MEASURING EARTHUQAKES
Earthquakes have a greater effect on society than most people think. These effects
range from economical to structural to mental. An earthquake only occurs for a few brief
moments. But the aftershocks can continue for weeks; the damage can continue for years.
This made us to understand more about earthquakes by measuring it.
Measuring Earthquakes
History: The earliest seismologists were the Chinese who worked hard to record their
quakes in detail. They even developed a means to predict earthquakes by filling a ceramic
jar to the brim with water and leaving it set. If the water overflowed the jar, then an
earthquake was imminent. Of course, this means of prediction was unreliable and uncertain.
It is thought that some animals may feel vibrations from a quake before humans,
and that even minutes before a quake dogs may howl and birds fly erratically. Aristotle was
one of the first Europeans to create a theory about the origin of Earthquakes. He thought
that they were the result of heavy winds.
The First Seismograph
The first seismograph was invented by the Chinese astronomer and mathematician,
Chang Heng. He called it an "earthquake weathercock”. It had eight dragons and each of
the eight dragons had a bronze ball in its mouth below the dragons, at the base of the
weather cock are eight toads with their mouths open representing eight directions.
Whenever there was even a slight earth tremor, a mechanism inside the seismograph would
open the mouth of one dragon.
The bronze ball would fall into the open mouth of one of the toads, making enough
noise to alert someone that an earthquake had just happened. Imperial watchmen could tell
the direction from which the earthquake came by seeing which dragon's mouth was empty.
Fig. 3: The First Seismograph – “earthquake weathercock” – A Chinese invention.
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In the 1850 Robert Mallet, figured out a means to measure the velocity of seismic
waves. Meanwhile, in Italy, Luigi Palmieri invented an electromagnetic seismograph, one of
which was installed near Mount Vesuvius and another at the University of Naples. These
seismographs were the first seismic instruments capable of routinely detecting earthquakes
imperceptible to human beings. In 1872 a U.S. scientist named Grove Gilbert figured out
that earthquakes usually center around a fault line. It was after the 1906 earthquake in San
Francisco that Harry Reid hypothesized that, earthquakes were likely the result of a build-up
of pressure along these faults. It was about 1910 that Alfred Wegener published his theory
of plate tectonics to explain volcanic and seismic activity. Since then, seismologists have
continued to work at a furious pace, building better instruments, computer models, theories,
and forecast to study the causes and effects of earthquakes.
Modern Seismographs
Most seismographs today are electronic, but a basic seismograph is made of a drum
with paper on it, a bar or spring with a hinge at one or both ends, a weight, and a pen. The
one end of the bar or spring is bolted to a pole or metal box that is bolted to the ground.
The weight is put on the other end of the bar and the pen is stuck to the weight. By
studying the seismogram, the seismologist can tell how far away the earthquake was and
how strong it was. This record does not tell the seismologist exactly where the epicenter
was, just that the earthquake happened so many miles or kilometers away from that
seismograph.
In a seismogram, there will be wiggly lines all across it. These wiggly lines are
seismic waves that the seismograph has recorded. Most of these waves were so small
(microseisms) that nobody felt them. At the time of earthquake, the P-wave will be the first
wiggle, which is bigger than the microseisms. P-waves are the fastest seismic waves, and
are usually the first ones that a seismograph records. The next set of seismic waves on the
seismogram will be the S-waves and these are normally bigger than the P-waves. The
surface waves (Love and Rayleigh waves) are often larger waves marked on the
seismogram.
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Fig. 4: Simple schematic diagram of modern seismograph
Working
Fig. 5: Schematic representation of Modern Seismograph
The seismometer must be able to move with the vibrations, yet part of it must
remain nearly stationary. This is accomplished by isolating the recording device (like a pen)
from the rest of the Earth using the principal of inertia. For example, if the pen is attached
to a large mass suspended by a spring, the spring and the large mass move less than the
paper which is attached to the Earth, and on which the record of the vibrations is made.
