EARTHQUAKE

Definition of Earthquake:
An Earthquake is a sudden tremor or movement of the earth's crust, which originates naturally at or
below the surface.
Earthquakes are three dimensional events, the waves move outwards from the focus, but can travel in
both the horizontal and vertical plains. This produces three different types of waves which have their own
distinct characteristics and can only move through certain layers within the Earth.
Earthquakes occur when energy stored in elastically strained rocks is suddenly released. This release
of energy causes intense ground shaking in the area near the source of the earthquake and sends waves of
elastic energy, called seismic waves, throughout the Earth. Earthquakes can be generated by bomb blasts,
volcanic eruptions, and sudden slippage along faults. Earthquakes are definitely a geologic hazard for those
living in earthquake prone areas, but the seismic waves generated by earthquakes are invaluable for studying
the interior of the Earth.

Origin of Earthquakes
Most natural earthquakes are caused by sudden slippage along a fault zone. The elastic rebound
theory suggests that if slippage along a fault is hindered such that elastic strain energy builds up in the
deforming rocks on either side of the fault, when the slippage does occur, the energy released causes an
earthquake. This theory was discovered by making measurements at a number of points across a fault. Prior
to an earthquake it was noted that the rocks adjacent to the fault were bending. These bends disappeared
after an earthquake suggesting that the energy stored in bending the rocks was suddenly released during the
earthquake.

Tectonic Earthquakes
Tectonic earthquakes are triggered when the crust becomes subjected to strain, and eventually moves.
The theory of plate tectonics explains how the crust of the Earth is made of several plates, large areas of crust
which float on the Mantle. Since these plates are free to slowly move, they can either drift towards each other,
away from each other or slide past each other. Many of the earthquakes which we feel are located in the areas
where plates collide or try to slide past each other. The process which explains these earthquakes, known as
Elastic Rebound Theory can be demonstrated with a green twig or branch.
Major earthquakes are sometimes preceded by a period of changed activity. This might take the form of
more frequent minor shocks as the rocks begin to move, called foreshocks, or a period of less frequent shocks as
the two rock masses temporarily 'stick' and become locked together. Following the main shock, there may be
further movements, called aftershocks, which occur as the rock masses 'settle down' in their new positions. Such
aftershocks cause problems for rescue services, bringing down buildings already weakened by the main
earthquake.

Volcanic Earthquakes
Earthquake can be linked to explosive volcanic eruptions; they are in fact very common in areas of
volcanic activity where they either proceed or accompany eruptions. Volcanic earthquakes are far less common
than Tectonic ones. They are triggered by the explosive eruption of a volcano. Given that not all volcanoes are
prone to violent eruption, and that most are 'quiet' for the majority of the time, it is not surprising to find that
they are comparatively rare. When a volcano explodes, it is likely that the associated earthquake effects will be
confined to an area 10 to 20 miles around its base, where as a tectonic earthquake may be felt around the globe.

Seismology: The Study of Earthquakes

When an earthquake occurs, the elastic energy is released sending out vibrations that travel
throughout the Earth. These vibrations are called seismic waves. The study of how seismic waves behave in
the Earth is called seismology.

Seismograms – Seismic waves travel through the Earth as vibrations. A seismometer is an instrument used
to record these vibrations, and the resulting graph that shows the vibrations is called a seismogram. The
seismometer must be able to move with the vibrations, yet part of it must remain nearly stationary.

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 The source of an earthquake is called the focus, which is an exact location within the Earth were
seismic waves are generated by sudden release of stored elastic energy. The epicentre is the point on the
surface of the Earth directly above the focus. Seismic waves emanating from the focus can travel in several
ways, and thus there are several different kinds of seismic waves.

Body Waves – emanate from the focus and travel in all directions through the body of the Earth. There are
two types of body waves: P –waves and S-waves:

P - waves - are Primary waves. They travel with a velocity that depends on the elastic properties of the rock
through which they travel.

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 
VP  3


Where, Vp is the velocity of the P-wave,  is the incompressibility of the material, is the rigidity of the
material, and is the density of the material.
P-waves are the same thing as sound waves. They move through the material by compressing it, but
after it has been compressed it expands, so that the wave moves by compressing and expanding the material
as it travels. Thus the velocity of the P-wave depends on how easily the material can be compressed (the
incompressibility), how rigid the material is (the rigidity), and the density of the material. P-waves have the
highest velocity of all seismic waves and thus will reach all seismographs first.


