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The final destructive phenomenon that can happen in
association with an earthquake is a giant wave called a tsunami.
Now what is a tsunami?
In the old days, they were referred to as tidal waves.
It's actually a misnomer,
because they have nothing to do with the daily tides that take place.
But they are similar to tides in the sense that they are not
like strong waves where big wave front comes in and then is gone.
Rather, they are more like a situation where the water level rises and rises and
rises as if a giant plateau of water is coming in.
So, it's like a mega tide that does move inland and
submerge areas of the coast that are at lower elevations.
So how does this work?
Well, let's imagine that we have a fault that
undergoes sudden displacement beneath a portion of the sea.
When that fault takes place, the surface of the sea is actually displaced.
Now the area that can be displaced can be huge if it's a very large earthquake.
We can be talking about 1000 of square kilometers of
the seafloor being displaced.
Not by much, maybe by centimeters to a few tens of centimeters at most.
But such a large area is affected by this displacement that a huge
volume of seawater is affected.
Now, what happens is that volume of sea water rises up.
And then because of gravity,
begins to migrate away from where the displacement took place at depth.
And that surface of elevated seawater that migrates away is the tsunami.
Now out in the open ocean, perhaps above where that,
that earthquake has happened, the amount of sea level rise is minimal.
Maybe centimeters, at most 10 centimeters or so.
But the volume of water that's moving,
the volume of water that's being affected by the passage of that wave is huge.
So, it can travel across huge distances of the ocean at about a speed that a jet
will travel and arrive at distant shorelines in minutes to hours.
When that wave begins to approach shore, it begins to interact with the seafloor.
The base of the wave starts to slow down, because the friction between the,
the volume of moving water and the seafloor below.
And as that happens,
the more distant parts of the wave begin to catch up with the front of the wave.
And as a result, the elevation of the wave surface begins to rise.
So where as the wave maybe only centimeters or 10 centimeters high
out in the open ocean, it may build to meters high as it approaches shore.
Now this point, let's make a distinction between tsunami and storm generated waves.
If you've ever seen the ocean during a storm or
have seen photographs of the ocean during a storm,
you may have seen very large waves approaching as breakers.
A wall of water that maybe 10, 20, 30 meters high as it approaches the shore.
How is that different from a tsunami?
Well, the difference lies in the wavelength of the wave.
A storm wave maybe very high.
It may have a high amplitude, but the width of it,
effectively the wavelength between adjacent wave crest is relatively small.
And that means that the volume of water in a single storm wave is relatively small.
So that storm wave comes to shore, it has enough water so
that it washes onto the beach it, the water will rise up the beach.
And if it's a really huge wave, it may go beyond the edge of the beach and
affect the areas surrounding the beach.
But eventually, it runs out of water and it begins, the water,
under the force of gravity, begins to move back to sea and then the next wave comes.
The wavelength of a typical storm wave is on the order of 70 meters,
50 to 70 meters or so.
In contrast, the wavelength of a tsunami is on
the order tens to as much as a 150 kilometers.
That means that instead of a wave that's that wide, the wave is that wide.
It's huge.
And so when that elevated water approaches shore,
it doesn't stop at the end of the beach.
It keeps going until it has completely submerged all
areas that are lower than the surface of the water.
Some tsunamis have affected the land as far as
10 kilometers inland from the edge of the sea.
There have been two particularly catastrophic tsunamis in recent years.
One of them occurred in 2004 due to a giant earthquake, a magnitude 9.2
earthquake that happened near Sumatra along the coast of the Indian Ocean.
And is estimated to have killed approximately 250,000 people,
a quarter of a million people.
People who were living along the coast of,
of Indonesia, India, Sri Lanka and even as far west as Africa.
The next major tsunami was the 2011 tsunami that
accompanied the Japanese earthquake that occurred off North Japan.
Here we see a map of the Pacific Ocean,
again showing the height of the wave as a function of
distance from where the earthquake occurred along the coast of Japan.
Again, many people had video cameras and
those that were in safe positions were able to video the wave as it came in.
What we're looking at here,
is the wave entering a harbor, where it's actually being caused to rise even higher.
Because the harbor focuses the, the volume of water, so
that it, it rises higher than it would have otherwise.
In fact, the word tsunami comes from the Japanese word, meaning harbor wave.
But again, you can see that there's no back side to this wave.
The entire harbor is filled with high water.
And so as it builds over the sea wall, it continues to move and
submerges much of the local community.
Now the power of the churning water is enough to move huge amounts of material.
But what happens during a tsunami is that the water picks up debris.
It picks up sediment.
It picks up cars.
It picks up wood.
And eventually, it becomes a moving slurry of material,
which is much stronger than just pure water.
In fact, the slurry that can tran, be transported by
a tsunami can sometimes almost have the viscosity of, of wet concrete.
And as a consequence, it just flattens everything in its path.
So, it's not like a clean water wave coming in from a storm.
It's more like a wall of concrete moving through an area, because of its density.
And in the aftermath, hardly anything is left.
In the case of the Japan 2011 event,
there were actually three aspects to the disaster.
The first was the ground shaking, the second was the, was the tsunami and
the third was the Fukushima nuclear disaster.
That's because the tsunami struck the Fukushima nuclear power plant,
which was built right along the coast.
The plant was designed on a way.
That its emergency power generators were diesel,
diesel engines that were at ground level.
So when the wave came in, not only did it knock out the power from power lines,
but it also knocked out the emergency power.
And as a consequence, it was impossible to keep the nuclear reactors cool.
And eventually, hydrogen gas built up and there were hydrogen explosions,
which spread nuclear radioactive material around the vicinity of the plant.
So we see in conclusion that earthquakes are very destructive.
They cause destruction.
Not only because of ground shaking and not only because the ground
changes shape as a consequence of fault motion, but
also because of other phenomena, such as fires, landslides and tsunami.
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Dr. Stephen MarshakProfessor and Director of the School of Earth, Society, and Environment
Dr. Eileen HerrstromLecturer