Surface waves travel a little slower than S-waves, so they tend to arrive at the
seismograph just after the S-waves. For shallow earthquakes, the surface waves may be the
largest waves recorded by the seismograph. Often they are the only waves recorded at long
distance, from medium-sized earthquakes.
Fig. 6: Wiggly lines of seismograph during an earthquake
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Different types of Scales:
Scales
Unscientific
Methods
Mercalli Scale
Modified
Mercalli
Intensity Scale
Scientific
Method
Richter Scale
Fig. 7: Flow chart showing different type of scales
The Richter scale
The magnitude of most earthquakes is measured on the Richter scale, invented by
Charles F. Richter in 1934. The Richter magnitude is calculated from the amplitude
of the largest seismic wave recorded for the earthquake, no matter what type of
wave was the strongest. The Richter magnitudes are based on a logarithmic scale
(base 10).
So it means that for each whole number, up on the Richter scale, the amplitude of
the ground motion recorded by a seismograph goes up ten times. On this scale, an
earthquake of magnitude 5 would result in ten times the level of ground shaking as an
earthquake of magnitude 4 and it is 32 times as much as energy would be released.
MAGNITUDE
CHANGE
1.0
0.5
0.3
0.1
GROUND MOTION CHANGE
(DISPLACEMENT)
10.0 TIMES
3.2 TIMES
2.0 TIMES
1.3 TIMES
ENERGY CHANGE
3.2 TIMES
5.5 TIMES
3 TIMES
1.4 TIMES
Table 1: Showing comparison between Magnitude Vs Ground Motion & Energy
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For example, a magnitude of 6.0 earthquake produces 10 times more ground motion
than a magnitude of 5.0 earthquake. The energy difference is about 32 times. The energy
release is the best indicator of destructive power of earthquake.
Richter Scale
(Magnitudes)
Earthquake Effects
Less than 3.5
Generally not felt, but recorded.
3.5-5.4
Often felt, but rarely causes damage.
5.5 - 6.0
At most slight damage to well-designed buildings. Can
cause major damage to poorly constructed buildings
over small regions.
6.1-6.9
Can be destructive in areas up to about 100 km across
where people live.
7.0-7.9
Major earthquake. Can cause serious damage over
larger areas.
8 or greater
Great earthquake. Can cause serious damage in areas
several hundred kilometers across.
Table 2: Showing possible effect for different magnitudes of Richter scale.
Comparison between Bhuj 2001 and Sumatra 2004
Bhuj 2001 Earthquake: Magnitude 7.7 on Richter scale
Sumatra 2004 Earthquake: Magnitude 9.1 on Richter scale
Since, the magnitude scale is logarithmic scale.
(109.1/107.7) = 101.4 = 25.1189
i.e. Sumatra earthquake is 25.1189 times greater than Bhuj earthquake. In other words,
Sumatra earthquake is equal to 25.1189 Bhuj earthquakes.
Energy difference calculation
Based on empirical formula log (E) is proportional to 1.5M. Where, “E” is energy and
“M” is magnitude. 101.5 is approximately 32 times.
((101.5)9.1)/ ((101.5)7.7) = 10(1.5 x 1.4) = 125.8925
The energy released is equal to 125.8925 times.
The Mercalli Scale
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Another way to measure the strength of an earthquake is to use the Mercalli scale.
Invented by Giuseppe Mercalli in 1902, this scale uses the observations of people who
experienced the earthquake to estimate its intensity. The Mercalli scale is not considered as
scientific as the Richter scale. Some witnesses of the earthquake might exaggerate just how
bad things were during the earthquake. Therefore, the amount of damage caused by the
earthquake may not accurately record how strong it was either.
The Modified Mercalli Intensity Scale
The effect of an earthquake on the earth's surface is called the intensity. Although
numerous intensity scales have been developed over the last several hundred years to
evaluate the effects of earthquakes, the one currently used in the United States is the
Modified Mercalli (MM) Intensity Scale. It was developed in 1931 by the American
seismologists, Harry Wood and Frank Neumann. This scale, composed of 12 increasing
levels of intensity that range from imperceptible shaking to catastrophic destruction, is
designated by Roman numerals. It does not have a mathematical basis; instead, it is an
arbitrary ranking based on observed effects.
