INTRODUCTION
An earthquake is a sudden natural movement of a portion of the earth's crust
that produces trembling or shaking of the ground. During earthquakes, rocks
suddenly release energy. An earthquake originates from a particular region
and spreads out in all directions.
Among the different natural hazards that our earth is subjected to, an
earthquake has the most devastating natural impact, resulting in huge loss of
human life and material. Those of us who have experienced an earthquake
know that there is a rumbling sound of doors windows rattle and beds and
tables in the room shake. While standing, we feel the floor move away from
under our feet.
DEFINITION OF EARTHQUAKE
The ground vibrations, both feeble and strong, produced on the surface of
the earth due to any reason what so ever are described as earthquakes.
Whenever these vibrations traverse, an earthquake is said to have taken
place.
It may also be defined as "Violent shaking of earth's surface due to abrupt
release of large amount of energy that has accumulated over a long time
inside the earth".
It is important to mention here that about 90 percent of all earthquakes are
caused naturally whereas 10 percent earthquakes are related to man made
sources such as nuclear explosions, reservoir induced tremors, etc.
EARTHQUAKE ENGINEERING OR SEISMOLOGY
The branch of science that deals with the study of earthquake and the
structure of the earth is termed as earthquake engineering or seismology.
Seismology includes study of generation, propagation and recording of
seismic waves that occur inside the earth along with the sources that produce
them. The research and development carried out in the area of earthquakeresistant constructions is also a part of earthquake engineering.
IMPORTANT TERMS ASSOCIATED WITH ENGINEERING
SEISMOLOGY.
Some of the important terms related to earthquake engineering are described
below.
(a) Focus or Seismic centre: The exact point or place inside the surface of
the earth at which an earthquake originates is termed as focus or seismic
centre. It is also termed as Hypocentre. It is actually a point on the fault
plane where slip or movement of tectonic plate occurs. Most of the
damaging earthquakes have shallow focus with focal depths less than 70
kilometers. Its position is determined with the help of seismographic
records.
(b) Epicentre: The point or line on the surface of the earth lying vertically
above the focus is termed as 'epicentre' or 'epicentral line'. As earthquake
cannot possibly originate at a mere point alone, therefore whole of the
disturbed area inside the earth is 'focus' and as a consequence, the epicentre
is a tract and not an isolated point located vertically above the focus.
(c) Focal depth: The depth of focus from the epicentre is termed as focal
depth or simply the vertical distance between epicentre and hypocentre is
called focal depth.
(d) Epicentral depth: The horizontal distance from the epicentre to any
place of interest or discussion is termed as epicentral distance. Intensity of
vibrations are more intense near the epicentre and as the epicentral distance
increases, these become feeble and weak and ultimately die out.
(e) Anticentre: The point on the surface of the earth diametrically opposite
to the epicentre is called anticentre. In seismology, anticentre is of little
importance, as focus is rarely determined accurately. Therefore, most
frequently the term epicentre or epicentral area is indicated in earthquake
engineering.
(f) Seismic waves : The waves transmitted in all directions due to large
strain energy released from the focus during an earthquake are termed as
seismic waves. These are of two types, body waves and surface waves,
which obey the laws of reflection and refraction as they pass through the
earth materials of different densities.
(g) Seismograph: An instrument which used to record ground vibrations or
surface displacements is termed as a 'Seismograph'. These seismographs are
used only to record weak motions and are unsuitable to record vibrations of
very high intensity. For recording higher intensities, special strong motion
instruments called accelerographs are employed.
(i) Isoseismal: An imaginary line on the surface of the earth along which the
intensity of a measured seismic shock is equal at every point is termed as
'isoseismal'. It is just like a contour line which joins points of same
elevation. In an earthquake hit area, places of similar intensity can be easily
marked and a line joining these points is an 'isoseismal line'.
