Explain The Causes, Classification Of Earthquake
The physical forces the surfaces are rearranging rock materials by shifting magmas about altering the structures of solid rocks. The adjustment beneath the surface however involve various crystal movements, some of which because of suddenness and intensity produce tremors in the rocks and they are known as earthquake. The science dealing with the study of earthquakes in all their aspects is called seismology.
Focus and epicenter:
The exact spot underneath the
earth surface at which an earthquake originates is known as its focus. These
waves first reach the point at the surface, which is immediately above the
focus or origin of the earthquake. This point is called epicenter. The point
which is diametrically opposite to the epicenter is called anticenter.
Intensity and magnitude:
Intensity of an earthquake may be defined as the ratio of an
earthquake based on actual effects produced by the quakes on the earth.
Magnitude of a tectonic
earthquake may be defined as the rating of an earthquake based on the total
amount of energy released when the over strained rocks suddenly rebound,
causing the earthquake.
Causes of earthquake:
The earthquake may be caused due
to various reasons, depending upon it intensity. Following causes of earthquake
are important:
1. Earthquakes due to superficial movements:
The feeble earthquakes are caused
due to superficial movements.i.e, dynamic agencies, and operation upon surface
of the earth.
The dashing waves cause vibrations along the seashore.
Water descending along high water falls, impinges the valley
floor and causes vibrations along the neighbouring areas.
At high altitudes the snow falling down is an avalance.also
causes vibrations along the neighbouring areas.
2. Earthquake
due to volcanic eruptions:
Most of the volcanoes erupt quietly
and as consequence, initiate no vibration on the adjoining area. But a few of
them when erupt, cause feeble tremors in the surface of the earth. But there
may be still a volcanic eruption may cause a severe vibration on the adjoining
area and have really disastrous effects.
3. Earthquake due to folding or faulting:
The earthquakes are also caused
due to folding of the layers of the earth's crust.
if the earthquakes are caused due to folding or faulting then such earthquakes
are more disastrous and are known as tectonic earthquakes and directly or
indirectly change the structural features of the earth crust.
Classification of earthquakes:
Earthquakes are classified on a
no. Of basis. Of these the depth of focus, the cause of origin and intensity are
important.
a) Depth of focus:
Three classes of earthquakes are
recognized on this basis, shallow, intermediate and deep seated. In the shallow
earthquakes the depth of focus lies anywhere up to 50 km below the surface. The
intermediate earthquakes originate between 50 and 300 km depth below the
surface.
b) Cause of
origin:
i) Tectonic
earthquakes are originated due to relative movements of crystal block on
faulting, commonly, earthquakes are of this type.
ii) Non
tectonic earthquakes: that owes their origin to causes distinctly different
from faulting, such as earthquakes arising due to volcanic eruptions or
landslides.
C) Intensity as basis:
Initially a scale of earthquakes
intensity with ten divisions was given by Rossi and ferel. Which was based on
the sensation of the people and the damage caused. However it was modified by
Mercalli and later by wood and Neumann.
Engineering considerations:
The time and intensity of the earthquake
can never be predicted. The only remedy that can be done at the best, it is
provide additional factors in the design of structure to minimize the losses
due to shocks of an earthquake. This can be done in the following way:
To collect sufficient data, regarding the previous seismic
activity in the area.
To assess the losses, which are likely to take place in
furniture due to earthquake shocks To provide factors of safety, to stop or
minimize the loss due to sever earth shocks.
Following
are the few precautions which make the building sufficiently earthquake proof.
The foundation of a building should rest on a firm rock bed.
Grillage foundations should preferably be provided.
Excavation of the foundation should be done up to the same
level, throughout the building.
The concrete should be laid in rich mortar and continuously
Masonry should be done with cement mortar of not les than 1:4
max. Flat R.CC slab should be provided.
All the parts of building should be tied firmly with each
other. Building should be uniform height.
Cantilivers, projections, parapets, domes etc, should be provided. Best materials should be used.
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