The Modified Mercalli Intensity value assigned to a specific site after an earthquake
has a more meaningful measure of severity to the nonscientist than the magnitude because
intensity refers to the effects actually experienced at that place.
Table 3: Showing comparison between Richter scale and Modified Mercalli Intensity
I.
II.
III.
IV.
V.
VI.
VII.
VIII.
Not felt, except by a very few, under especially favorable conditions.
Felt only by a few persons at rest, especially on upper floors of buildings.
Felt quite noticeably by persons indoors, especially on upper floors of buildings.
Many people do not recognize it as an earthquake. Standing motor cars may rock
slightly. Vibrations are similar to the passing of a truck.
Felt indoors by many, outdoors by few during the day. At night, some awakened.
Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy
truck striking building. Standing motor cars rocked noticeably.
Felt by nearly everyone; many awakened. Some dishes, windows broken.
Unstable objects overturned. Pendulum clocks may stop.
Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen
plaster.
Damage negligible in buildings of good design and construction; slight to
moderate in well-built ordinary structures; considerable damage in poorly built or
badly designed structures; some chimneys broken.
Damage slight in specially designed structures; considerable damage in ordinary
substantial buildings with partial collapse. Damage great in poorly built structures.
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IX.
X.
XI.
XII.
Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture
overturned.
Damage considerable in specially designed structures; well-designed frame
structures thrown out of plumb. Damage great in substantial buildings, with partial
collapse. Buildings shifted off foundations.
Some well-built wooden structures destroyed; most masonry and frame structures
destroyed with foundations. Rails bent.
Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent
greatly.
Damage total. Lines of sight and level are distorted. Objects thrown into the air.
Locating and Measuring Earthquakes
Earthquake Distance:
The epicenter is located using the difference in the arrival times between P and S
wave recordings, which are related to distance. By comparing the two equations given
below it can be inferred that the velocity of P wave is greater than S wave velocity.
P wave velocity depends on a material's "plane wave modulus" and its density. Since
solids, liquids and gasses have a finite bulk modulus; P waves can travel through any of
these medium. S-wave velocity depends on a material's shear modulus (μ), and density ().
Since fluids (liquids and gasses) have zero shear modulus, S waves cannot travel through
fluids.
It has been established that velocity of "P" waves is 1.68 to 1.75 times than "S"
waves, the velocity is depending on the particular soil construction. Since there is difference
in velocity between the waves, the arrival time these waves to a particular seismic station
will also differ. By knowing velocity and time, distance can be calculated by plotting distance
and time travel graph.
For example, from the fig. 8, the first P – wave was arrived the seismic station on 7th
minute and the S – wave was recorded on 12th minute. Plot the graph between travel time
along Y – axis and distance to the epicenter along X – axis. For the 5 minute time difference
it is found from the graph that the epicenter is 3800 km away from the particular seismic
station. It can be verified that one cannot find the same 5 minute time gap between P –
wave and S - wave for another epicenter distance.
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Fig. 8: Travel – time graph to find epicenter distance from the seismic station
Earthquake Direction:
Three or more seismic stations can be used to find exact location of an earthquake
epicenter. Using compass, draw a circle with a radius equal to the distance measured
earlier. The center of the circle will be the location of the seismograph. The epicenter of the
earthquake is somewhere on the edge of that circle. Do the same thing for the distance to
the epicenter that the other seismograms recorded. All of the circles should intersect at
point. The point where all of the circles intersect is the approximate epicenter of the
earthquake.
For example, in Fig. 9, three seismic stations located at different distances from the
epicenter, are kept as the center of circle and circles are drawn, after finding the distances
by using above mentioned method. The intersection three circles indicates that the
earthquake was triggered along the mid-Atlantic ridge, i.e. in between South America and
Africa.
