Page 129 - SST Class 07

The second belt of volcanoes is found along the Secondary Waves : The secondary waves (S
Mediterranean Sea and is called the Mid-World waves) or shake waves are the next to arrive.
Mountain Belt. This belt has mostly dormant and Their rate of movement is less than that of the P
extinct volcanoes. waves. These waves produce a strong shaking

Fujiyama in Japan is one of the world’s most action. They do not pass through liquids.
beautiful volcanic mountain. The Deccan plateau Surface Waves : The surface waves (L waves)
of India was probably formed due to fissure are the last to arrive and travel over the surface
eruption. of the earth. These waves are very powerful and
EARTHQUAKES cause most of the damage. Their effect is not
Earthquakes are the vibration of the earth’s seen at great depths.
surface caused by the movements of lithospheric
plates. There are three types of earthquake
S wave
waves— P waves or longitudinal waves, S waves
Prior to seismic Motion produced by S
or transverse waves and L waves or surface
disturbance. A straight waves. Particles move
waves. The place in the crust where the move- and fence line provides a back and fourth at right
ment starts is called the focus. The place on the good reference. angles to the line of
wave progression.
surface above the focus is called the epicenter.
Vibrations travel outwards from the epicenter as
waves. Greatest damage is usually closest to the
P wave
epicenter and the strength of the earthquake L wave
decreases away from the centre. Motion produced by Motion produced by
P waves. Particles are L waves. Particles
compressed and then move L in a circular path
expanded in the line at the surface. The motion
of wave progression. diminishes with depth.

Measuring Earthquakes
The intensity earthquake waves are measured on
the Richter Scale. The earthquake tremors are
recorded by an instrument called seismograph.
The science that deals with earthquakes is called
seismology and the earthquake scientists are
called seismologists.
Earthquake
The Richter Scale ranges from zero to nine. An

Types of Earthquake Waves increase in one point on the Richter Scale indi-
According to the mode of travel, earthquake cates that the force of the earthquake is 10 times
greater than the strength of the previous number.
waves are classified into three main types:
The earthquakes of intensity six and above are
Primary Waves : The primary waves (P waves)
very destructive.
or push waves are the first to arrive as they are
More than 10,00,000 earthquakes are recorded
the fastest. These waves usually travel at a speed
every year. An earthquake’s intensity is mea-
of about six km per second. These waves cause
relatively small displacements. sured in relation to its effect on human life.

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