McGillX | MCGATOCXT314-G003500_TCPT

PROF JOHN STIX: Today we're going to start talking about the topic of tsunamis, big waves. This is a

famous painting done by Hokusai showing a big wave and Mount Fuji, in Japan, in

the distance. A very famous painting, iconic classic view. And some tsunami waves

look like that, and many waves don't look like that. And we'll look at the reasons why

that is during this lecture on tsunamis.

This is an amazing picture of damage after the March 2011 Tohoku earthquake in

Japan, which created a big wave, a big tsunami. And you can see this area of flat-

lying coastline, the inland part of which has been completely flooded by the tsunami

wave. A fire occurring in the distance. Major, major destruction from this event.

So what is a tsunami? It's a wave that is generated when there is some sort of offset

in the ocean floor. So the ocean floor is offset. And the water in the ocean is also

offset. So when the water in the ocean is displaced, the wave, the tsunami wave, is

generated. So we have to have some mechanism to cause that offset, and there's a

number of mechanisms which displace water.

Tsunami is a Japanese word, it means harbor wave in Japanese. And the reason

it's named so is because the wave, when it is in open water, it is moving. It's very

small and it's moving harmlessly. And it's only when the wave gets close to the

shoreline and enters a harbor, for example, that it becomes very destructive.

And people have called these tsunami waves tidal waves. And that term is

completely wrong. So mark a big X where it says, when you see tidal wave, when

they mean tsunami wave. Because it has absolutely nothing to do with tides. It is

true that if you have a high tide situation and a tsunami comes in the impact is going

to be greater than if the tide was low. But the tides, the movement of the tides, are

completely separate from anything to do with the tsunami wave. Very important

point.

And tsunamis are very dangerous. And another important point is that they are

actually quite common. They kill typically thousands of people over a decade, and

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sometimes many, many more people. So during the 1990's alone, a few thousand

people were killed. And these are some occurrences of tsunami waves that came in

and killed people around different parts of the world.

And you will see that almost all these occurrences are occurring where? They are

occurring in the Pacific Ocean basin. Around the Pacific Rim, or the Pacific Ring of

Fire, where there are subduction zones. So that association between tsunamis and

subduction zones is very important. Mainly driven by earthquake activity, but also by

volcanoes.

And a really important epic event, which occurred on the 26th of December, 2004,

was the Boxing Day tsunami. Which was generated by a very powerful, greater than

magnitude 9.0 earthquake, off the coast of Sumatra in Indonesia. And this wave

radiated right across the Indian Ocean basin. And you can see from this slide the

countries that were affected by the wave.

Essentially the entire Indian Ocean basin was affected by this wave sweeping

across the ocean. And it's hard to believe, but a quarter of a million people were

killed by this wave. Mainly in Indonesia, but a lot of people lost their lives in Sri

Lanka and Indonesia and India and other countries as well.

And then more recently, very recently, the March 2011 magnitude 9.0 earthquake,

the Tohoku earthquake off the coast of Japan created another very, very large

tsunami. Which impacted greatly upon the coast of Japan. But also swept across

the Pacific Ocean basin, hitting South America, Hawaii, North America, and so forth.

So most of the impact was in Japan, but it was felt across the entire ocean.

And let's keep in mind as we talk about tsunamis the very, very important Pacific

Ring of Fire subduction zone association in terms of generating these waves. In

terms of tsunamis this is where most of the action is. Tsunamis can occur in the

Atlantic Ocean. They can occur in the Indian Ocean, as we've just seen. But really

the focus here is on subduction zones, big earthquakes, and sometimes volcanoes

as well.

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And let's finish this introductory part of the lecture by looking at Cascadia, the Pacific

Northwest of Canada and the US, where there is a subduction zone. Here is the

subduction zone right here. And this area has the potential for generating large

earthquakes, magnitude 9.0 and greater earthquakes. And so if you can generate

such a large earthquake the potential for generating a tsunami also is great.

And if you generate a tsunami in this region right here, this has a lot of hazard

impacts. Because it doesn't take very much time for the wave to be generated and

then hit the coast. And of course there are a lot of people living along the coast in

the Pacific Northwest. So this is a very important point, and people who live in

Cascadia are certainly thinking about what to do, how to mitigate the effects of a

large earthquake and tsunami.

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