The epicenter is the point on the Earth's surface that is directly above the hypocenter
or focus, the point where an earthquake or underground explosion originates. In the case of
earthquakes, the epicenter is directly above the point where the fault begins to rupture, and
in most cases, it is the area of greatest damage. But for larger events, the length of the
fault rupture is much longer, and damage can be spread across the rupture zone.
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Fig. 9: Finding epicenter of an earthquake
For example, in the magnitude 7.9, 2002 Denali earthquake in Alaska, the epicenter
was at the western end of the rupture. But the greatest damage occurred about 330 km
away at the eastern end of the rupture zone.
Finding the magnitude of an earthquake
Fig.10: Showing reference scale to find location and magnitude an earthquake
Measure the amplitude of the strongest wave. The amplitude is the height of the
strongest wave. On this seismogram, the amplitude is 23 millimeters. Find 23 millimeters on
the right side of the chart and mark that point. Place a ruler (or straight edge) on the chart
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between the points you marked for the distance to the epicenter and the amplitude. The
point where your ruler crosses the middle line on the chart marks the magnitude (strength)
of the earthquake. This earthquake had a magnitude of 5.
Focal Depth of an earthquake
Earthquakes can occur anywhere between the Earth's surface and about 700
kilometers below the surface. For scientific purposes, this earthquake depth range of 0 - 700
km is divided into three zones: Shallow, Intermediate, and Deep.
Shallow earthquakes are between 0 km and 70 km in deep. Intermediate
earthquakes, normally occur at the depth of 70 - 300 km and Deep seated earthquakes
occur at a depth of 300 - 700 km. In general, the term "deep-focus earthquakes" is applied
to earthquakes deeper than 70 km.
The focal depth can be calculated from measurements based on seismic wave
phenomena. As with all wave phenomena in physics, there is uncertainty in such
measurements that grows with the wavelength. So the focal depth of the source of these
long-wavelength waves is difficult to determine exactly. Very strong earthquakes radiate a
large fraction of energy is released in seismic waves. This is associated with very long
wavelengths. Therefore a stronger earthquake involves the release of energy from a larger
mass of rock.
Calculating Depth of an earthquake
The most obvious indication on a seismogram that a large earthquake has a deep
focus is the small amplitude of the recorded surface waves. The surface waves do generally
indicate that an earthquake is either shallow or may have some depth. The most accurate
method of determining the focal depth of an earthquake is to read a depth phase recorded
on the seismogram.
The depth phase is the characteristic phase (pP). “pP” initially goes up from the
earthquake source, reflects off the Earth's surface. Then it follows closely behind the P wave
to arrive at the seismograph. At distant seismograph stations, the “pP” follows the P wave by
a time interval that changes slowly with distance but rapidly with depth. This time interval,
pP - P (pP minus P), is used to compute depth-of-focus tables. Then the additional travel
time for pP is simply twice the vertical travel time from hypocenter to the surface.
(i.e.) The extra travel time as (pP - P) = 2d/v,
Where,
(pP - P) is the travel time difference,
‘d’ is hypo central depth, and
‘v’ is the average P wave velocity above the source.
Using the time difference of pP-P as read from the seismogram and the distance
between the epicenter and the seismograph station, the depth of the earthquake can be
determined from published travel-time curves or depth tables.
EFFECTS OF EARTHQUAKES
Earthquakes really pose little direct danger to a person. People can't be shaken to
death by an earthquake. Some movies show scenes with the ground suddenly opening up
and people falling into fiery pits, but this just doesn't happen in real life. Earthquakes really
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pose little direct danger to human beings. Their effects like ground shaking, ground
displacement, liquefaction, flooding and fire are posing danger to human beings.
The Effect of Ground Shaking
Ground Shaking: The first main earthquake danger is the effect of ground shaking. Buildings
can be damaged by the shaking itself. Also it can be due to the ground beneath them
settling to a different level than it was before the earthquake, called as subsidence.
