EARTHQUAKES AND LANDSLIDES
An
Earthquake is the result of the release of the stored energy due sudden
vibrations of the earth’s crust that creates seismic waves.
An
Earthquake may caused by the tectonics
plates getting stuck and putting strain on the ground . The strain becomes so crest
rocks give way by fault planes breaking. Smaller earthquakes may also caused by
the volcanoes.
EFFECTS OF THE EARTHQUAKES
1- Shaking and ground rupture
2- Landslides and Avalanches
3- Fires
4- Soil liquefaction
5- Tsunami
6- Human impact
7- River water
CAUSES OF THE EARTHQUAKES
-
Movements
of tectonic plates
-
Volcanic
eruption
-
Anthropogenic
source
-
Dams
-
Uses
of explosives
-
Removal
of natural gases
CLASSIFICATION OF THE EARTHQUAKES
The
classification of the earthquakes is broadly divided into two categories
1- Natural earthquake
2- Artificial / man-made earthquake
Natural
earthquakes are also divided into three categories :-
1- TECTONIC EARTHQUAKE
The word
tectonic means any structural change in rocks caused due to their deformation
or displacement. The term “TECTONIC” has derived from the Greek word which
means a builder.
The main
cause of the tectonic earthquake is the building up stresses in the rocks until
they are strained to the breaking point.These are caused by the sudden earth
movement , generally along faults , usually at the depth varying from about 4.5
km to 24 km below the earth surface.
The
majority of the natural earthquakes including the most powerful and damaging
ones belong to this categories.
PLUTONIC EARTHQUAKES
These are
the deep focus earthquakes , the depth of disturbance beimg between 250km to
700km.
VOLCANIC EARTHQUAKES
These are
the associated with the explosive eruption of volcanoes . Naturally , they
occurs in volcanic zones of the world.
The circum
pacific belt is example volcanic earthquakes, Volcanic earthquake are common hazards. The earthquake of volcanic
origin are less severe and more limited in extent than those caused by the
fracturing if the earthquake crust.
2- ARTIFICIAL EARTHQUAKE
Artificial
earthquake which occurs due to human activities causes disturbance in the earth
crust producing vibrations of various magnitude.
TERMINOLOGY USED IN THE EARTHQUAKES
-
FOCUS
-
EPICENTER
-
FOCAL
DEPTH
-
EPICENTRAL
DISTANCE
-
SEISMIC
NOISE
-
ISOSEISMAL
LINES
-
MAGNITUDE
OF EARTHQUAKE
-
INTENSITY
OF EARTHQUAKE
FOCUS :-
the focus of the earthquake is also called its “hypocentre”.The focus is the
point below ground level where earthquake originates and generates shock waves
which generates in all direction.
EPICENTER
:- The epicentre is the point at the ground level , immediately above the
focus.
EPICENTRAL
DISTANCE :- It is the distance between the epicentre and the point of interest.
FOCAL DEPTH
:- It is the depth of the focus from the epicenter . Most of the damaging
earthquake have shallow depth with focal depth 70kms.
SEISMIC
NOISE :- Many faults give rise to the numerous very small and , to human senses
imperceptible earthquakes which may be described as seismic noise.
ISOSEISMAL
LINES :- It the line joining the places with equal seismic intensity i.e. lines
joining the points where similar ground motion were experienced ; they are
assessed by examining the damage done .
It is
observed that isoseismal lines are circle if the focus is the centre as in case
of homogeneous ground and these are elliptical if the focus is the line in case
of heterogeneous ground.
MAGNITUDE
OF EARTHQUAKE :- The magnitude of the earthquake is measured with help of the Richter scale,
body wave magnitude , surface wave magnitude and wave energy magnitude.
INTENSITY
OF MAGNITUDE:-
LANDSLIDES
In general
landslides are viewed as slope failure resulting in sudden slipping of enormous
volume of material along downward slope.
TYPES OF
THE LANSLIDES
DEBRIS
SLIDE / SLUMP
ROCK SLIDES
ROCK FALL
DEBRIS
SLIDE / SLUMP :- Failure of unconsolidated material along a shearing surface
which slides down rapidly is called a slump or debris.
ROCK SLIDES
:- when detached block of bed rocks moves down the hill , it is called a rock
slide .
In rock
slides the movements takes place on bedding planes , joints or any other planes
of weakness.
ROCK FALL:-
From steep rock slopes , blocks of rocks of varying size which are loosed by
weathering , suddenly fall downward under the influence of the gravity. This is
called rock fall.
CAUSES OF LANDSLIDES
1- WATER
SLOPE
NATURE OF ROCK / LITHOLOGY
STRUCTURE OF ROCK
ANTHROPOGENIC CAUSES
WATER :-
Water is an most important factor in causing landslides. It act in three ways :
-
Water
diminishes the strength of rock soil and thus help in their movement.
-
The
water that seeps into the rocks or soil not only produce lubricants but also
exerts addition force on the grains forcing them to displace from their
position.
-
It
adds weight to the material , hence many landslides occurs after rains. On freezing
it exerts an expensive force.
SLOPE :- It
has been observed that majority of of the earth or rocks failure are
continental slopes .
This indicates
that slopes are directly responsible for the landslides. As a rule , stteper
the slopes greater the availability of the landslides.
NATURE OF
ROCKS / LITHOLOGY :- Unconsolidated sediments , such as clay sand , gravel etc
can not stand permanently along the
steeper slope greater than their angle of repose (about 70degrees)and are
likely to be effected by the landslides.
STRUCTURE
OF THE ROCKS:- Joints , fracture , shear zones, and bedding planes usually
becomes the slip surface in case of landslides , landslides are particularly common
hill slopes where dip of rock bed is also in the direction.
ANTHROPOGENIC
CAUSES (MAN MADE CAUSES):- Fallowing are the anthropogenic causes
-
Excavation
of slopes
-
Loading
of slopes
-
Deforestation
-
Irrigation
-
Artificial
vibrations
PREVENTION MEASURES FOR THE LANDSLIDES
Fallowing
are the preventive measures for the landslides:-
1- To counter the effect of landslides
2- To counter the effect of water
3- To counter structural defect
0 Comments