The term "earthquake" is used to
describe any seismic event which generates seismic
waves. An earthquake occurs as a result of a sudden
release of energy in the Earth's crust which creates
seismic waves. They occur along faults (fault is a
planar breaks in rock along which there is
displacement of one side relative to the other). An
earthquake is
what happens when two blocks of the earth suddenly
slip past one another. The surface where they slip
is called the fault
or fault plane.
Every day our Earth
experiences earthquakes in different parts of the
world. More than a million earthquakes occurs all
around the world each year but most of them are too
small to cause any damage and are barely felt by
humans. Earthquakes with magnitude of about
2.0 or less are usually called microearthquakes.
They are generally
recorded only on local seismographs.
Focus
and Epicenter:
The point on a fault at which the
first movement or break occurs during an earthquake
is called Earthquake's Focus. The point
directly above the focus on the surface of Earth is
called epicenter. Major earthquakes
dramatically demonstrates that Earth is a dynamic,
ever changing system.
Seismic Waves:
When an earthquake
occurs it releases its stored up energy in the form
of three types of waves. These waves are called
seismic waves. There are three main types of
seismic waves - P S and L. P and S waves travel
through the interior of the Earth. P are
compressional waves. P waves are also called
Primary waves and they travels through earth
as sound waves travel through the air. S waves are
also called Secondary waves or shear waves,
involving side to side motion of molecules. L waves
are called longitudinal waves are the slowest as
they travel from Earth's surface, they cause the
maximum structural damage.
How Earthquakes are measured?
Earthquakes
are measured using an instrument
called Seismograph. Seismograph measures the
P,S and L waves. Seismograph records
the time and intensity of earthquakes.
Seismogram
The record
created by Seismograph is called seismogram.
Seismogram shown above in the picture
is a simulation. The actual records of
earthquake waves are far more complicated
than what is presented here. As P and S
waves travel through the earth, they are
reflected by various layers of the earth
(such as the core- mantle boundary). This
interaction produces additional seismic
waves which will be detected by
seismographs.
Causes of
Earthquake:
Crust of the
Earth is constantly moving. This
sections of the crust are called plates and
the process which moves these plates is
described in more detail in
Plate tectonics page.
To learn more about Plate tectonics
click here.
These plates constantly pushing against each
other due to forces produced by its semi
viscous interior. The areas where these
plates collide often have increased volcanic
and earthquake activity.
There are about 20 plates along the surface
of the earth that move continuously and
slowly past each other. When the plates gets
crushed or stretched, huge rocks form at
their edges and the rocks shift with great
force, releasing tremendous amount of energy
which in turn causes an earthquake.
Watch Video
Plate Tectonics
Other causes
of Earthquakes are Volcanic activity,
Underground nuclear explosions , landslides
etc. About 80 percent of all the Earth's
earthquakes occur along the rim of the
Pacific Ocean. This is also called as the
"Ring of Fire" because of extreme volcanic
activity there. Most earthquakes takes place
at fault zones, where tectonic plates
collide or slide against each other.
Earth's Plates
( Photo Source: Nasa)
In a sequence
of earthquakes there is normally one event
significantly larger than the other which is
called the first shock or main shock, any
earthquakes after that are called
aftershocks.
How do earthquakes
cause damage?
Structural damage in an earthquake is
caused by the ground shaking. The amount of
ground shaking that might be produced
depends on magnitude or energy released in
an earthquake, distance from the focus or
source, focal depth, type of faulting, and
type of material are important factors.
In general, large earthquakes produce ground
motions with large amplitudes and long
durations. Large earthquakes also produce
strong shaking over much larger areas than
do smaller earthquakes. In addition, the
amplitude of ground motion decreases with
increasing distance from the focus of an
earthquake. The frequency content of the
shaking also changes with distance. The
frequency of ground motion is an important
factor in determining the severity of the
damage to structures.
Can
Earthquakes be predicted?
The answer to
this question is no. Until now we do not
have any concrete technology which can
predict exact timing and place of any
earthquake.
What
should be done to avoid Earthquake
?
Nothing much can be done to avoid
earthquakes as earthquakes happen
all the time everywhere on Earth.
However most severe earthquakes
occur along the major fault lines
and plate boundaries around the
world. If possible do not live in
the areas which fall within these
boundaries or fault lines. But if
you happen to live around these
places then try to build the home
which are earth quake proof. For
more info and tips...
click
here
