Earthquakes occur due to movements within the Earth's crust caused by plate tectonics or faulting. The point of origin is called the focus or hypocenter, located below the surface. The location directly above on the surface is the epicenter, where shaking is strongest. Other causes include volcanic eruptions and human activity such as nuclear explosions. Earthquake intensity is measured using an instrument called a seismograph.

1. EARTHQUAKE

3.  How The Earthquakes Occur?
CRITICAL QUESTION

4. OBJECTIVES
oThe pupil will understand what is earthquake.
oThey will be able to know how earthquake
occurs?
oThey will know about seismograph.

5. EARTHQUAKE
Earthquakes are tremors or vibrations on the
Earth’s surface produced by internal forces or
man-made stresses. Earthquakes result from
sudden movements of the Earth’s Crust along
line of weakness.
The point of origin of the earthquake is called the
seismic focus which usually has a depth of less
than 60km. The point on the Earth’s surface
vertically above the seismic focus is called the
epicenter. The intensity of the tremors is
maximum near the epicenter and decreases with
distance from the epicenter.

7. CAUSES OF EARTHQUAKE
The main causes of earthquake are as
under:
 Volcanic Eruption
 Faulting
 Plate Tectonics
 Anthropogenic Factors

8. VOLCANIC ERUPTION
Volcanic earthquakes
are caused by gas
explosions or the
updomining and
fissuring of volcanic
structure.

9. FAULTING
Earthquake occur
when movement of
the earth take place
along a line of
fracture called a
fault.

10. PLATE TECTONICS
Major earthquake are
caused by the
movement of tectonic
plates. The sliding of
two plates over each
other, in a horizontal
or vertical direction,
produces violent
waves which cause
the earthquake.

11. ANTHROPOGENIC FACTORS
Human Over-interaction with
nature is also causes
earthquake.
These are responsible for
disturbance of the
equilibrium in the Earth’s
Crust. Nuclear explosions or
implosions, blasting of rocks
by dynamite, impounding of
large quantities of water in
dams and reservoirs are
responsible for earthquake.

15. SEISMOGRAPH
The intensity of an earthquake is measured by
an instrument called a seismograph.

18. Thank you