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As in the ancient times,
even today earthquakes continue to exercise the human imagination
powerfully.It is difficult to imagine another phenomenon that is
at once subject to such wide scientific scrutiny, and yet capable
of arousing such profound dread as well as curiosity in the popular
mind. In all of history, humans have constructed all manners of hypotheses
in way of understanding it. The earliest belief was that the earth
was supported by an animal whose occasional movement resulted in earthquakes.
In Japan the animal was thought to be a spider; in India, a mole;
in certain parts of South America, a whale; and for North American
Indians it was a tortoise. But it was the great empiricist Aristotle
who framed, what is perhaps the first "rational" explanation: that earthquakes
occurred due to sudden release of entrapped air from within earth's
body, which caused the ground to shake and open.
Of course, it is now known that an earthquake is caused by
a slip on a fault, i.e. a fracture between two large blocks
of rock. Such faults can extend from a few centimeters to thousands
of kilometers. They allow the two blocks of rock to move relative
to each other,either slowly,over geologic times; or suddenly, which
results in an earthquake. Some of the most "famous" faults
across the world include the Californian San Andreas Fault
that runs about 650 miles long and 10 miles deep. Closer to
home, amongst others, a significant section of the Himalayan belt,
is known to be situated on a fault line of relatively
high susceptibility.
The
reason why fault lines give away suddenly can be understood in terms of
earth's inner structure. Our planet was formed nearly 4 billion
years ago by a massive conglomeration of materials from space,
and is cooling off even today.Over the aeons, heavier materials
like iron sank towards centre of the earth; while lighter silicates,
other oxygen compounds and water rose to the surface.
As we see it today, the earth is comprised of four concentric
layers: the inner core, outer core, mantle and crust. The core
contains mostly iron and about 10% sulfur. Being under extreme pressure,the
inner core is solid, while the outer core exists in a highly
hot and molten state. Beyond the core, the mantle is a semi-solid
mass (composed of iron, magnesium and silicate compounds), which is
capable of plastic deformation. The crust,which is the outermost layer,
is the thinnest and least dense of the layers. It is relatively cold,
rocky (i.e. deposits of calcium, sodium and aluminium-silicate minerals)
and brittle; and, thus, can fracture easily during earthquakes.
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For nearly
half a century it was thought that the faults in the earth's crust
were the main reason behind earthquakes. It now appears that a very
significant proportion of earthquakes have their origin in the earth's
mantle.And that, the opening of faults,in the crust is a secondary
phenomenon: it is damage produced by deformation of the more mobile
mantle below.It is this continual slow movement of the outer layer
of earth - technically known as "plate tectonics"-that eventually
lead to earthquakes. It is responsible for causing volcanoes as
well. The very same motions have also created the greatestmountain
ranges across the world!
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