Assignment On Earthquake

Assignment
On
“EARTHQUAKE’’
Name: Mahir Tajwar
Class Roll: AE-025
Registration No: 2013-716-677
Session: 2013-14
Department of Geology
University of Dhaka
Dhaka – 1000
Date: 14.11.2018

Earthquake
Earthquake, any sudden shaking of the ground caused by the passage of seismic
waves through Earth’s rocks. Seismic waves are produced when some form of energy stored in
Earth’s crust is suddenly released, usually when masses of rock straining against one another
suddenly fracture and “slip.” Earthquakes occur most often along geologic faults, narrow zones
where rock masses move in relation to one another. The major fault lines of the world are located
at the fringes of the huge tectonic plates that make up Earth’s crust. (See the table of major
earthquakes.)
Residents of an earthquake-damaged neighborhood of Port-au-Prince, Haiti, seeking safety in a
sports field, Jan. 13, 2010. The magnitude-7.0 earthquake struck the region the day before.
Little was understood about earthquakes until the emergence of seismology at the beginning of
the 20th century. Seismology, which involves the scientific study of all aspects of earthquakes,
has yielded answers to such long-standing questions as why and how earthquakes occur.

Encyclopædia Britannica, Inc.
About 50,000 earthquakes large enough to be noticed without the aid of instruments occur
annually over the entire Earth. Of these, approximately 100 are of sufficient size to produce
substantial damage if their centres are near areas of habitation. Very great earthquakes occur on
average about once per year. Over the centuries they have been responsible for millions of deaths
and an incalculable amount of damage to property.
Crowds watching the fires set off by the earthquake in San Francisco in 1906, photo by Arnold
Genthe.

Causes
Earth’s major earthquakes occur mainly in belts coinciding with the margins of tectonic plates.
This has long been apparent from early catalogs of felt earthquakes and is even more readily
discernible in modern seismicity maps, which show instrumentally determined epicentres. The
most important earthquake belt is the Circum-Pacific Belt, which affects many populated coastal
regions around the Pacific Ocean—for example, those of New Zealand, New Guinea, Japan,
the Aleutian Islands, Alaska, and the western coasts of North and South America. It is estimated
that 80 percent of the energy presently released in earthquakes comes from those
whose epicentres are in this belt. The seismic activity is by no means uniform throughout the
belt, and there are a number of branches at various points. Because at many places the Circum-
Pacific Belt is associated with volcanic activity, it has been popularly dubbed the “Pacific Ring
of Fire.”
A second belt, known as the Alpide Belt, passes through the Mediterranean region eastward
through Asia and joins the Circum-Pacific Belt in the East Indies. The energy released in
earthquakes from this belt is about 15 percent of the world total. There also are striking
connected belts of seismic activity, mainly along oceanic ridges—including those in the Arctic
Ocean, the Atlantic Ocean, and the western Indian Ocean—and along the rift valleys of East
Africa. This global seismicity distribution is best understood in terms of its plate tectonic setting.
Natural forces
Earthquakes are caused by the sudden release of energy within some limited region of the rocks
of the Earth. The energy can be released by elastic strain, gravity, chemical reactions, or even the
motion of massive bodies. Of all these the release of elastic strain is the most important cause,
because this form of energy is the only kind that can be stored in sufficient quantity in the Earth
to produce major disturbances. Earthquakes associated with this type of energy release are called
tectonic earthquakes.
Tectonics
Tectonic earthquakes are explained by the so-called elastic rebound theory, formulated by the
American geologist Harry Fielding Reid after the San Andreas Fault ruptured in 1906, generating
the great San Francisco earthquake. According to the theory, a tectonic earthquake occurs when
strains in rock masses have accumulated to a point where the resulting stresses exceed the
strength of the rocks, and sudden fracturing results. The fractures propagate rapidly through the
rock, usually tending in the same direction and sometimes extending many kilometres along a
local zone of weakness. In 1906, for instance, the San Andreas Fault slipped along a plane 430
km (270 miles) long. Along this line the ground was displaced horizontally as much as 6 metres
(20 feet).

Earthquakes are caused by a sudden fracture of rock masses along a fault line.
As a fault rupture progresses along or up the fault, rock masses are flung in opposite directions
and thus spring back to a position where there is less strain. At any one point this movement may
take place not at once but rather in irregular steps; these sudden slowings and restartings give rise
to the vibrations that propagate as seismic waves. Such irregular properties of fault rupture are
now included in the modeling of earthquake sources, both physically and mathematically.
Roughnesses along the fault are referred to as asperities, and places where the rupture slows or
stops are said to be fault barriers. Fault rupture starts at the earthquake focus, a spot that in many
cases is close to 5–15 km under the surface. The rupture propagates in one or both directions
over the fault plane until stopped or slowed at a barrier. Sometimes, instead of being stopped at
the barrier, the fault rupture recommences on the far side; at other times the stresses in the rocks
break the barrier, and the rupture continues.
Earthquakes have different properties depending on the type of fault slip that causes them (as
shown in the figure). The usual fault model has a “strike” (that is, the direction from north taken
by a horizontal line in the fault plane) and a “dip” (the angle from the horizontal shown by the
steepest slope in the fault). The lower wall of an inclined fault is called the footwall. Lying over
the footwall is the hanging wall. When rock masses slip past each other parallel to the strike, the
movement is known as strike-slip faulting. Movement parallel to the dip is called dip-slip
faulting. Strike-slip faults are right lateral or left lateral, depending on whether the block on the
opposite side of the fault from an observer has moved to the right or left. In dip-slip faults, if the
hanging-wall block moves downward relative to the footwall block, it is called “normal”
faulting; the opposite motion, with the hanging wall moving upward relative to the footwall,
produces reverse or thrust faulting.