(j) Isoseismal map: A map showing different isoseismals for a particular
earthquake is termed as an isoseismal map. Such an isoseismal map is quiet
helpful to scientists and engineers in the development of seismic zones and
is used to carry out long term planning on earthquake preparedness in the
absence of earthquake prediction.
(k) Seismic zones (Seismic belt): The regions of earth's crust where
earthquakes occur frequently and repeatedly are termed as seismic zones or
seismic belt. Depending on the intensity of seismic activity, these zone are
further divided into highly seismic, moderately seismic and poorly seismic
zones. The Himalayan ranges and the Indo-gangetic plains are the areas
which are considered as highly seismic zones.
(l) Fault: A fracture or a crack along which blocks of earth's mass move
relatively on either side parallel to the fracture is termed as a fault. This
sliding of earth's mass takes place in pieces called "Tectonic plates'. This
sudden slip of rock masses along the fault plane releases large strain energy
stored in the interface rocks generating seismic shocks. It is important to
note that the energy released during 2001 Bhuj earthquake (Gujrat, India)
was 200 times of that of energy released by Atom bomb dropped on
Hiroshima (Japan) in the year 1945.
(m) Active fault: A fault is termed as an active, if it is likely to have another
earthquake or seismic shock in the near future. Remember that all faults are
considered as active if they have caused movement of blocks of crust once
or more times in the past. The term 'aseismic refers to a fault where no
earthquakes or seismic shocks have been observed.
(n) Foreshocks and Aftershocks: Foreshocks are defined as relatively
smaller earthquakes or minor seismic shocks that precede the major tectonic
earthquake known as "main shock. Foreshocks are generally caused by
small ruptures or plastic deformations. "Aftershocks are defined as
earthquakes of smaller intensity that follow the mainshock. Aftershocks
generally occur in sequence and continue for a long period due to fresh
ruptures or readjustments of the fractured mass.
(0) Tsunamis : Tsunamis refer to giant sea waves or harbour waves. It is a
Japanese term composed of two words "Tsu' meaning harbour and 'nami'
meaning wave. Tsunami are a series of large sea waves caused by
earthquake or seismic activity near the coastal regions or at the ocean floor.
(p) Earthquake hazard: Any phenomenon associated with an earthquake
such as Tsunamis, landslides, groundshaking, surface faultings, and ground
deformation etc. that may affect the normal life of people is termed as
earthquake hazard.
NECESSITY OF EARTHQUAKE ENGINEERING
(SIGNIFICANCE OF SEISMOLOGY)
The current seismic zone map indicates that more than 60 percent of the
land area in India is considered prone to earthquakes. Many recent
earthquakes of high magnitude which occurred in India during the last
decade sound an alarming bell that the seismic risk in the country has been
increasing day by day. Most deaths during an earthquake are caused by
collapse of man made structures. The basic knowledge of civil engineering
and structural engineering without any exposure to earthquake engineering
is not sufficient to build earthquake resistant structures. Hence the most
important step towards an earthquake-resistant India is to train the practicing
professional engineers in the subject of earthquake engineering. The
knowledge of earthquake engineering is essential for a civil engineer to
enable him to build earthquake resistant constructions that can appropriately
withstand earthquakes. The vast devastation in Gujrat during the Bhuj
earthquake of 2001, clearly shows that seismic design, detailing and related
earthquake resistant practices are not being followed in the construction of
buildings and other structures. The inadequate preparedness of the country
to face damaging earthquakes is due to poor knowledge of science of
seismology. Thus a basic knowledge of earthquake engineering' is needed to
make a significant impact towards earthquake safety in our country.
The knowledge of earthquake engineering or seismology is thus essential:
(i) To design and build earthquake resistant structures for earthquake
safety.
(ii) To carry out advanced research and development for effective
earthquake management.
(iii) For earthquake preparedness f.e. to learn how to prepare facing
earthquakes.
(iv) For seismic evaluation and retrofitting. The term retrofitting refers
to upgrading the strength and structural capacity of a seismically
deficient building to enable it to safely withstand the effect of
seismic shocks in future.
