CONVECTIONAL CURRENT:
“ Convection is the process by which less dense
material rises and more dense material sinks.”
·
Rocks, water and air indeed most materials expand and thus
become less dense as temperature increases.
·
So convection is typically driven by temperature differences.
·
In the earth’s mantle, hot rock rises and slightly cool rock
sinks
·
The high pressure and temperature gradient between the crust
and core cause convection currents to develop in the viscous mantle.
CAUSES OF EARTHQUAKE:
1.Natural
2.Man-made
NATURAL CAUSES OF
EARTHQUAKES:
1. Tectonic plate movement:
·
This happens when one crustal plate collides against each
other.
·
To speak about this tectonic movement, the important
consideration is that the continental plate collides oceanic plate
·
The oceanic plate is overridden by the continental plate by
the process of subduction jerky movements.
EXAMPLE:
The Indian plate and Himalayan
Earthquakes:
·
Indian plate was originally part of ancient continent of
Gondwana from which it is split off eventually became major plate.
·
Initially it was fused with the adjacent Australian plate. It
includes most of South Asia.
·
The collision with Eurasian plate along the boundary between
India and Nepal created Tibetan plateau and Himalayas.
·
The Indian plate is currently moving North east 5cm/ year.
While the Eurasian plate is moving only 2cm/year leading to collision of both
plates due to difference in moving speeds
·
The junction of India and Eurasian tectonic plates it causes
of earthquakes in Himalayan region.
·
US Geological survey seismologist, Paul Earle explains that
this movement of two tectonic plates cause the formation of Himalayas.
·
Despite of being located on the boundary of two major
tectonic plates, Nepal has suffered relatively from large earthquakes in recent
history.
FAULTS:
“A Fault is a fracture or
break in earth’s crust (Lithosphere) Faulting occurs when shear stresses on a
rock overcomes the forces which hold it together.”
Typically faults are associated with or form the boundaries
between earth’s tectonic plates .
The phenomenon of earthquakes caused by a sudden displacement along the sides of a
fault can be summarized as follows.
o
The strain which has accumulated in the fault for a long time
reaches its maximum limit
o
A slip occurs at the fault and causes a rebound
o
A push and pull force initiates at the fault.
o
The situation is equivalent to two pairs of coupled forces
acting suddenly.
o
This action causes radial wave propagation.
SEISMIC WAVES:
These are the waves of energy caused by sudden braking
of the rock within the earth or due to an explosion.
They are the energy that travels through the earth and is
recorded on seismographs.
TYPES OF SEISMIC WAVES:
Fig: Types of seismic waves
1. BODY WAVES:
These waves can travel through the earth’s inner layer
Travelling through the interior of earth, body waves arrive
before the surface waves emitted by earthquakes
A) P-Waves:
Also known as P-wave or compression waves
Fastest kind of seismic waves
Can move through solid rock and fluids like water or liquid
layers of the earth.
These waves can be sometimes identified by some kind of
animals rather than human.
B) S-Waves:
Also known as secondary wave
This is the wave, we feel it as earthquake
It is slower than P-wave and can only move through solid
rock, not through any liquid medium.
The S-waves are characterized by particle motion that is
perpendicular to propagation direction.
2. SURFACE WAVES:
- Travelling only through crust, surface waves are of lower
frequency than body waves and are easily distinguished on a seismogram as a
result.
- Though they arrive after body waves, it is surface waves that
are almost entirely responsible for the damage and destruction associated with
earthquakes.
- This damage and the strength of surface waves are reduced in
deeper earthquakes.
A) Love waves:
First kind of surface wave is called a Love wave named after
A.E.H.Love, a British mathematician who worked out the mathematical model for
this kind of wave in 1911
It is the fastest surface wave and moves the ground from side
to side.
Confined to the surface of the crust love waves produce
entirely horizontal motion.
B) Reyleigh Waves:
-
Named after John William Strutt, Lord Reyleigh who
mathematically predicted the existence of this kind of wave in 1885
-
A Reyleigh wave rolls along the ground just like a wave rolls
across a lake 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
Reyleigh wave, which can be much larger than other waves.
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