Types of faulting in tectonic earthquakes In normal and reverse faulting, rock masses slip
vertically past each other. In strike-slip faulting, the rocks slip past each other horizontally.
All known faults are assumed to have been the seat of one or more earthquakes in the past,
though tectonic movements along faults are often slow, and most geologically ancient faults are
now aseismic (that is, they no longer cause earthquakes). The actual faulting associated with an
earthquake may be complex, and it is often not clear whether in a particular earthquake the total
energy issues from a single fault plane.
Observed geologic faults sometimes show relative displacements on the order of hundreds of
kilometres over geologic time, whereas the sudden slip offsets that produce seismic waves may
range from only several centimetres to tens of meters. In the 1976 Tangshan earthquake, for
example, a surface strike-slip of about one metre was observed along the causative fault east
of Beijing, and in the 1999 Taiwan earthquake the Chelung-pu fault slipped up to eight metres
vertically.
Volcanism
A separate type of earthquake is associated with volcanic activity and is called a volcanic
earthquake. Yet it is likely that even in such cases the disturbance is the result of a sudden slip of
rock masses adjacent to the volcano and the consequent release of elastic strain energy. The
stored energy, however, may in part be of hydrodynamic origin due to heat provided
by magma moving in reservoirs beneath the volcano or to the release of gas under pressure.
There is a clear correspondence between the geographic distribution of volcanoes and major
earthquakes, particularly in the Circum-Pacific Belt and along oceanic ridges. Volcanic vents,
however, are generally several hundred kilometres from the epicentres of most major shallow
earthquakes, and many earthquake sources occur nowhere near active volcanoes. Even in cases
where an earthquake’s focus occurs directly below structures marked by volcanic vents, there is

probably no immediate causal connection between the two activities; most likely both are the
result of the same tectonic processes.
Volcanoes and thermal fields that have been active during the past 10,000 years.
Artificial induction
Earthquakes are sometimes caused by human activities, including the injection of fluids into
deep wells, the detonation of large underground nuclear explosions, the excavation of mines, and
the filling of large reservoirs. In the case of deep mining, the removal of rock produces changes
in the strain around the tunnels. Slip on adjacent, preexisting faults or outward shattering of rock
into the new cavities may occur. In fluid injection, the slip is thought to be induced by premature
release of elastic strain, as in the case of tectonic earthquakes, after fault surfaces are lubricated
by the liquid. Large underground nuclear explosions have been known to produce slip on already
strained faults in the vicinity of the test devices.
Reservoir induction
Of the various earthquake-causing activities cited above, the filling of large reservoirs is among
the most important. More than 20 significant cases have been documented in which
local seismicity has increased following the impounding of water behind high dams. Often,
causality cannot be substantiated, because no data exists to allow comparison of earthquake
occurrence before and after the reservoir was filled. Reservoir-induction effects are most marked
for reservoirs exceeding 100 metres (330 feet) in depth and 1 cubic km (0.24 cubic mile) in

volume. Three sites where such connections have very probably occurred are the Hoover Dam in
the United States, the Aswan High Dam in Egypt, and the Kariba Dam on the border
between Zimbabwe and Zambia. The most generally accepted explanation for earthquake
occurrence in such cases assumes that rocks near the reservoir are already strained from regional
tectonic forces to a point where nearby faults are almost ready to slip. Water in the reservoir adds
a pressure perturbation that triggers the fault rupture. The pressure effect is perhaps enhanced by
the fact that the rocks along the fault have lower strength because of increased water-pore
pressure. These factors notwithstanding, the filling of most large reservoirs has not produced
earthquakes large enough to be a hazard.
The specific seismic source mechanisms associated with reservoir induction have been
established in a few cases. For the main shock at the Koyna Dam and Reservoir in India (1967),
the evidence favours strike-slip faulting motion. At both the Kremasta Dam in Greece (1965) and
the Kariba Dam in Zimbabwe-Zambia (1961), the generating mechanism was dip-slip on normal
faults. By contrast, thrust mechanisms have been determined for sources of earthquakes at
the lake behind Nurek Dam in Tajikistan. More than 1,800 earthquakes occurred during the first
nine years after water was impounded in this 317-metre-deep reservoir in 1972, a rate amounting
to four times the average number of shocks in the region prior to filling.
Seismology and nuclear explosions
In 1958 representatives from several countries, including the United States and the Soviet Union,
met to discuss the technical basis for a nuclear test-ban treaty. Among the matters considered
was the feasibility of developing effective means with which to detect underground nuclear
explosions and to distinguish them seismically from earthquakes. After that conference, much
special research was directed to seismology, leading to major advances in seismic signal
detection and analysis.
Recent seismological work on treaty verification has involved using high-
resolution seismographs in a worldwide network, estimating the yield of explosions, studying
wave attenuation in the Earth, determining wave amplitude and frequency spectra discriminants,
and applying seismic arrays. The findings of such research have shown that underground nuclear
explosions, compared with natural earthquakes, usually generate seismic waves through the body
of the Earth that are of much larger amplitude than the surface waves. This telltale difference
along with other types of seismic evidence suggest that an international monitoring network of
270 seismographic stations could detect and locate all seismic events over the globe of
magnitude 4 and above (corresponding to an explosive yield of about 100 tons of TNT).

