An unexpected movement of the
earth's surface is called an earthquake. When
two parts of the earth's surface move suddenly in relation to each
other a long fall fly due to tectonic forces an
earthquake occurs. A tremendous amount of energy is released in
the form of tremors and vibrations such earthquakes are called tectonic earthquakes.
The point of origin of an earthquake is called the focus while
the point directly above it on the surface of the earth is called
the epicenter. Owing to the strong vibrations around the
point of origin the degree of destruction is greater around
the epicenter.
The branch of knowledge that deals
with the study of earthquakes is called seismology and
the experts who study seismic waves are called seismologists. There
are three types of seismic waves that spread in all directions from
the focus. They are:- The P, S and L waves.
The compressional waves are the first
to reach the earthquake recording station as they travel 1.7
times faster than the shear waves and are therefore called primary
or P waves. While the shear waves are called secondary or
S waves. The primary waves cause the particles of rock to vibrate
forwards and backwards in the direction of the waves. Secondary waves
on the other hand cause the particles to oscillate at right angles to
the direction of the waves. When p and s reach the surface of
the earth they get converted into L long waves that travel along
the surface vibrating horizontally at right angles to the direction
of the waves called love waves or rayleigh waves. As
they also travel like sea waves. The most violent shocks and
subsequent destruction is caused by the L waves.
The instrument used to detect and
record seismic waves is called a seismograph. The recorded
material appears like a graph and is called a size per gram.
Based on the depth of the foci earthquakes are classified into
three types. They are :-
Deep focused earthquakes that occur at depths exceeding 300 kilometers
the reasons for which have not yet been identified.
Intermediate focused earthquakes with depths of foci ranging between 55 and 300
kilometers focus earthquakes with depths lesser than 55
kilometers to measure the magnitude of the earthquakes.
A Richter scale is
used the point scale is arranged in such a way that each of the units
on the scale is equivalent to 30 times the energy released by the
previous unit. While a magnitude of 2 is seldom felt a magnitude of 7
indicates the lower limit of an earthquake that has a devastating effect
over a large area.
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