Earthquakes - Everything You Need To Know

Earthquakes: Everything you need to know
clearias.com/earthquakes/
ClearIAS Team March 6, 2016
Earthquakes and volcanoes are examples of sudden movements which originate from inside
the earth. We have already seen the types of volcanoes. But, what is an earthquake? To put it
simple, earthquake is the shaking of the earth. An earth quake can be defined as a sudden
violent shaking of the ground as a result of movements in the earth’s crust or volcanic
action. These movements result in the release of energy along a fault and cause the earth to
shake. An earthquake, like volcanoes is a type of endogenic processes.
Table of Contents
Earthquakes: Points you should not miss!

Earthquake waves or Seismic waves
Body waves
Primary waves (p-waves):
Secondary waves (s-waves):
Surface Waves
Love waves:
Rayleigh waves:
Shadow regions of waves
Measuring earthquakes
Mercalli Scale:
Richter Scale:
Distribution of Earthquakes
Earthquakes: Points you should not miss!

The network of seismographic (seismograph is the instrument used to measure
earthquakes) stations all over the world record dozens of earthquakes every day. Most
of them are not felt by human beings as they are minor quakes only.
The occurrence of a severe earthquake is limited to a few regions in the world.
The point within the earth’s crust where an earthquake originates is called as the focus
or hypocenter or seismic focus.
It generally lies within a depth of 6 kms in the earth crust.
The point vertically above the focus on the earth’s surface is called as the epicenter.
The intensity of earthquake will be highest in the epicenter and decreases as one
moves away.
All natural earthquakes take place in the lithosphere (i.e, the region which constitutes
the earth’s crust and rigid upper part of the mantle).
Earthquake waves or Seismic waves
The earthquake which originates in the lithosphere propagates different seismic waves
or earthquake waves.
Earthquake waves are basically of two types – body waves and surface waves.
Body waves
They are generated due to the release of energy at the focus and moves in all
directions traveling through the body of the earth. Hence, the name – body waves.
They travel only through the interior of the earth.
Body waves are faster than surface waves and hence they are the first to be detected
on a seismograph.
There are two types of body waves as primary waves and secondary waves.
Primary waves (p-waves):
Primary waves are the fastest body waves (twice the speed of s-waves) and are the
first to reach during an earthquake.
They are similar to sound waves, i.e, they are longitudinal waves, in which particle
movement is in the same direction of wave propagation.
They travel through solid, liquid and gaseous materials.
They create density differences in the earth material leading to stretching and
squeezing.
Secondary waves (s-waves):
They arrive at the surface with some time-lag after primary waves.
They are slower than primary waves and can pass only through solid materials.
This property of s-waves led seismologists to conclude that the earth’s outer core is in
a liquid state. (the entire zone beyond 105o from the epicenter does not receive S-
waves)
They are transverse waves in which directions of particle movement and wave
propagation are perpendicular to each other.
Surface Waves
When the body waves interact with surface rocks, a new set of waves is generated
called as surface waves.
These waves move along the earth surface.
Surface waves are also transverse waves in which particle movement is perpendicular
to the wave propagation.
Hence, they create crests and troughs in the material through which they pass.
Surface waves are considered to be the most damaging waves.
Two common surface waves are Love waves and Rayleigh waves.
Love waves:
This kind of surface waves causes horizontal shifting of the earth during an
earthquake.
They have much slower than body waves but are faster than Rayleigh.
They exist only in the presence of semi-infinite medium overlain by an upper finite
thickness.
Confined to the surface of the crust, Love waves produce entirely horizontal motion.
Rayleigh waves:
These waves follow an elliptical motion.

A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean.
Because it rolls, it moves the ground up and down and side-to-side in the same
direction that the wave is moving.
Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be
much larger than the other waves.
Shadow regions of waves
We already discussed that p-waves pass through all medium while S-waves passes
only through solid medium.
With the help of these properties of primary waves, seismologists have a fair idea
about the interior of the earth.
Even though p-waves pass through all mediums, it causes reflection when it enters
from one medium to another.
The variations in the direction of waves are inferred with the help of their record on
seismographs.
The area where the seismograph records no waves is called as ‘shadow zone’ of that
wave.
Accordingly, it is observed that the area beyond 1050 does not receive S-waves and the
area in between 1050 to 1400 does not receive P-waves.
Measuring earthquakes
Seismometers are the instruments which are used to measure the motion of the
ground, which including those of seismic waves generated by earthquakes, volcanic
eruptions, and other seismic sources.
A Seismograph is also another term used to mean seismometer though it is more
applicable to the older instruments.
The recorded graphical output from a seismometer/seismograph is called as a
seismogram. (Note: Do not confuse seismograph with seismogram. Seismograph is an
instrument while seismogram is the recorded output)
There are two main scales used in the seismometers: (1) Mercalli Scale and Richter
Scale.
Mercalli Scale:
The scale represents the intensity of earthquake by analyzing the after effects like how
many people felt it, how much destruction occurred etc. The range of intensity is from 1-12.
Richter Scale:
The scale represents the magnitude of the earthquake. The magnitude is expressed in
absolute numbers from 1-10. Each whole number increase in Richter scale represents a ten
times increase in power of an earthquake.
Distribution of Earthquakes
There are two well-defined belts where earthquakes frequently occur – The Circum-
Pacific Belt and The Mid-World Mountain Belt.
About 68% of earthquakes in the world occur in the Circum-Pacific Belt.

Mid-World Mountain belt extends from the Alps with their extension into
Mediterranean, the Caucasus, and the Himalayan region and continues to Indonesia.
21% of earthquakes are occurring in this belt.
The remaining 11% occur in the other parts of the world.
Next in the series: Exogenic Forces and their classification.
Article by: Jijo Sudarshan

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Earthquakes: Everything you need to know

ClearIAS Team March 6, 2016

Earthquakes: Points you should not miss!

Earthquakes: Points you should not miss!

Primary waves (p-waves):

Secondary waves (s-waves):

These waves follow an elliptical motion.

About 68% of earthquakes in the world occur in the Circum-Pacific Belt.

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