S-Waves - Secondary waves, also called shear waves. VS 


Surface Waves - Surface waves differ from body waves in that they do not travel through the Earth, but
instead travel along paths nearly parallel to the surface of the Earth. Surface waves behave like S-waves in
that they cause up and down and side to side movement as they pass, but they travel slower than S-waves
and do not travel through the body of the Earth. Surface waves are often the cause of the most intense
ground motion during an earthquake.

Locating the Epicentres of Earthquakes - In order to determine the location of an earthquake epicentre,
we need to have recorded a seismograph of the earthquake from at least three seismographic stations at
different distances from the epicentre of the quake. In addition, we need one further piece of information -
that is the time it takes for P-waves and S-waves to travel through the Earth and arrive at a seismographic
station. Such information has been collected over the last 80 or so years, and is available as travel time
curves.
From the seismographs at each station one determines the S-P interval (the difference in the time of arrival
of the first S-wave and the time of arrival of the first P-wave. Note that on the travel time curves, the S-P
interval increases with increasing distance from the epicentre.
Thus the S-P interval tells us the distance to the epicentre from the seismographic station where the
earthquake was recorded. Thus at each station we can draw a circle on a map that has a radius equal to the
distance from the epicentre. Three such circles will intersect in a point that locates the epicentre of the
earthquake.

Magnitude of Earthquakes - The size of an earthquake is usually given in terms of a scale called the
Richter Magnitude. Richter Magnitude is a scale of earthquake size developed by a seismologist named
Charles Richter. The Richter Magnitude involves measuring the amplitude (height) of the largest recorded
wave at a specific distance from the earthquake. While it is correct to say that for each increase in 1 in the
Richter Magnitude, there is a tenfold increase in amplitude of the wave, it is incorrect to say that each
increase of 1 in Richter Magnitude represents a tenfold increase in the size of the Earthquake.
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 A better measure of the size of an earthquake is the amount of energy released by the earthquake.
While this is much more difficult to determine, Richter gave a means by which the amount of energy
released can be estimated:
Log E = 11.8 + 1.5 M
Where Log refers to the logarithm to the base 10, E is the energy released in ergs, and M is the Richter
Magnitude.

The rules of Plate Tectonics

1. Continental crust is less dense, or lighter, than Oceanic crust so it doesn't sink. It is never destroyed and is
permanent.
2. Oceanic crust is heavier so it can sink below Continental crust. It is constantly being formed and destroyed at
ocean ridges and trenches.
3. Continental crust can carry on beyond the edges of the land and finally end far below the sea. This explains
why the edges of all the continents don't have deep trenches right up against their coastlines.
4. Plates can never overlap. This means that they must either collide and both be pushed up to form mountains,
or one of the plates must be pushed down into the mantle and be destroyed.
5. There can never be gaps between plates, so if two plates move apart, as in the middle of the Atlantic, new
rock will be formed to fill the space.
6. We know the Earth isn't getting bigger or smaller, so the amount of new crust being formed must be the same
as the amount being destroyed.
7. Plate movement is very slow. This is partly why Wegener's original ideas were ignored. Nobody could 'see' the
continents moving. When the plates make a sudden movement we call it an Earthquake, and it's the only time
we are directly aware of the plates moving.

Stress:
Stress is a force applied over an area. One type of stress that we are all used to is a uniform stress, called
pressure. A uniform stress is where the forces act equally from all directions. In the Earth the pressure due to
the weight of overlying rocks is a uniform stress and is referred to as confining stress. If stress is not equal
from all directions then the stress is a differential stress. Three kinds of differential stress occur.
1. Tensional stress (or extensional stress), which stretches rock;
2. Compressional stress, which squeezes rock; and
3. Shear stress, which result in slippage and translation.

Stages of Deformation
When a rock is subjected to increasing stress it changes its shape, size or volume. Such a change in shape, size or
volume is referred to as strain. When stress is applied to rock, the rock passes through 3 successive stages of
deformation.
Elastic Deformation -- wherein the strain is reversible.
Ductile Deformation -- wherein the strain is irreversible.
Fracture -- irreversible strain wherein the material breaks.

We can divide materials into two classes that depend on their relative behavior under stress.
Brittle materials have a small to large region of elastic behavior, but only a small region of ductile behavior
before they fracture.
Ductile materials have a small region of elastic behavior and a large region of ductile behavior before they
fracture.