Effects
Earthquakes have varied effects, including changes in geologic features, damage to man-made
structures, and impact on human and animal life. Most of these effects occur on solid ground,
but, since most earthquake foci are actually located under the ocean bottom, severe effects are
often observed along the margins of oceans.
Surface phenomena
Earthquakes often cause dramatic geomorphological changes, including ground movements—
either vertical or horizontal—along geologic fault traces; rising, dropping, and tilting of the
ground surface; changes in the flow of groundwater; liquefaction of sandy ground; landslides;
and mudflows. The investigation of topographic changes is aided by geodetic measurements,
which are made systematically in a number of countries seriously affected by earthquakes.
Earthquakes can do significant damage to buildings, bridges, pipelines, railways, embankments,
and other structures. The type and extent of damage inflicted are related to the strength of the
ground motions and to the behaviour of the foundation soils. In the most intensely
damaged region, called the meizoseismal area, the effects of a severe earthquake are usually
complicated and depend on the topography and the nature of the surface materials. They are
often more severe on soft alluvium and unconsolidated sediments than on hard rock. At distances
of more than 100 km (60 miles) from the source, the main damage is caused by seismic waves
traveling along the surface. In mines there is frequently little damage below depths of a few
hundred metres even though the ground surface immediately above is considerably affected.
Earthquakes are frequently associated with reports of distinctive sounds and lights. The sounds
are generally low-pitched and have been likened to the noise of an underground train passing
through a station. The occurrence of such sounds is consistent with the passage of high-
frequency seismic waves through the ground. Occasionally, luminous flashes, streamers, and
bright balls have been reported in the night sky during earthquakes. These lights have been
attributed to electric induction in the air along the earthquake source.
Tsunamis
Following certain earthquakes, very long-wavelength water waves in oceans or seas sweep
inshore. More properly called seismic sea waves or tsunamis (tsunami is a Japanese word for
“harbour wave”), they are commonly referred to as tidal waves, although the attractions of
the Moon and Sun play no role in their formation. They sometimes come ashore to great
heights—tens of metres above mean tide level—and may be extremely destructive.

After being generated by an undersea earthquake or landslide, a tsunami may propagate
unnoticed over vast reaches of Open Ocean before cresting in shallow water and inundating a
coastline.
The usual immediate cause of a tsunami is sudden displacement in a seabed sufficient to cause
the sudden raising or lowering of a large body of water. This deformation may be the fault source
of an earthquake, or it may be a submarine landslide arising from an earthquake.
Large volcanic eruptions along shorelines, such as those of Thera (c. 1580 BCE)
and Krakatoa (1883 CE), have also produced notable tsunamis. The most
destructive tsunami ever recorded occurred on December 26, 2004, after an earthquake displaced
the seabed off the coast of Sumatra, Indonesia. More than 200,000 people were killed by a series
of waves that flooded coasts from Indonesia to Sri Lanka and even washed ashore on the Horn of
Africa.
Following the initial disturbance to the sea surface, water waves spread in all directions. Their
speed of travel in deep water is given by the formula (Square root of√gh), where h is the sea
depth and g is the acceleration of gravity. This speed may be considerable—100 metres per
second (225 miles per hour) when h is 1,000 metres (3,300 feet). However, the amplitude (that
is, the height of disturbance) at the water surface does not exceed a few metres in deep water,
and the principal wavelength may be on the order of hundreds of kilometres; correspondingly,
the principal wave period—that is, the time interval between arrival of successive crests—may
be on the order of tens of minutes. Because of these features, tsunami waves are not noticed by
ships far out at sea.