Shaking of the ground caused by the passage of seismic waves near the epicenter of
the earthquake is responsible for the collapse of most structures. The intensity of ground
shaking depends on distance from the epicenter and on the type of bedrock underlying the
area.
In general, loose unconsolidated sediment is subject to more intense shaking than
solid bedrock. Damage to structures from shaking depends on the type of construction.
Concrete and masonry structures, because they are brittle are more susceptible to damage
than wood and steel structures, which are more flexible.
Fig. 11: Showing destruction due to ground shaking during 2010 Haiti earthquake
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Fig. 12: Showing Ground Deformation-Differential Settling during 1967 Caracas, Venezuelan
earthquake. Photo Credit: National Geophysical Data Center
Liquefaction: It is the process of mixing sand or soil and groundwater (water underground)
during the shaking of a moderate or strong earthquake. Liquefaction is a process that occurs
in water-saturated unconsolidated sediment due to shaking. In areas underlain by such
material, the ground shaking causes the grains to get loose the grain to grain contact, and
thus the material tends to flow.
Buildings can even sink into the ground if soil liquefaction occurs. When the water
and soil are mixed, the ground becomes very soft and acts similar to quicksand. If
liquefaction occurs under a building, it may start to lean, tip over, or sink several feet. The
ground firms up again after the earthquake has past and the water has settled back down to
its usual deeper place in the ground. Liquefaction is a hazard in areas that have
groundwater near the surface and sandy soil.
Buildings can also be damaged by strong surface waves making the ground heave
and lurch. Any buildings in the path of these surface waves can lean or tip over from all the
movement. The ground shaking may also cause landslides, mudslides, and avalanches on
steeper hills or mountains, all of which can damage buildings and hurt people.
Fig. 12: Explaining process of liquefaction
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Fig. 13: Buildings got toppled and sunk due to an 1964, Niigata, Japan earthquake. About
1/3 of the city subsided by as much as 2 meters as a result of sand compaction. Photo
Credit: National Geophysical Data Center
Ground Displacement
The second main earthquake hazard is ground displacement (ground movement)
along a fault. If a structure (a building, road, etc.) is built across a fault, the ground
displacement during an earthquake could seriously damage or rip apart that structure.
Fig. 14: Soils failed and moved down slope. The soil failure left a part of the school on
unmoved ground and dropped the remainder into a wide trough during 1964 Prince William
Sound, Alaska earthquake Photo Credit: National Geophysical Data Center.
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Flooding
The third main hazard is flooding. An earthquake can rupture (break) dams or levees
along a river. The water from the river or the reservoir would then flood the area, damaging
buildings and maybe sweeping away or drowning people. Tsunami also can cause flooding,
when earthquake occurred in oceanic or in coastal region.
Physics of Tsunami
The term tsunami comes from the Japanese, meaning "harbor" (tsu, 津) and "wave"
(nami, 波). This type of tidal wave is a series of water waves caused by the displacement of
a large volume of a body of water, usually an ocean, sometimes in large lakes, is rapidly
displaced on a massive scale.
Generation mechanisms
The principal generation mechanism (or cause) of a tsunami is the displacement of a
substantial volume of water or perturbation of the sea. This displacement of water is usually
attributed to earthquakes, landslides, volcanic eruptions, or more rarely by meteorites and
nuclear tests. The waves formed in this way are then sustained by gravity. It is important to
note that tides do not play any part in the generation of tsunamis; hence referring to
tsunamis as 'tidal waves' is inaccurate.
Tsunamis can be generated when the sea floor abruptly deforms and vertically
displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that
are associated with the earth's crustal deformation; when these earthquakes occur beneath
the sea, the water above the deformed area is displaced from its equilibrium position. More
specifically, a tsunami can be generated when thrust faults associated with convergent or
destructive plate boundaries move abruptly, resulting in water displacement, due to the
vertical component of movement involved. Movement on normal faults will also cause
displacement of the seabed, but the size of the largest of such events is normally too small
to give rise to a significant tsunami.