How a material behaves will depend on several factors. Among them are:
Temperature - At high temperature molecules and their bonds can stretch and move, thus materials will behave
in more ductile manner. At low Temperature, materials are brittle.
Confining Pressure - At high confining pressure materials are less likely to fracture because the pressure of the
surroundings tends to hinder the formation of fractures. At low confining stress, material will be brittle and tend
to fracture sooner.
Strain rate -- Strain rate refers to the rate at which the deformation occurs (strain divided by time). At high
strain rates material tends to fracture. At low strain rates more time is available for individual atoms to move and
therefore ductile behavior is favored.
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Composition -- Some minerals, like quartz, olivine, and feldspars are very brittle. Others, like clay minerals,
micas, and calcite are more ductile. This is due to the chemical bond types that hold them together. Thus, the
mineralogical composition of the rock will be a factor in determining the deformational behavior of the rock.
Another aspect is presence or absence of water. Water appears to weaken the chemical bonds and forms films
around mineral grains along which slippage can take place. Thus wet rock tends to behave in ductile manner,
while dry rocks tend to behave in brittle manner.

Evidence of Former Deformation

Evidence of deformation that has occurred in the past is very evident in crustal rocks. For example, sedimentary
layers and lava flows generally are deposited on a surface parallel to the Earth's surface (nearly horizontal). Thus,
when we see such layers inclined instead of horizontal, evidence of an episode of deformation is present. In
order to uniquely define the orientation of a planar feature, we first need to define two terms – strike and dip.
For an inclined plane the strike is the compass direction of any horizontal line on the plane. The dip is the
angle between a horizontal plane and the inclined plane, measured perpendicular to the direction of strike.

Fracture of Brittle Rocks

Faults - Faults occur when brittle rocks fracture and there is an offset along the fracture. When the offset is
small, the displacement can be easily measured, but sometimes the displacement is so large that it is difficult to
measure.

Types of Faults
Faults can be divided into several different types depending on the direction of relative displacement.
Since faults are planar features, the concept of strike and dip also applies, and thus the strike and dip of a fault
plane can be measured. One division of faults is between dip-slip faults, where the displacement is measured
along the dip direction of the fault, and strike-slip faults where the displacement is horizontal, parallel to the
strike of the fault.
Dip Slip Faults - Dip slip faults are faults that have an inclined fault plane and along which the relative
displacement or offset has occurred along the dip direction. Note that in looking at the displacement on any
fault we don't know which side actually moved or if both sides moved, all we can determine is the relative sense
of motion.
For any inclined fault plane we define the block above the fault as the hanging wall block and the block below
the fault as the footwall block.
Normal Faults - are faults that result from horizontal tensional stresses in brittle rocks and where the hanging-
wall block has moved down relative to the footwall block.
Horsts & Grabens - Due to the tensional stress responsible for normal faults, they often occur in a series, with
adjacent faults dipping in opposite directions. In such a case the down-dropped blocks form grabens and the
uplifted blocks form horsts. In areas where tensional stress has recently affected the crust, the grabens may
form rift valleys and the uplifted horst blocks may form linear mountain ranges.
Reverse Faults - are faults that result from horizontal compressional stresses in brittle rocks, where the
hanging-wall block has moved up relative the footwall block.
A Thrust Fault is a special case of a reverse fault where the dip of the fault is less than 15o.
Thrust faults can have considerable displacement, measuring hundreds of kilometers, and can result in older
strata overlying younger strata.
Strike Slip Faults - are faults where the relative motion on the fault has taken place along a horizontal
direction. Such faults result from shear stresses acting in the crust. Strike slip faults can be of two varieties,
depending on the sense of displacement. To an observer standing on one side of the fault and looking across the
fault, if the block on the other side has moved to the left, we say that the fault is a left-lateral strike-slip fault.
If the block on the other side has moved to the right, we say that the fault is a right-lateral strike-slip fault.
Transform-Faults are a special class of strike-slip faults. These are plate boundaries along which two plates
slide past one another in a horizontal manner. The most common type of transform faults occur where oceanic
ridges are offset. Note that the transform fault only occurs between the two segments of the ridge. Outside of
this area there is no relative movement because blocks are moving in the same direction. These areas are called
fracture zones.

