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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 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.

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 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.

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