When tsunamis approach shallow water, however, the wave amplitude increases. The waves may
occasionally reach a height of 20 to 30 metres above mean sea level in U- and V-shaped
harbours and inlets. They characteristically do a great deal of damage in low-lying ground
around such inlets. Frequently, the wave front in the inlet is nearly vertical, as in a tidal bore, and
the speed of onrush may be on the order of 10 metres per second. In some cases there are several
great waves separated by intervals of several minutes or more. The first of these waves is often
preceded by an extraordinary recession of water from the shore, which may commence several
minutes or even half an hour beforehand.
1946 Hilo tsunami Vintage newsreels show the terrible destruction that a tsunami brought to
Hilo, Hawaii, in 1946.
Organizations, notably in Japan, Siberia, Alaska, and Hawaii, have been set up to provide
tsunami warnings. A key development is the Seismic Sea Wave Warning System, an
internationally supported system designed to reduce loss of life in the Pacific Ocean. Centred
in Honolulu, it issues alerts based on reports of earthquakes from circum-Pacific seismographic
stations.
Seiches
Seiches are rhythmic motions of water in nearly landlocked bays or lakes that are sometimes
induced by earthquakes and tsunamis. Oscillations of this sort may last for hours or even for a
day or two.
The great Lisbon earthquake of 1755 caused the waters of canals and lakes in regions as far away
as Scotland and Sweden to go into observable oscillations. Seiche surges in lakes in Texas, in the
southwestern United States, commenced between 30 and 40 minutes after the 1964 Alaska
earthquake, produced by seismic surface waves passing through the area.
A related effect is the result of seismic waves from an earthquake passing through the seawater
following their refraction through the seafloor. The speed of these waves is about 1.5 km (0.9
mile) per second, the speed of sound in water. If such waves meet a ship with sufficient intensity,
they give the impression that the ship has struck a submerged object. This phenomenon is called
a seaquake.

Intensity & Magnitude
Intensity scales
The violence of seismic shaking varies considerably over a single affected area. Because the
entire range of observed effects is not capable of simple quantitative definition, the strength of
the shaking is commonly estimated by reference to intensity scales that describe the effects in
qualitative terms. Intensity scales date from the late 19th and early 20th centuries, before
seismographs capable of accurate measurement of ground motion were developed. Since that
time, the divisions in these scales have been associated with measurable accelerations of the
local ground shaking. Intensity depends, however, in a complicated way not only on ground
accelerations but also on the periods and other features of seismic waves, the distance of the
measuring point from the source, and the local geologic structure. Furthermore, earthquake
intensity, or strength, is distinct from earthquake magnitude, which is a measure of the
amplitude, or size, of seismic waves as specified by a seismograph reading. See
below Earthquake magnitude.
measuring magnitude and intensityThe Richter scale measures the magnitude of earthquakes,
and the Mercalli scale measures their intensity.
A number of different intensity scales have been set up during the past century and applied to
both current and ancient destructive earthquakes. For many years the most widely used was a 10-
point scale devised in 1878 by Michele Stefano de Rossi and Franƈois-Alphonse Forel. The scale
now generally employed in North America is the Mercalli scale, as modified by Harry O. Wood
and Frank Neumann in 1931, in which intensity is considered to be more suitably graded. A 12-
point abridged form of the modified Mercalli scale is provided below. Modified Mercalli
intensity VIII is roughly correlated with peak accelerations of about one-quarter that of gravity
(g = 9.8 metres, or 32.2 feet, per second squared) and ground velocities of 20 cm (8 inches) per

second. Alternative scales have been developed in both Japan and Europe for local conditions.
The European (MSK) scale of 12 grades is similar to the abridged version of the Mercalli.
Modified Mercalli scale of earthquake intensity
 I. Not felt. Marginal and long-period effects of large earthquakes.
 II. Felt by persons at rest, on upper floors, or otherwise favourably placed to sense
tremors.
 III. Felt indoors. Hanging objects swing. Vibrations are similar to those caused by the
passing of light trucks. Duration can be estimated.
 IV. Vibrations are similar to those caused by the passing of heavy trucks (or a jolt similar
to that caused by a heavy ball striking the walls). Standing automobiles rock. Windows,
dishes, doors rattle. Glasses clink, crockery clashes. In the upper range of grade IV,
wooden walls and frames creak.
 V. Felt outdoors; direction may be estimated. Sleepers awaken. Liquids are disturbed,
some spilled. Small objects are displaced or upset. Doors swing, open, close. Pendulum
clocks stop, start, change rate.
 VI. Felt by all; many are frightened and run outdoors. Persons walk unsteadily. Pictures
fall off walls. Furniture moves or overturns. Weak plaster and masonry cracks. Small
bells ring (church, school). Trees, bushes shake.
 VII. Difficult to stand. Noticed by drivers of automobiles. Hanging objects quivering.
Furniture broken. Damage to weak masonry. Weak chimneys broken at roof line. Fall of
plaster, loose bricks, stones, tiles, cornices. Waves on ponds; water turbid with mud.
Small slides and caving along sand or gravel banks. Large bells ringing. Concrete
irrigation ditches damaged.
 VIII. Steering of automobiles affected. Damage to masonry; partial collapse. Some
damage to reinforced masonry; none to reinforced masonry designed to resist lateral
forces. Fall of stucco and some masonry walls. Twisting, fall of chimneys, factory stacks,
monuments, towers, elevated tanks. Frame houses moved on foundations if not bolted
down; loose panel walls thrown out. Decayed pilings broken off. Branches broken from
trees. Changes in flow or temperature of springs and wells. Cracks in wet ground and on
steep slopes.
 IX. General panic. Weak masonry destroyed; ordinary masonry heavily damaged,
sometimes with complete collapse; reinforced masonry seriously damaged. Serious
damage to reservoirs. Underground pipes broken. Conspicuous cracks in ground.
In alluvial areas, sand and mud ejected; earthquake fountains, sand craters.
 X. Most masonry and frame structures destroyed with their foundations. Some well-built
wooden structures and bridges destroyed. Serious damage to dams, dikes, embankments.
Large landslides. Water thrown on banks of canals, rivers, lakes, and so on. Sand and
mud shifted horizontally on beaches and flat land. Railway rails bent slightly.
 XI. Rails bent greatly. Underground pipelines completely out of service.
 XII. Damage nearly total. Large rock masses displaced. Lines of sight and level distorted.
Objects thrown into air.