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Distribution of Earthquakes
The distribution of earthquakes is called seismicity. Seismicity is highest along relatively narrow belts
that coincide with plate boundaries. This makes sense, since plate boundaries are zones along which lithospheric
plates move relative to one another.
Earthquakes along these zones can be divided into shallow focus earthquakes that have focal depths less
than about 100 km and deep focus earthquakes that have focal depths between 100 and 700 km.

1. Earthquakes at Diverging Plate Boundaries. Diverging plate boundaries are zones where two plates move
away from each other, such as at oceanic ridges. In such areas the lithosphere is in a state of tensional stress and
thus normal faults and rift valleys occur. Earthquakes that occur along such boundaries show normal fault
motion, have low Richter magnitudes, and tend to be shallow focus earthquakes with focal depths less than
about 20 km. Such shallow focal depths indicate that the brittle lithosphere must be relatively thin along these
diverging plate boundaries.
Examples - all oceanic ridges, Mid-Atlantic Ridge, East Pacific rise, and continental rift valleys such as the basin
and range province of the western U.S. & the East African Rift Valley.

2. Earthquakes at Transform Fault Boundaries. Transform fault boundaries are plate boundaries where
lithospheric plates slide past one another in a horizontal fashion. The San Andreas Fault of California is one of
the longer transform fault boundaries known. Earthquakes along these boundaries show strike-slip motion on
the faults and tend to be shallow focus earthquakes with depths usually less than about 100 km. Richter
magnitudes can be large.
Examples - San Andreas Fault, California, South Island of New Zealand.

3. Earthquakes at Converging Plate Boundaries. Convergent plate boundaries are boundaries where two
plates run into each other. Thus, they tend to be zones where compressional stresses are active and thus reverse
faults or thrust faults are common. There are two types of converging plate boundaries. (1) subduction
boundaries, where oceanic lithosphere is pushed beneath either oceanic or continental lithosphere; and (2)
collision boundaries where two plates with continental lithosphere collide.

Subduction boundaries –At subduction boundaries cold oceanic lithosphere is pushed back down into the
mantle where two plates converge at an oceanic trench. Because the subducted lithosphere is cold it remains
brittle as it descends and thus can fracture under the compressional stress. When it fractures, it generates
earthquakes that define a zone of earthquakes with increasing focal depths beneath the overriding plate. This
zone of earthquakes is called the

Benioff Zone. Focal depths of earthquakes in the Benioff Zone can reach down to 700 km.
Examples - Along coasts of South American, Central America, Mexico, Northwestern U.S., Alaska, Japan,
Philippines, Caribbean Islands.

Collision boundaries - At collisional boundaries two plates of continental lithosphere collide resulting in fold-
thrust mountain belts. Earthquakes occur due to the thrust faulting and range in depth from shallow to about
200 km.
Examples - Along the Himalayan Belt into China, along the Northern edge of the Mediterranean Sea through
Black Sea and Caspian Sea into Iraq and Iran.

Intraplate Earthquakes - These are earthquakes that occur in the stable portions of continents that are not
near plate boundaries. Many of them occur as a result of re-activation of ancient faults, although the causes of
some intraplate earthquakes are not well understood.
Examples - New Madrid Region, Central U.S., Charleston South Carolina, Along St. Lawrence River - U.S. -
Canada Border.

Hazards Associated with Earthquakes

Possible hazards from earthquakes can be classified as follows:
1. Ground Motion - Shaking of the ground caused by the passage of seismic waves, especially surface waves,
near the epicenter of the earthquake are responsible for the most damage during an earthquake. The intensity of
ground shaking depends on:
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 Local geologic conditions in the area. In general, loose unconsolidated sediment is subject to more intense
shaking than solid bedrock.
 Size of the Earthquake. In general, the larger the earthquake, the more intense is the shaking and the
duration of the shaking.
 Distance from the Epicenter. Shaking is most severe near the epicenter and drops off away from the
epicenter. The distance factor depends on the type of material underlying the area. There are, however,
strange exceptions.
 Damage to structures from shaking depends on the type of construction. Concrete and masonry structures
are brittle and thus more susceptible to damage wood and steel structures are more flexible and thus less
susceptible to damage.