CAUSES OF EARTHQUAKES
We all know that earthquakes originate due to a sudden impact on the body
of the earth. When the impact is of lighter intensity, milder or feeble
vibrations are set up, whereas a heavier impact generates strong tremors
having disastrous effects.
The causes of earthquakes may be broadly classified into following three
categories.
1. Superficial or surface causes
2. Volcanic causes
3. Tectonic causes
1. Superficial causes (surface causes): Earthquakes of mild intensity which
occur over the ground surface caused by the dynamic agencies operating
upon the surface of the earth are termed as Superficial ones. The various
surface causes producing seismic tremors of low intensity are enlisted
below:
(a) A huge landslide or a rock fall along hill slopes.
(b) Giant sea waves and crashing breakers along sea shores.
(c) Running water, descending falls and cascades upon valley floor.
(d) Heavy vibrating machinery in Industrial area.
(e) Movement of locomotives and other heavy vehicles on earth surface.
(f) Man made explosions and other nuclear tests.
(g) Mining blasts in mining areas. heavy excavations causing land
subsidence, which sets minor tremors in
(h) Large scale heavy excavations causing land subsidence, which sets
minor tremors in the vicinity.
Some of the other microseismic causes producing feeble seismic shocks are
due to cultural noise or disturbed vibrations generated by Industry and the
traffic.
From the causes enlisted above, it is quiet clear that the superficial agencies
initiate earthquakes no doubt, but very few of them are strong enough to be
perceptible to our senses and none of them can possible have any destructive
effect on human life.
2. Volcanic causes: Volcanic activities taking place in different parts of the
world, oftenly produce volcanic outbursts or explosions during which the
surface of the earth trembles. The impact thus generated is sometimes so
strong that it causes earthquakes or seismic shocks in the nearby areas. But
however it is not necessary to be concluded that all sorts of volcanic
eruptions on earth produce earthquakes. There are number of volcanoes
which erupt frequently and yet initiate no earthquakes on the earth's surface.
People living in countries like Japan, Italy, Newzealand, Iceland, Java,
Sumatra etc. have enough experience of such volcanic activities.
Volcanic earthquakes, caused by high pressures exerted by movement of hot
molten lava known as magma with in the earth's crust are generally shallow
earthquakes of mild intensity. In India, there are no active volcanoes.
However, the peninsular and extra-peninsular India has observed volcanic
activity on a large scale in the past, some millions of years back. There are
even traces of dormant and extinct volcanoes in the islands of Bay of Bengal
and Andaman regions. (An active volcano is one which erupts frequently
whereas extinct volcano shows no volcanic activity one with rare eruption is
called dorment volcano)
3. Tectonic causes: Tectonic causes are those which originate within the
earth's crust and are necessarily associated with the relative movements of
rock masses forming the crust of the earth. The seismic shocks which occur
due to sudden release of enormous strain owing to crustal movements are
termed as tectonic earthquakes. It is an established fact that all major
earthquakes causing large scale devastation on the surface of the earth are of
tectonic origin. Tectonic earthquakes occur due to :
(i)
(ii)
Displacement or rupturing of rock masses along pre existing
cracks or faults.
Development of new fault planes.
The term 'tectonic' relates to structural distortions occurring in the earth's
crust caused by upheavals and other movements within it, whereas 'plate
tectonics' refers to interrelating movements of the rigid plates or sections
that make up the earth's crust, riding on the semi-molten rock mass of the
interior. The rocks forming the earth's crust when subjected to stress, yield
only upto a certain limit. However, when the accumulating stress increases
beyond the said limit, the rocks are subjected to fracturing and the fractured
blocks suffer relative displacement accompanied by enormous strain
released by rupturing of blocks along confined regions. This produces
vibrations giving rise of earthquakes whose magnitude depends upon the
amount of strain energy released. The disastrous Bhuj earthquake which
occurred in Gujrat (India), in the year 2001 was also of tectonic origin.