Fig. 15: Vertical downward movement due to subduction causes sudden
displacement of water
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Fig. 16: Vertical uplift of a fault causes sudden rise of water
Shallow undersea earthquakes are responsible for most tsunamis though at time
landslides triggered by smaller seismic events can also generated potentially lethal waves.
Strong earthquakes cause a displacement of the crust. When they occur underwater, this
crustal movement disturbs a large volume of water like a giant paddle and ripples spread
out in all directions at speeds of 600-800 kilometers per hour.
History of Tsunami Devastation
1. In 1703 at Awa, Japan with more than 100,000 people dead.
2. On April 24, 1771, a tsunami struck the Japanese island of Ishigaki. It was 85 meters
high. It hurled a 750 ton piece of coral to a distance of 2.5 kilometers inland.
3. Again in Japan, 27,000 people drowned in 1896, in a giant tsunami.
4. An underwater volcanic eruption destroyed island of Krakatau (Krakatoa) on August
26-27, 1883, a wave 35 meters high swept across the East Indies killing in excess of
36,000 people.
5. The 1960 earthquake in Chile created tsunami waves that traveled more than 10,000
kilometers to Hilo, Hawaii. The 12 meters high water wall killed 61 people and
destroyed many buildings.
6. In 2004 earthquake in the Indonesian region triggered tsunami and more than
200,000 people died. Most of the devastation due to tsunami was in counties like
Indonesia, Thailand, Malaysia, Singapore, India and Sri Lanka.
The Strength of Tsunami
A tsunami at El Popoyo, Indonesia, a large rock was carried from an offshore region
to 50 m inland and raised 1.85 m above sea level. The rock measured 2.3 m in length, 1.6
m in breadth and 0.5 m in thickness and provides a silent testimony of the force of the
tsunami waves.
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Theory of Waves
Waves propagates by transferring energy, they does not move particles in the
direction of their motion. The relation between energy, frequency and velocity of a wave can
be understood from the following equation.
E = h = hv / 
Fig. 17: showing basic physical measurements for a wave
Ocean Waves (Tidal waves)
Energy from the oceans is also available in the form of oceanic waves or sea-waves.
Due to the blowing of wind on the surface of ocean, very fast sea-waves (or water waves)
move on its surface. As wind grabs the water molecules, the friction causes ripples. Wind
continues to push against these ripples in a snowball effect that eventually creates a large
wave. The size and momentum of the waves increases, as they approach the sea shore.
Simply saying, the wave energy moves on top of the water.
Fig. 18: As wind hits the water, ripples form. Then, wind gets an even better grip and
continues to push the ripples until they grow to be large waves. Photo Curtsey: How stuff
works
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Fig. 19: Friction due to grabbing of water molecules increases friction, which causes ripples
and wave starts propagate.
A few factors determine how strong an individual wave will be. These include:



Speed of wind: The faster the wind is traveling, the bigger a wave will be.
Time of wind: The wave will get larger the longer the length of time the wind is
hitting it.
Distance of wind: The farther the wind travels against the wave (known as fetch),
the bigger it will be.
Where, “” is frequency of a wave, “g” is acceleration due to gravity, “” is wavelength of
the tidal waves and “d” is depth of a wave (amplitude)
But,
 = (2v /)
Therefore, by substituting in the above equation,
 = g (2v /) / 2
= (g v) /
Where, “v” is the velocity of the wind and “” is the angular frequency of the wave.
The frequency of a wave increases with acceleration due to gravity and velocity of
the wind. This is the reason the regular oceanic wave height is more along sea shores,
which are located away from the equator, since the acceleration due to gravity increases.
Also the velocity of the wind, increases the frequency of the wave get increased. This is the
reason the wave heights are more during cyclone formation. If the depth of the wave
(amplitude of the wave) increases, the frequency of the wave gets increased. But the
wavelength of the wave decreases with increase in frequency.
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Characteristics of tsunami waves
Tsunamis are characterized as shallow-water waves. Shallow-water waves are
different from wind-generated waves, the waves many of us have observed on a beach.