2. Faulting and Ground Rupture - Ground rupture generally occurs only along the fault zone that moves
during the earthquake. Thus structures that are built across fault zones may collapse, whereas structures built
adjacent to, but not crossing the fault may survive.
3. Aftershocks - These are usually smaller earthquakes that occur after a main earthquake, and in most cases
there are many of these (1260 were measured after the 1964 Alaskan Earthquake). Aftershocks occur because
the main earthquake changes the stress pattern in areas around the epicenter, and the crust must adjust to these
changes. Aftershocks are very dangerous because they cause further collapse of structures damaged by the main
shock.
4. Fire - Fire is a secondary effect of earthquakes. Because power lines may be knocked down and because
natural gas lines may rupture due to an earthquake, fires are often started closely following an earthquake. The
problem is compounded if water lines are also broken during the earthquake since there will not be a supply of
water to extinguish the fires once they have started. In the 1906 earthquake in San Francisco more than 90% of
the damage to buildings was caused by fire.
5. Landslides - In mountainous regions subjected to earthquakes ground shaking may trigger landslides, rock
and debris falls, rock and debris slides, slumps, and debris avalanches.
6. Liquefaction - Liquefaction is a processes that occurs in water-saturated unconsolidated sediment due to
shaking. In areas underlain by such material, the ground shaking causes the grains to lose grain to grain contact,
and thus the material tends to flow. You can demonstrate this process to yourself next time your go the beach.
Stand on the sand just after an incoming wave has passed. The sand will easily support your weight and you will
not sink very deeply into the sand if you stand still. But, if you start to shake your body while standing on this
wet sand, you will notice that the sand begins to flow as a result of liquefaction, and your feet will sink deeper
into the sand.
7. Changes in Ground Level - A secondary or tertiary effect that is caused by faulting. Earthquakes may cause
both uplift and subsidence of the land surface. During the 1964 Alaskan Earthquake, some areas were uplifted
up to 11.5 meters, while other areas subsided up to 2.3 meters.
8. Tsunamis - Tsunamis are giant ocean waves that can rapidly travel across oceans, as will be discussed in
more detail later. Earthquakes that occur beneath sea level and along coastal areas can generate tsunamis, which
can cause damage thousands of kilometers away on the other side of the ocean.
9. Flooding - Flooding is a secondary effect that may occur due to rupture of human made dams, due to
tsunamis, and as a result of ground subsidence after an earthquake.

Earthquake prediction

Long-Term:
Long-term forecasting is based mainly on the knowledge of when and where earthquakes have occurred in the
past. Thus, knowledge of present tectonic setting, historical records, and geological records are studied to
determine locations and recurrence intervals of earthquakes. Two aspects of this are important.

Paleoseismology - The study of prehistoric earthquakes. Through study of the offsets in sedimentary layers
near fault zones, it is often possible to determine recurrence intervals of major earthquakes prior to historical
records. If it is determined that earthquakes have recurrence intervals of say 1 every 100 years, and there are no
records of earthquakes in the last 100 years, then a long-term forecast can be made and efforts can be
undertaken to reduce seismic risk.

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Seismic gaps - A seismic gap is a zone along a tectonically active area where no earthquakes have occurred
recently, but it is known that elastic strain is building in the rocks. If a seismic gap can be identified, then it
might be an area expected to have a large earthquake in the near future.

Short-Term:
Short-term predication involves monitoring of processes that occur in the vicinity of earthquake prone faults for
activity that signifies a coming earthquake.
Anomalous events or processes that may precede an earthquake are called precursor events and might signal a
coming earthquake.

Despite the array of possible precursor events that are possible to monitor, successful short-term earthquake
prediction has so far been difficult to obtain. This is likely because:
 the processes that cause earthquakes occur deep beneath the surface and are difficult to monitor.
 earthquakes in different regions or along different faults all behave differently, thus no consistent patterns
have so far been recognized.

Among the precursor events that may be important are the following:

1. Spatial prediction:
Almost all earthquakes occur along active faults. Therefore a key to identifying the location of possible future
earthquakes is field mapping at sufficient scale (say, 1:24000), showing known or inferred active faults.