With the use of an intensity scale, it is possible to summarize such data for an earthquake by
constructing isoseismal curves, which are lines that connect points of equal intensity. If there
were complete symmetry about the vertical through the earthquake’s focus, isoseismals would be
circles with the epicentre (the point at the surface of the Earth immediately above where the
earthquake originated) as the centre. However, because of the many unsymmetrical geologic
factors influencing intensity, the curves are often far from circular. The most probable position of
the epicentre is often assumed to be at a point inside the area of highest intensity. In some cases,
instrumental data verify this calculation, but not infrequently the true epicentre lies outside the
area of greatest intensity.
Magnitude scales
Earthquake magnitude is a measure of the “size,” or amplitude, of the seismic waves generated
by an earthquake source and recorded by seismographs. (The types and nature of these waves are
described in the section Seismic waves.) Because the size of earthquakes varies enormously, it is
necessary for purposes of comparison to compress the range of wave amplitudes measured on
seismograms by means of a mathematical device. In 1935 the American seismologist Charles F.
Richter set up a magnitude scale of earthquakes as the logarithm to base 10 of the
maximum seismic wave amplitude (in thousandths of a millimetre) recorded on a
standard seismograph (the Wood-Anderson torsion pendulum seismograph) at a distance of 100
km (60 miles) from the earthquake epicentre. Reduction of amplitudes observed at various
distances to the amplitudes expected at the standard distance of 100 km is made on the basis
of empirical tables. Richter magnitudes ML are computed on the assumption that the ratio of the
maximum wave amplitudes at two given distances is the same for all earthquakes and is
independent of azimuth.
Richter first applied his magnitude scale to shallow-focus earthquakes recorded within 600 km of
the epicentre in the southern California region. Later, additional empirical tables were set up,
whereby observations made at distant stations and on seismographs other than the standard type
could be used. Empirical tables were extended to cover earthquakes of all significant focal
depths and to enable independent magnitude estimates to be made from body- and surface-wave
observations. A current form of the Richter scale is shown in the table.

Earthquake energy
Energy in an earthquake passing a particular surface site can be calculated directly from the
recordings of seismic ground motion, given, for example, as ground velocity. Such recordings
indicate an energy rate of 105
watts per square metre (9,300 watts per square foot) near a
moderate-size earthquake source. The total power output of a rupturing fault in a shallow
earthquake is on the order of 1014
watts, compared with the 105
watts generated in rocket motors.
The surface-wave magnitude Ms has also been connected with the surface energy Es of an
earthquake by empirical formulas. These give Es= 6.3 × 1011
and 1.4 × 1025
ergs for earthquakes
of Ms = 0 and 8.9, respectively. A unit increase in Ms corresponds to approximately a 32-fold
increase in energy. Negative magnitudes Ms correspond to the smallest instrumentally recorded
earthquakes, a magnitude of 1.5 to the smallest felt earthquakes, and one of 3.0 to any shock felt
at a distance of up to 20 km (12 miles). Earthquakes of magnitude 5.0 cause light damage near
the epicentre; those of 6.0 are destructive over a restricted area; and those of 7.5 are at the lower
limit of major earthquakes.
The total annual energy released in all earthquakes is about 1025
ergs, corresponding to a rate of
work between 10 million and 100 million kilowatts. This is approximately one one-thousandth
the annual amount of heat escaping from the Earth’s interior. Ninety percent of the total seismic
energy comes from earthquakes of magnitude 7.0 and higher—that is, those whose energy is on
the order of 1023
ergs or more.
Frequency
There also are empirical relations for the frequencies of earthquakes of various magnitudes.
Suppose N to be the average number of shocks per year for which the magnitude lies in a range
about Ms. Thenlog10 N = a − bMsfits the data well both globally and for particular regions; for
example, for shallow earthquakes worldwide, a = 6.7 and b = 0.9 when Ms > 6.0. The frequency
for larger earthquakes therefore increases by a factor of about 10 when the magnitude is
diminished by one unit. The increase in frequency with reduction in Ms falls short, however, of
matching the decrease in the energy E. Thus, larger earthquakes are overwhelmingly responsible
for most of the total seismic energy release. The number of earthquakes per year with Mb > 4.0
reaches 50,000.

