McGillX | MCGATOCXT314-G003600_TCPT

PROF JOHN STIX: Now I would like to start discussing some of the principles behind these very large

waves. So let's define some terms here. This figure shows the structure of a wave.

And in this case, it could be any wave. It could be a wave in a bathtub, or a wave on

the shoreline, or a tsunami wave.

But we have the crest or the peak of the wave located right here, for example, right

there. And then there's the trough of the wave. Here's one trough and here's

another trough here.

And the amplitude of the wave is from the still water line to the crest of the wave or

the peak of the wave. And then the wave height is simply from the trough to the

peak. And then the wave length of a wave is the distance from one peak, OK, to the

next, or from one trough to the next trough, OK? That's the distance.

So the take-home point here, in terms of tsunamis, is that the wavelengths of

tsunamis-- especially in open water, especially in the open ocean where the water is

deep and so forth, not near land-- these wavelengths can be very, very large, 10s to

100s of kilometers. And that's actually hard to conceive of. So this is something you

cannot actually see in the open ocean. This distance is just too large for you to

actually see the wave length, compared to a normal ocean wave which might have

a wave length of 50 meters, or 100 meters, or something like that. So that's maybe

the most important thing that distinguishes tsunamis from other types of waves.

And when we look at how fast a wave goes, we can look at the distance it takes for

the wave to move from point A to point B. So here, for example, we have the crest

of a wave at point A. And that crest is going to travel to point B. So that travel time,

basically the wavelength of the wave, that travel time from point A to point B is

called the wave period. And so to calculate the speed of a wave, how fast the wave

is going, it's the wavelength of the wave divided by its period. Very important point.

And so now, let's look at the speed of a wave near shorelines, near the coast, and

then in open water. So in shorelines, we'll use this equation here. The speed of a

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wave is related to the square root of the gravitational constant multiplied by the

water depth. That gives you a pretty good approximation for how fast the wave is

moving close to shore.

But then when we go to open water, we look at this equation here. We use this

equation here, where the speed of a wave is related to the square root of the

gravitational constant, not multiplied by the water depth, but multiplied by the

wavelength of the wave. Which of course, in the case of tsunami, are very, very

large. Divided by 2 pi.

So it turns out when you do these calculations, you see that tsunami waves in the

open ocean are moving at hundreds of kilometers per hour very, very fast.

Typically, 700, maybe, to 1,000 kilometers per hour. And in a sense, that's hard to

conceive of, but think of a jet airplane. A jet airplane is flying somewhere between

700 and 1,000 kilometers per hour. So a jet airplane is flying at about the same

speed as a tsunami does as it's moving in the open ocean.

So in the open ocean, the wavelengths are very, very large. And because of that,

they can sense the bottom of the sea floor. So the sea floor may be very, very deep.

It may be a few thousand meters deep. And yet, because of this very, very large

scale of the wave, the wave senses the bottom of the sea, the sea floor, and the

topography of the sea floor.

For example, a mid-ocean ridge. The mid-ocean ridge might influence, in part, the

direction of a tsunami. So the tsunami might be propagating this way, but it can also

be steered somewhat by the nature of the topography of the sea floor. Of course, if

the topography is completely flat, there's no influence. But in a mid-ocean ridge, for

example, that can help funnel, or channel a tsunami wave.

And it's very important to make the distinction between what is a distant tsunami

and a local tsunami. A distant tsunami is a tsunami that's generated far away. For

example, a tsunami that's generated by an earthquake in Japan, where the wave

moves across the Pacific Ocean and then arrives in Central America, for example.

There's plenty of time to warn people and to let people know that a tsunami is on

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the way and they need to get out of the way of the tsunami.

But a local tsunami is one that's generated very close to where people live, for

example. So think of a subduction zone. Here's a subduction zone, and an

earthquake is created, is generated by the subduction process. And then a tsunami

is created from the earthquake, and the tsunami propagates to the beach, which is

right here in the subduction zone.

It happens very, very quickly. The tsunami moves so quickly that you only have

about a half hour to get out of the way. So it's a real problem, in the case of a local

situation, to warn people properly. It's very difficult to do that. So this is a real issue,

in terms of trying to mitigate against tsunamis when the tsunami is local and very

closely located to the shoreline.

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