2. Temporal prediction:
a) Seismic wave velocity- Micro-earthquakes swarms may be precursors of a large earthquake. It has also
been observed that the difference in arrival time between P and S waves sometimes decreases from normal.
It is believed that stress causes microfracturing about 1 year before an earthquake, so that the seismic
velocity changes.
b) Foreshocks - Prior to a 1975 earthquake in China, the observation of numerous foreshocks led to
successful prediction of an earthquake and evacuation of the city of the Haicheng. The magnitude 7.3
earthquake that occurred, destroyed half of the city of about 100 million inhabitants, but resulted in only a
few hundred deaths because of the successful evacuation..
c) Emission of Radon Gas - Radon is an inert gas that is produced by the radioactive decay of uranium and
other elements in rocks. Because Radon is inert, it does not combine with other elements to form
compounds, and thus remains in a crystal structure until some event forces it out. Deformation resulting
from strain may force the Radon out and lead to emissions of Radon that show up in well water. The newly
formed microcracks could serve as pathways for the Radon to escape into groundwater. Increases in the
amount of radon emissions have been reported prior to some earthquakes.
d) Water Level in Wells - As rocks become strained in the vicinity of a fault, changes in pressure of the
groundwater (water existing in the pore spaces and fractures in rocks) occur. This may force the
groundwater to move to higher or lower elevations, causing changes in the water levels in wells.
e) Ground Uplift and Tilting - Measurements taken in the vicinity of active faults sometimes show that prior
to an earthquake the ground is uplifted or tilts due to the swelling of rocks caused by strain building on the
fault. This may lead to the formation of numerous small cracks (called microcracks). This cracking in the
rocks may lead to small earthquakes called foreshocks.
f) Seismic activity- With an abundance of operating seismograph stations, it is possible to notice any unusual
seismic activity. These anomalies may be precursors of an earthquake. Seismic gaps, i.e. places where
anomolous quiescence occurs, can be harbingers of large earthquakes.
g) Changes in the Electrical Resistivity of Rocks - Electrical resistivity is the resistance to the flow of
electric current. In general rocks are poor conductors of electricity, but water is more efficient a conducting
electricity. If microcracks develop and groundwater is forced into the cracks, this may cause the electrical
resistivity to decrease (causing the electrical conductivity to increase). In some cases a 5-10% drop in
electrical resistivity has been observed prior to an earthquake.
h) Unusual Radio Waves - Just prior to the Loma Prieta Earthquake of 1989, some researchers reported
observing unusual radio waves. Where these were generated and why, is not yet known, but research is
continuing.

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i) Strange Animal Behavior - Prior to a magnitude 7.4 earthquake in Tanjin, China, zookeepers reported
unusual animal behavior. Snakes refusing to go into their holes, swans refusing to go near water, pandas
screaming, etc. This was the first systematic study of this phenomenon prior to an earthquake. Although
other attempts have been made to repeat a prediction based on animal behavior, there have been no other
successful predictions.
j) Strain rate extrapolation-If sufficient reliable past rates of strain are known, a reasonable inference can
be made as to future displacement.

Planning for earthquake:

At any specific site, a first step in assessing earthquake hazards is the inspection of a general geological map.
Site-specific information is needed, such as bedrock type, topography, proximity to faults, landslide history, etc.
A second step is a detailed assessment of the subsurface conditions at the site in question. For example, earth
materials such as saturated, loose, granular material (silt, sand) are subject to liquefaction, where earthquake
deformation transforms stable material into a fluid like state. Other materials are subject to landslides.
Unconsolidated surficial deposits have greater wave amplitude than bedrock, and earthquake vibrations last
longer in them.
Many civil engineers emphasize that it is not earthquakes that kill people, but the failure of buildings. Structural
engineers maintain the best earthquake strategy is not to try to control earthquakes, but simply to build sound
structures on relatively safe sites. In general, tall building should be avoided.
Steel skeleton buildings are superior to other types. Older masonry construction is hazadrous; most ware
designed by masons with no knowledge of earthquake engineering. Steel buildings can be secured by more bolts
or welds. Bearing walls, which act to resist twisting caused by lateral earth displacements, are being encouraged
in seismic areas. Base isolation, large rubber shocks absorbers placed between a structure and the foundation are
used in new buildings and bridges. The successful use of shear walls as a means of stiffening buildings and
resisting earthquake forces has been observed in many earthquakes. A shear wall acts to stiffen the structure so
that the deflection of upper floors is reduced. Shear walls typically include walls with no openings such as central
elevator corridors or outer walls of buildings with no windows.

Aseismic design:
The problem of designing and construction of practically quake-resistant structures at economical costs has
been studied intensively and extensively in the recent past in all major countries of the world. In the simple
assumption, a structure is designed to resist the vertical acceleration of 1 g by virtue of its weight only. As
such, most of the aseismic designs take into consideration only the horizontal component of ground
acceleration due to an earthquake. The magnitude of these lateral stresses would be function of a number of
factors, most important of which are:
i) the ground acceleration due to an expected shock during the designed life of the project;
ii) the weight of the structure;
iii) the type of the construction.