Earthquakes around the world
Table: 10 Worst Earthquake of the World
Location Date Magnitude
1 Chile May 22, 1960 9.5
2 Prince William Sound, Alaska March 28, 1964 9.2
3 Andreanof Islands, Aleutlan Islands March 9, 1957 9.1
4 Kamchatka Nov 4, 1952 9.0
5 Off western coast of Sumatra, Indonesia Dec 26, 2004 9.0
6 Off the coast of Ecuador Jan 31, 1906 8.8
7 Rat Islands, Aleutian Island Feb 4, 1965 8.7
8 Northern Sumatra, Indonesia March 28, 2005 8.7
9 India-China border Aug 15, 1950 8.6
10 Kamchatka Feb 3,1923 8.5
North America
There are several major earthquake zones in North America. One of the most notable is found on
Alaska's central coast, extending north to Anchorage and Fairbanks. In 1964, one of the most
powerful earthquakes in modern history, measuring 9.2 on the Richter scale, struck Alaska's
Prince William Sound.
Another zone of activity stretches along the coast from British Columbia to the Baja California
Peninsula, where the Pacific plate rubs against the North American plate. California's Central
Valley, San Francisco Bay Area, and much of Southern California are crisscrossed with active
fault lines that have spawned a number of notable quakes, including the magnitude 7.7 temblor
that leveled San Francisco in 1906.
In Mexico, an active quake zone follows the western Sierras south from near Puerta Vallarta to
the Pacific coast at the Guatemala border. In fact, most of the western coast of Central America
is seismically active, as the Cocos plate rubs against the Caribbean plate. The eastern edge of
North America is quiet by comparison, though there is a small zone of activity near the entry to
the St. Lawrence River in Canada.
South America
South America's most active earthquake zones stretch the length of the continent's Pacific border.
A second notable seismic region runs along the Caribbean coast of Colombia and Venezuela.
Activity here is due to a number of continental plates colliding with the South American plate.
Four of the 10 strongest earthquakes ever recorded have occurred in South America.
In fact, the most powerful earthquake ever recorded took place in central Chile in May 1960,
when a magnitude 9.5 quake hit near Saavedra. More than 2 million people were left homeless
and almost 5,000 killed. A half century later, a magnitude 8.8 temblor struck near the city of
Concepcion in 2010. About 500 people died and 800,000 were left homeless, and the nearby

Chilean capital of Santiago sustained serious damage. Peru has also had its share of earthquake
tragedies.
Asia
Asia is a hotbed of earthquake activity, particularly where the Australian plate wraps around the
Indonesian archipelago, and also in Japan, which lies astride three continental plates. More
earthquakes are recorded in Japan than in any other place on earth. The nations of Indonesia, Fiji,
and Tonga also experience record numbers of earthquakes annually. When a 9.1 earthquake
struck the western coast of Sumatra in 2014, it generated the largest tsunami in recorded history.
More than 200,000 people died in the resulting inundation. Other major historical quakes include
a 9.0 quake on Russia's Kamchatka Peninsula in 1952 and an 8.6 magnitude quake that struck
Tibet in 1950. Scientists as far away as Norway felt that quake.
Central Asia is another of the world's major earthquake zones. The greatest activity occurs along
a swath of territory extending from the eastern shores of the Black Sea down through Iran and
along the southern shores of the Caspian Sea.
Europe
Northern Europe is largely free of major earthquake zones, except for a region around western
Iceland known also for its volcanic activity. The risk of seismic activity increases as you move
southeast toward Turkey and along portions of the Mediterranean coast.
In both instances, the quakes are caused by the African continental plate pushing upward into the
Eurasian plate beneath the Adriatic Sea. The Portuguese capital of Lisbon was practically leveled
in 1755 by a magnitude 8.7 quake, one of the strongest ever recorded. Central Italy and western
Turkey are also epicenters of quake activity.
Africa
Africa has far fewer earthquake zones than other continents, with little to no activity across much
of the Sahara and central part of the continent. There are pockets of activity, however. The
eastern Mediterranean coast, including Lebanon, is one noteworthy region. There, the Arabian
plate collides with the Eurasian and African plates.
The region near the Horn of Africa is another active area. One of the most powerful African
earthquakes in recorded history occurred in December 1910, when a 7.8 quake struck western
Tanzania.
Australia and New Zealand
Australia and New Zealand are a study in seismic contrast. While the continent of Australia has a
low to moderate risk of quakes overall, its smaller island neighbor is one of the world's
earthquake hot spots. New Zealand's most powerful temblor stuck in 1855 and measured 8.2 on
the Richter scale. According to historians, the Wairarapa quake caused some parts of the
landscape to become 20 feet higher in elevation.
Antarctica
At the centre is Webb Island. On the left are some ice cliffs from the Wormald Ice Piedmont
(also on Adelaide Island). The distant mountain behind the ice piedmont is probably the Mount