Wind-generated waves usually have period (T) of 5 to 20 second and a wavelength () of
about 100 to 200 meters (300 to 600 feet). The tidal wave Height regular wind generated
waves around 3 metres (10 feet) and the wave speed are in the range between 16 and 32
km/hr (10 to 20 miles/hr).
Fig. 20: Details of regular wind generated wave
Unlike a normal wave, energy of a tsunami moves through the water, not on top of
it. A tsunami can have a period in the range of 10 minutes to 2 hours and a wavelength in
excess of 500 km (300 miles). It is because of their long wavelengths that tsunamis behave
as shallow-water waves. From equation (given in page 18), one can understand that a wave
is characterized as a shallow-water wave when the ratio between the water depth and its
wavelength gets very small. Hence in very deep water, a tsunami will travel at high speeds
and travel great transoceanic distances with limited energy loss. For example, when the
ocean is 20,000 feet (6100 m) deep, unnoticed tsunami travel about 550 miles per hour
(890 km/hr), the speed of a jet airplane. And they can move from one side of the Pacific
Ocean to the other side in less than one day.
The speed of a shallow-water wave is equal to the square root of the product of the
acceleration of gravity (9.8 m/s2) and the depth of the water.
Velocity
v  g d
As a tsunami leaves the deep water of the open sea and propagates into the more
shallow waters near the coast, it undergoes a transformation. Since the speed of the
tsunami is related to the water depth, as the depth of the water decreases, the speed of the
tsunami diminishes. The rate at which a wave loses its energy is inversely related to its
wavelength. Since a tsunami has a very large wave length, it will lose little energy as it
propagates.
Energy loss rate = 1/
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Therefore, the speed of the tsunami decreases as it enters shallower water, and the
height of the wave grows. Because of this "shoaling" effect, a tsunami that was
imperceptible in deep water may grow in height. Since the energy is conserved, the change
of total energy of the tsunami to be remains constant. As the kinetic energy decreases the
potential energy increases. Also from the height it attained, when come down its velocity
increases due to transformation of energy from P.E. to K.E. So with the sheer weight of
water and velocity the tsunami pulverize objects in its path, often reducing buildings to their
foundations. Large objects such as ships and boulders can be carried several miles inland
before the tsunami subsides.
Fig. 21: Details of tsunami wave in Deep Ocean
Fig. 22: Details of the tsunami wave, when it approaches the shore
Fig. 23: Showing change in amplitude and wavelength of a tsunami wave
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Detecting tsunamis is a very difficult thing to do. Wave begins with the height of
about 12 - 23 inches and look like nothing more than the gentle rise and fall of the sea
surface. Tsunamis in deep water can have a wavelength greater than 300 miles (500
kilometers) and a period of about an hour. As they approach the shore the height of the
wave increases due to decrease in water depth and wave length. When a tsunami moving
away from the beach, it is fast and short amplitude, therefore its velocity “v” and
wavelength “” increases, so its amplitude “A” get decreased. When a Tsunami moving
toward the beach, it is slow and gained higher amplitude, therefore its velocity “v” and
wavelength “” decreases, with its amplitude “A” get decreased.
Fire
The fourth main earthquake hazard is fire. These fires can be started by broken gas
lines and power lines, or tipped over wood or coal stoves. They can be a serious problem,
especially if the water lines that feed the fire hydrants are broken, too. For example, after
the Great San Francisco Earthquake in 1906, the city burned for three days. Most of the city
was destroyed and 250,000 people were left homeless.
Fig. 24: Fire at Valdez, Alaska. The tank fire was triggered by failure of oil storage tanks at
the Union Oil tank farm. Photo Credit: National Geophysical Data Center.
The photograph was taken around 10:30 p.m., about 5 hours after the quake; the
whole waterfront was burning furiously. Some buildings along Front Street and Standard
Oil's pumping control station also caught fire. The Union Oil tank farm continued to burn for
two weeks.
Dr. N. Venkatanathan
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