During an earthquake, when the ground tends to move in one direction, the lateral force exerts a shearing
effect on the building above and hence is often referred as base shear force. This has to be annulled and
made ineffective if the structure above is to remain stable during the shock.
Quantitatively, the Base Shear Force on a single storied structure is given by the relationship
F = a/g .W, where a = ground acceleration due to an expected earthquake, g = acceleration due to gravity, W =
weight of the structure. This is however a simple empirical relationship. In actual practice, the Base Shear Force,
F, (the lateral force) may be function of a number of factors and be obtained by relationship such as
F = (SKZIRN) x W
Where, S = factor depending on the “response spectra” of the structures and whose values vary between 0.12
and 3.00;
K = factor depending on nature of damage in the past earthquakes; value for masonry construction lie between
2 – 4 and for reinforced construction between 0.6 to 1.6;
Z = factor depending on “seismic coefficient”, the ratio between ground acceleration due to gravity; values lie
between 0.15 and 0.02 depending on the location of the structure vis-à-vis seismic zoning;

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I = factor depending on “importance” of the structure; the construction of public importance such as education,
hospitals, cinema halls have greater importance;
R = “risk factor”; which is greater than 1 for factories, especially producing poisonous gases, atomic reactors,
reservoirs, overhead tanks;
N = factor depending on nature of soil;
The W normally signifies dead load but in residential buildings, 25 per cent values of W are added for the live
load in calculating the lateral stresses during an earthquake.
The base shear force or the overturning moment, M, can be then calculated by using the relationship
M = F.Y
Where, Y is the vertical distance of the C.G. of structure (portion of structure) above the horizontal section
under consideration.
The overturning moment thus known, a safety factor can now be introduced into the design of the building.
In case of Multi-storied Building allowances must also be provided for increased flexibility of the structure. The
base shear force in the case of such buildings can be calculated by
i) multiplying the seismic factor, a/g, by a factor 4.5/(N + 4.5) where, N is the number of storeys above
the surface under consideration;
ii) using the corrected seismic factor in calculating the base shear force and overturning moment.

Quake resistant building:

a) The foundations
i) Structures built on loose soil or sediments or weak rocks will have to withstand greater risk compared to
those founded on solid bed rocks in the same region, other things being the same. This is primarily due
to the fact that soil particles undergo a lot of compaction during seismic shocks thereby causing
settlement. Rocks already sufficiently compacted because of their mode of formation, will not normally
suffer from such an effect and hence offer better resistance to shocks.
ii) Foundations for concrete and masonry buildings should be excavated to the same level through out the
building, and should be as far as possible of continuous type.
iii) The superstructure should be thoroughly tied up with the foundations by introducing keys or
reinforcements to offer maximum resistance against sliding at that level.

b) The body
i) The walls should be, unless property designed for resisting lateral forces, as light in weight as possible
and made up of wood or lightweight concrete. Stronger and resistant walls should be designed with
reinforced rather than plain concrete.
ii) Continuity of cross walls should be maintained as far as possible and in such a way that different
parts of the building behave as integrals of the same structure and not at random.
iii) In masonry walls, keys should be inserted in a proper style during each course so that danger of
sliding a past the horizontal joints in minimized. It is of course desirable that only the strongest
mortars, preferably cement sand mortars are used for masonry work in the seismic regions.

c) The roof
i) Flat, R.C.C. roofs give better resistance against shocks compared to the sloping roofs or those designed
with different slop angles.
ii) Even in flat roofs, when sales and tiles and corrugated sheets are used, all the case should be taken to
minimize development of lateral stresses.
iii) Projections above or beyond the roof level such as chimneys, should be altogether avoided or kept to
the minimum.

d) General
i) All the parts of the same building- the foundations, superstructure and the roof, should be firmly tied
together so that the entire structure acts as a unit during a shock and not as multiple of independently
behaving units.
ii) As far as possible, uniform height should be given to the structure.

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iii) Architectural fancies like parapets, cantilevers, arches and domes etc. should be avoided as far as
possible. When deemed absolutely essential, these should be designed with extra care and for the
maximum risk as these are among the first parts to fall apart in the building during a shock.

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