes over the last 100 years, about 5 of them have happened in the last decade. You would've said the same had you been around in the 1950's and you do get this clustering of big earthquakes. It's hard to tell whether it's really random or not based on statistics, there may be good reasons why one big earthquake can trigger another. If you actually shake the rocks around, you move things around, that may be the last tiny bit of effect you need to actually make some other fault slip in a different part of the planet. It's not impossible. It's rather difficult to prove, but we do know that one earthquake can trigger another one from the activity that you see actually around volcanoes very often. After a big earthquake, little earthquakes around volcanoes very often increase in frequency a lot and that's probably because you're knocking fluids which are sitting around in the Earth and making them move.

Jess - My name is Jess from St. Ives. During this last century, have the regularity of earthquakes increased and the size of them increased, or has it just been because we've been collecting more data?

James - No. There's no sign that the Earth has changed its behaviour. There's a lot of sign that we have changed our behaviour. So, earthquakes in the last century have killed far more than earthquakes in all the previous thousand years put together. And that's because we concentrate cities in dangerous places. About half the world now lives in major cities and many of those cities are places that have been destroyed in the past. But in the past, it was quite difficult to kill even 10,000 people. Now, we routinely kill 100,000 people in these big earthquakes in major cities in dangerous places.