Physics of Tsunami (Part 1 of 2)


Uploaded by laputa1729 on 20.03.2011

Transcript:
So I think since we had the tsunami,
let's discuss a little bit of that.
Some of the physics connected with that very briefly.
Because we heard its magnitude was 8.9 in Richter scale.
So what energy released does it correspond to?
You know all these Richter scales are logarithmic,
as most other things in Physics,
like magnitude of stars. That's also logarithmic.
And you know the sound intensities,
they are in decibels, that's also logarithmic.
Wind speeds are also logarithmic.
Beaufort scale they call it.
So here also Richter and Gutenberg introduced the logarithmic scale
to quantify the earthquakes.
So the formula, they generally use this for,
suppose you know the earthquake of Richter 8.9 or 8 or whatever
or 6 or whatever,
what is the energy released?
How do they estimate the energy released?
So, i'll just give you the formula they use normally.
So this log 10 E in Joules. This is in Joules.
So, this is given by
This is just a coincidence that we have a similar thing for the luminosity scale for stars
You know 2.5 log I2 by I1
So here, 2.5M, M is the magnitude and they put a minus 1.2.
So that explains, suppose it is...
this is in Joules, that's why J.
So you can calculate now, this is 8.9, almost 9
so it works out to about 10^20 Joules.
This is roughly the energy released.
So this works out to about...
if you take the typical 1 Megaton nuclear weapon
this works out to be the intensity of about fifty thousand nuclear bombs.
So one hundred megaton, which is the most powerful nuclear weapon tested
so far by Russia. Russia once tested a sixty megaton nuclear bomb.
So that's about five into 10^24 ergs.
So this is equivalent to something like about thirty thousand or forty thousand
typical nuclear weapons
of one megaton.
So that is why you would have seen that it is said that it was equivalent to several tens of thousands of nuclear weapons.
About hundred times weaker.
So the most powerful earthquake recorded was about 8.9 or 9.
And this also, of course, triggered a Tsunami.
And for that you know you need a little more information.
So what's happening is these underground earthquakes, normally, it comes about because of these tectonic plates.
One tectonic plane subducts, it goes down under another tectonic plate.
So the material in between is pushed up.
And, it so happens, only on Earth we have this plate tectonics.
It doesn't happen on any other planet.
Venus doesn't have plate tectonics, Mars doesn't have.
So, there's an essential condition for this that there should be a lot of water on the planet.
This water goes deep inside. In fact this phenomenon effects only the
crust and the mantle, the upper part of the mantle.
So you know the Earth is made up in layers.
So you've got the outer crust, the lithosphere. Lithosphere is the crust and the mantle.
The upper mantle goes upto about twenty kilometers.
So most of the earthquakes have the origin around ten kilometers deep or twenty kilometers deep.
Not very much deeper than that.
So, essentially, as far as the diameter of the Earth is concerned, this is basically a surface phenomenon.
So what's happening is, because of the presence of water
we have got all this molten rock
because there is an increase of pressure with depth,
and temperature also.
So we go a few kilometers down, a temperature gradient is there.
So the rock becomes quite molten.
Originally these things became molten because of the radioactivity in the Earth's crust.
A lot of isotopes like 40 Calcium, 40 potassium, so many isotopes are there.
So these initially provided much of the heat to keep the earth hot.
So what has happened is that these rocks get molten and they form what is called magma.
Not plasma but magma, nothing but molten or semi-molten rock.
So it's a highly visco-elastic fluid.
Both viscous and elastic.
And it can move.
So the continents are literally floating on this.
The continents are nothing but continental plates floating on this magma.
Every continent has got its own plate. Even other landmasses have got their plate.
India has got its own plate called the Indian plate.
And Asia has got its own, China has its own plate.
So apparently India was at one time near the Atlantic.
About two hundred million years ago, India was very much in the southern hemisphere.
Then the Indian plate broke up.
So initially, there were only two huge continents, Laurentian and Gondwanaland.
And even before that there was only one super continent Panagea.
All the landmasses were in one piece.
And it was Alfred Wegener who suggested that
all the continents were part of only one land mass,
because he noticed that like a jigsaw puzzle,
the west coast of Africa fits nicely in the east coast of South America,
and similarly, he could fit India and Australia and Africa, and so all these, like a jigsaw puzzle.
He could fit all that into one piece.
So he suggested the continents are actually drifting.
But nobody believed in that time.
But now it has become a reality with GPS and everything because they are actually able to measure the small drift in continents.
In fact Indian plate is still going northwards.
They are pushing against China.
And, so, what's happening is that the Himalayas are growing taller.
And the Himalayas were the result of the Indian plate colliding with the Asian plate
about forty million years ago.
So the Himalayas are pretty young mountains because they formed only forty million years ago.
And as evidence for this we see even fossils of marine animals on top of Mount Everest.
This shows that at one time it was all part of the sea.
So all this mountain building and movement of continents are all connected.
So the Earth's surface is in a state of flux.
Continuous flux. So all the continents are moving around.
So even the Mediterranean is closing because the African plate is pushing northwards again.
And similarly Europe is trying to merge with America. So the Atlantic is shrinking.
In another hundred million years, maybe sometime I will talk about it.
People have actually estimated what all will happen after another hundred million years.
The face of the continents will be very different.
So you see after all humanity has been there only for a few...
you know, when we started doing science, technology on a vast scale
we are hardly a few centuries in existence as far as these things are concerned.
Whereas these things have been going on for several millions of years or hundreds of millions of years.
So earthquakes are a result of all these. Several continental plates are there.
The American plate is there. South America has got one plate. Africa has got one plate.
Australia has got one plate. They are all drifting.
And especially around the Pacific, you know, there is a highly...
all these plates are sort of merging.
So it is the biggest ocean right now.
But maybe it won't be the biggest ocean after say, hundred million years.
So actually there is a ring of fire around Japan and all these islands in the Pacific
So that's why they are highly earthquake prone regions.
Because these plates, every now and then they collide.
And the intensity of the collision depends on how much matter is thrust up and so on.
In fact, in 2004, there was another big underground earthquake in Indonesia
that led to again a big tsunami
which caused two lakh people to die
including several in the Sri Lanka coast, Chennai also. Lot of people died.
So again, that happened hardly around seven years back in 2004
Six years back.
And even last year there was Chile. In 2010 again, there was a big earthquake. Haiti, you know.
And in Chile there was a big earthquake and a tsunami.
So, in the past year there have been three or four such events.