The earth is constantly undergoing
change. Some changes are fast and
some are slow. Earthquakes and
volcanoes are endogenic forces which cause
sudden changes on the earth. Weathering and
agents of sculpture are exogenic forces which
bring about slow changes. In the following
pages a description has been given about the
earthquakes and volcanoes.
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EARTHQUAKE
Earthquake is the trembling of the earth
initiated by sudden shock. Sometimes it is so
violent that it damages or destroys strong
buildings. The devastation caused on
26 January 2001 in Bhuj, Gujarat speaks of
the violent nature of earthquakes. Most of the
shocks that cause the earth to tremble are
associated with diastrophic movements. The
sudden slippage of rocks along the faults and
also in some cases faults reaching the ground
surface, displace the objects on the ground. In
other cases, crustal rocks bend elastically and
set waves in motion.
The place where the earthquake originates
inside the earth is called its focus. The point
on the earth�s surface vertically above the
focus is called the epicentre. The waves travel
away through rocks in all the directions.
Shaking is the strongest near the epicentre.
Great accumulation of elastic strain is built up
constantly along active lithospheric plate
boundaries, particularly along converging
(where two plates meet) boundaries. Slippage
of opposed plates in these converging zones
release enormous quantities of energy in the
form of great earthquakes. The relationship
between earthquake activity or seismicity and
lithospheric plate boundaries is remarkably
strong.
TYPES OF SEISMIC WAVES
We have already discussed earthquake waves in
Chapter 3. In this section, we propose to examine
them in greater details. There are three types of
seismic waves generated by earthquake called
Primary, Secondary and Long waves designated
as P, S, and L respectively. P waves are fast
travelling waves; they arrive first. S waves are
slower; they take almost twice as long to travel.
In fast travelling P waves, the individual particles
vibrate to and fro in the direction of wave
propagation whereas in S waves the particles
vibrate up and down at right angles to the
direction of propagation.
The P waves travel through the body of the
earth at an average rate of 6 km/sec. They are
faster at depth (about 8 km/sec to 11 km/sec)
than near the surface. They travel through
both solid and liquid medium. The S waves also
go through the body of the earth but at a slower
rate of 3.5 km/sec. L waves follow the
circumference of the earth and travel at more
or less constant rate. They are also known as
surface waves. Thus, P and S waves travel by
a direct path through the earth while
L waves follow its circumference.
The Seismograph
The seismograph is a sensitive instrument that
records earthquakes thousands of kilometres
distant and so weak that their vibrations could
not possibly be recognised by the human
senses. The seismograph design is based on the
principle of inertia � the tendency of any mass
to resist a change in a state of rest or of uniform
motion in a straight line is greater, the greater
the mass of the object.
The first indication that a severe
earthquake has occurred at a distant point is
the sudden beginning of the Primary waves
(P waves). These waves then die down
somewhat for a few minutes; then a second
burst of activity sets in with the arrival of
Secondary waves (S waves) which are
somewhat larger in height than the P waves.
Following it, smooth waves that increase
greatly in amplitude to a maximum and then die
slowly are recorded. These are the surface
waves. Two measurements are made directly
from the seismograph:
amplitude in mm of the largest (highest) wave recorded; and
time in seconds elapsed between arrivals of the P and S waves.