St. Louis Massif (1280 m) on Arrowsmith Peninsula on the Antarctic mainland, 53 km from
Rothera. The somewhat darker mountains on the right are on Wyatt Island in Laubeuf Fjord.
Compared to the other six continents, Antarctica is the least active in terms of earthquakes. This
is because very little of its land mass lies on or near the intersection of continental plates. One
exception is the region around Tierra del Fuego in South America, where the Antarctic plate
meets the Scotia plate. Antarctica's biggest quake, a magnitude 8.1 event, occurred in 1998 in the
Balleny Islands, which are south of New Zealand. In general, though, Antarctica is seismically
quiet.
Earthquake in Bangladesh
Bangladesh is extremely vulnerable to earthquake activities. Four zones have been identified as
the severest zones in Bangladesh in terms of maximum ground surface acceleration and the
probable movements of the deep-seated crustal faults and lineaments. The severest zones include
northern part of Dinajpur, Rangpur, Mymensingh, Sylhet, Tangail, northern part of Dhaka,
Khulna, Jessor, Kushtia, and Chittagong. 1885 earthquake of Manikganj, 1897 earthquake of
Great Assam, 1918 earthquake of Srimangal, 1930 earthquake of Dhubri, and 1950 earthquake of
Assam all are quite matured to recur any time and may create devastation in Bangladesh. In
order to determine the exact level of seismicity in Bangladesh, an extensive programme to study
of earthquake activities should be undertaken and be institutionalized
Earthquake Zone of Bangladesh:
Zone 1: High Risk
Zone 2: Moderate Risk
Zone 3: Low Risk

Earthquake Zoning Map of Bangladesh
Earthquake History of Bangladesh:
1548: The first recorded earthquake was a terrible one. Sylhet and Chittagong were violently
shaken; the earth opened in many places and threw up water and mud of a Sulphurous smell.
1762: The great earthquake of April 2, which raised the coast of Foul island by 2.74m and the
northwest coast of Chedua island by 6.71m above sea level and also caused a permanent
submergence of 155.40 sq km near Chittagong. The earthquake proved very violent in Dhaka
and along the eastern bank of the meghna as far as Chittagong. In Dhaka 500 persons lost their
lives, the rivers and jheels were agitated and raised high above their usual levels and when they
receded their banks were strewn with dead fish. A large river dried up, a tract of land sank and
200 people with all their cattle were lost. Two volcanoes were said to have opened in the
Sitakunda hills.

1865: Terrible shock was felt, during the second earthquake occurred in the winter of 1865,
although no serious damage occurred.
1885: Known as the Bengal Earthquake. Occurred on 14 July with 7.0 magnitude and the
epicenter was at Manikganj. This event was generally associated with the deep-seated Jamuna
Fault.
1897: Known as the Great India Earthquake with a magnitude of 8.7 and epicenter at Shillong
Plateau. The great earthquake occurred on 12 June at 5.15 pm, caused serious damage to
masonry buildings in Sylhet town where the death toll rose to 545. This was due to the collapse
of the masonry buildings. The tremor was felt throughout Bengal, from the south Lushai Hills on
the east to Shahbad on the west. In Mymensingh, many public buildings of the district town,
including the Justice House, were wrecked and very few of the two-storied brick-built houses
belonging to zamindars survived. Heavy damage was done to the bridges on the Dhaka-
Mymensingh railway and traffic was suspended for about a fortnight. The river communication
of the district was seriously affected (brahmaputra). Loss of life was not great, but loss of
property was estimated at five million Rupees. Rajshahi suffered severe shocks, especially on the
eastern side, and 15 persons died. In Dhaka damage to property was heavy. In Tippera masonry
buildings and old temples suffered a lot and the total damage was estimated at Rs 9,000.
1918: Known as the Srimangal Earthquake. Occurred on 18 July with a magnitude of 7.6 and
epicenter at Srimangal, Maulvi Bazar. Intense damage occurred in Srimangal, but in Dhaka only
minor effects were observed.
1934: Known as the Bihar-Nepal Earthquake. Occurred on 15 January with a magnitude of 8.3
and the epicenter at Darbhanga of Bihar, India. The earthquake caused great damage in Bihar,
Nepal and Uttar Pradesh but did not affect any part of Bangladesh.
Another earthquake occurred on 3 July with a magnitude of 7.1 and the epicenter at Dhubri of
Assam, India. The earthquake caused considerable damages in greater Rangpur district of
Bangladesh.
1950: Known as the Assam Earthquake. Occurred on 15 August with a magnitude of 8.4 with the
epicenter in Assam, India. The tremor was felt throughout Bangladesh but no damage was
reported.
1997: Occurred on 22 November in Chittagong with a magnitude of 6.0. It caused minor damage
around Chittagong town.
1999: Occurred on 22 July at Maheshkhali Island with the epicenter in the same place, a
magnitude of 5.2. Severely felt around Maheshkhali island and the adjoining sea. Houses cracked
and in some cases collapsed.

