Download Earthquakes

Earthquakes
o Earthquakes are caused mainly due to disequilibrium in any
part of the crust of the earth.
o
It is a vibration of the surface of the earth caused by the
disequilibrium of the rocks beneath the surface of the
earth.
o The place of origin of an earthquake deep within the earth
is called focus.
o The place on the ground surface , which is perpendicular to
the focus, recording seismic waves for the first time is called
Epicentre.
Causes of Earthquakes
1. vulcanicity- The explosive violent gases during the process
of vulcanicity try to escape upward and hence they push the
crustal surface from below with great force & thus is caused
severe earth tremor of high magnitude.
2. Faulting- The horizontal & vertical movements caused by
endogenetic forces result in the formation of faults & folds
which in turn cause iso static disequilibrium in the crustal
rocks which ultimately cause earthquakes.
3. Elastic rebound theory- According to Reid the underground
rocks are elastic & expand when stretched & pulled. The
rocks continue to be stretched so long as the tensile forces
do not exceed the elasticity of the rocks but as the tensile
forces exceed the rocks elasticity, they are broken & the
broken rock blocks try to occupy their previous positions so
that they may adjust themselves.in this process earthquake
occurs.
4. Anthropogenic causes- Human activities like pumping of
ground water and oil, deep underground mining, blasting of
rocks by dynamites for constructing dams, roads, reservoirs etc.,
nuclear explosions cause earth tremors of serious
consequences.
5. Plate tectonic theory- The crust of the earth is composed of
solid and moving plates having either continental crust or
oceanic crust or even both. The earth’s crust consists of 6 major
plates and 20 minor plates. These plates are constantly moving
in relation to each other due to thermal convective currents
originating deep within the earth. Thus, all the tectonic events
take place along the boundaries of these moving plates. All sorts
of disequilibrium are caused due to different types of plate
motions and consequently earthquakes of varying magnitudes
are caused.
Classification of Earthquakes
Based on causative factors
Volcanic
Tectonic
Plutonic
Artificial
Based on focus
Moderate
Intermediate
Deep focus
Based on human casualties
Moderately
hazardous
Highly hazardous
Most hazardous
Classification of earthquakes
Based on focus
1. Moderate- focus is located
at the depth from ground
surface to 50 kms.
2. Intermediate- focus at the
depth between 50 – 250
km.
3. Deep focus- focus at the
depth between 250 – 700
km.
Based on human casualities
1. Moderately hazardoushuman deaths are below
50,000 mark.
2. Highly hazardous- human
deaths ranging between
51,000- 1,00,000.
3. Most hazardous- human
deaths above 1,00,000
mark.