2003: Occurred on 27 July at Kolabunia union of Barkal upazila, Rangamati district with
magnitude 5.1. The time was at 05:17:26.8 hours.
Status of earthquakes in Bangladesh
Bangladesh is surrounded by the regions of high seismicity which include the Himalayan Arc
and shillong plateau in the north, the Burmese Arc, Arakan Yoma anticlinorium in the east and
complex Naga-Disang-Jaflong thrust zones in the northeast. It is also the site of the Dauki Fault
system along with numerous subsurface active faults and a flexure zone called Hinge Zone.
These weak regions are believed to provide the necessary zones for movements within the basin
area.
In the generalised tectonic map of Bangladesh the distribution of epicentres is found to be linear
along the Dauki Fault system and random in other regions of Bangladesh. The investigation of
the map demonstrates that the epicentres are lying in the weak zones comprising surface or
subsurface faults. Most of the events are of moderate rank (magnitude 4-6) and lie at a shallow
depth, which suggests that the recent movements occurred in the sediments overlying the
basement rocks. In the northeastern region (surma basin), major events are controlled by the
Dauki Fault system. The events located in and around the madhupur tract also indicate shallow
displacement in the faults separating the block from the alluvium.
The first seismic zoning map of the subcontinent was compiled by the Geological Survey of
India in 1935. The Bangladesh Meteorological Department adopted a seismic zoning map in
1972. In 1977, the Government of Bangladesh constituted a Committee of Experts to examine
the seismic problem and make appropriate recommendations. The Committee proposed a zoning
map of Bangladesh in the same year.
In the zoning map, Bangladesh has been divided into three generalised seismic zones: zone-I,
zone-II and zone-III. Zone-I comprising the northern and eastern regions of Bangladesh with the
presence of the Dauki Fault system of eastern Sylhet and the deep seated Sylhet Fault, and
proximity to the highly disturbed southeastern Assam region with the Jaflong thrust, Naga thrust
and Disang thrust, is a zone of high seismic risk with a basic seismic co-efficient of 0.08.
Northern Bangladesh comprising greater Rangpur and Dinajpur districts is also a region of high
seismicity because of the presence of the Jamuna Fault and the proximity to the active east-west
running fault and the Main Boundary Fault to the north in India. The Chittagong-Tripura Folded
Belt experiences frequent earthquakes, as just to its east is the Burmese Arc where a large
number of shallow depth earthquakes originate. Zone-II comprising the central part of
Bangladesh represents the regions of recent uplifted Pleistocene blocks of the Barind and
Madhupur Tracts, and the western extension of the folded belt. The Zone-III comprising the
southwestern part of Bangladesh is seismically quiet, with an estimated basic seismic co-efficient
of 0.04.

Mitigation approach
The occurrence of earthquakes in an earthquake prone region cannot be prevented. Rather, all
that could be done is to make a prediction and issue a warning for minimising loss of life and
property. Although precise prediction is not always possible, an acceptable valid prediction of an
earthquake will certainly minimise the loss of life and property. However, as far as Bangladesh is
concerned a detailed geological map including the delineation of all crustal faults and lineaments
is of prime importance. The Aeromagnetic survey of Bangladesh has already provided the
pattern and distribution of such faults and lineaments. By now the delineation of faults within the
Tertiary sections are well established, but the situation within the Quaternary section is quite
uncertain. It is evident that Quaternary sediments are affected by various earthquake events in
Bangladesh pertaining to uplift, subsidence, ground deformation and massive liquefaction. Since
water plays an important role in fault creep and fault slip, a small amount of water can produce
an effect on a lubricated surface for fault displacement with a stress drop of only 10 to 100 bars.
The earthquake disaster mitigation approach should be followed by
(i) Pre-disaster physical planning of human settlements,
(ii) Building measures for minimising the impact of disaster and
(iii) Management of settlements.

References
1. Are Earthquakes Really on the Increase? Archived 2014-06-30 at the Wayback Machine.,
USGS Science of Changing World. Retrieved 30 May 2014.
2. Alam, M.K.1988. Geology of Madhupur Tract and its adjoining areas in Bangladesh. Records
of the Geological Survey of Bangladesh.Vol.5, pt.3.p.18.
3. Ambraseys,N.and R.Billham.2003.Reevaluated intensities for the Great Assam earthquake of
12 June 1897, Shillong, India.Bulletin of the. Seismological Society of America.93 (2):655–673.
4. Ambraseys,N.and J.J.Douglas.2004.Magnitude calibration of north Indian earthquakes.
Geophysical Journal International.159:165–206.
5. "Earthquake Facts and Statistics: Are earthquakes increasing?". United States Geological
Survey. Archived from the original on 2006-08-12. Retrieved 2006-08-14.
6. "Earth's gravity offers earlier earthquake warnings". Retrieved 2016-11-22.
7. Encyclopedia Britannica, Inc.
8. “Gravity shifts could sound early earthquake alarm". Retrieved 2016-11-23.
9. Islam, N. (2005) Dhaka Now: Contemporary Urban Development. Bangladesh Geographical
Society, Dhaka.
10. Ohnaka, M. (2013). The Physics of Rock Failure and Earthquakes. Cambridge University
Press. p. 148. ISBN 9781107355330
11. Rahman, S. (2003) ‘Dhaka: A disaster in waiting?’ The Daily Star (Dhaka). 3 August.
12. Spence, William; S. A. Sipkin; G. L. Choy (1989). "Measuring the Size of an Earthquake".
United States Geological Survey. Archived from the original on 2009-09-01. Retrieved 2006-11-
03.
13. https://www.banglapedia.org
14. https://en.wikipedia.org

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