Lecture earthquake-1

Earthquake
View as a Disaster
Composed by
H.M.A. Mahzuz
Assistant Professor (PhD Fellow),
Department of Civil and Environmental Engineering,
Shahjalal University of Science & Technology, Sylhet,
Bangladesh.
E-mail: mahzuz_211@yahoo.com

Outline of the lecture
1) What is Earthquake?
2) Earthquake causes, source, Seismic waves
3) Earthquake Measuring scales
4) Effects of Earthquake
5) Special Concern: Tsunami

1) What is Earthquake?
A sudden movement of ground that may cause
a huge loss of life, property.
It is an unpredictable event in which masses of
rock shift below Earth‘s surface, releasing
enormous amounts of energy and sending out
shock waves that sometimes cause the ground
to shake dramatically. Not all earthquakes are
enormous, but they can become one of Earth's
most destructive forces.

THE CAUSES OF EARTHQUAKES:
→Rock fall.
→Volcanic.
→Tectonic.
2) Earthquake causes, sources, Seismic waves

220 million years ago

The earths crust is divided into a number
moving of tectonic plates

Tectonic Plate Velocity
Plate
Absolute Velocity
(cm/yr)
Antarctic ~2.05
African ~2.15
Arabian ~4.65
Caribbean ~2.45
Cocos ~8.55
Eurasian ~0.95
Indian-australian ~6.00
Nazca ~7.55
North American ~1.15
Pacific ~8.10
Philippine ~6.35
South American ~1.45
Q: What kind of plate
boundary exists between
the Indian-Australian and
Eurasian plate?
Ans: Convergent. The
plates are moving toward
each other and the Indo-
Austrailian Plate is being
subducted under the
Eurasian Plate.

Plates move in different directions.
Match the diagram with the correct
caption
• Plates slide past each
other
• Plate slides under
another plate
• Plates move towards
each other
• Plates move away from
each other

Largest Earthquakes in the World Since 1900 (Magnitude 9.5-8.5)

Tectonic Earthquakes
Most earthquakes occur at plate
margins due to tension, compression
or shearing forces. Rocks at plate
margins are in constant motion and are
being pushed, pulled, bent, twisted
and folded.
Inevitably at some point they must
break or crack to produce faults, folds
and joints.

In geology, a fault is a planar fracture or discontinuity
in a volume of rock, across which there has been
significant displacement along the fractures as a
result of earth movement. Large faults within the
Earth's crust result from the action of plate
tectonic forces, with the largest forming the
boundaries between the plates, such as subduction
zones or transform faults. Energy release associated
with rapid movement on active faults is the cause of
most earthquakes.
A fault line is the surface trace of a fault, the line of
intersection between the fault plane and the Earth's
surface.

In geology, a fold is the undulation or wave like
pattern of rock layers. It may be produced due to
extreme pressure from the converging rock layers
towards each other. Folds may give the rise of joints
and faults. Therefore it also an unexpected
phenomena which may arise earthquakes in future.

In geology the term joint refers to fracture in rock where
the displacement associated with the opening of the
fracture is greater than the displacement due to lateral
movement in the plane of the fracture (up, down or
sideways) of one side relative to the other. Typically, there
is little to no lateral movement across joints. This makes
joints different from a fault which is defined as a fracture
in rock in which one side slides laterally past the other
with a displacement that is greater than the separation
between the blocks on either side of the fracture. Joints
normally have a regular spacing related to either the
mechanical properties of the individual rock or the
thickness of the layer involved. Joints generally occur as
sets, with each set consisting of joints sub-parallel to each
other.

Converging Plates towards each
other !!
Fault

Diverging Plates move away from
each other.
Fault

Sliding Plates passing each other !
Fault

Sliding Plates one over another!
Fault

In structural geology,
a syncline is a fold with
younger layers closer to the
center of the structure.
A synclinorium (pluralsynclinor
iums or synclinoria) is a large
syncline with superimposed
smaller folds. Synclines are
typically a downward fold,
termed a synformal syncline
(i.e. a trough); but synclines
that point upwards, or perched,
can be found when strata have
been overturned and folded
(an antiformal syncline).
Fold

In structural geology,
an anticline is a fold that
is convex up and has its
oldest beds at its core.
The term is not to be
confused with antiform,
which is a purely
descriptive term for any
fold that is convex up.
Therefore if age
relationships between
various strata are
unknown, the term
antiform should be used.

The area where energy is first released to cause an earthquake is called the focus. The focus
lies underground at a shallow, intermediate, or deep depth—down to about 430 miles (700
kilometers). On the basis of the depth of focus, an earthquake may be termed as shallow focus
(0-70 km), intermediate focus (70-300 km), and deep focus (>300 km).
The epicenter is the point on Earth's surface, usually almost directly above the focus, where
the seismic waves of an earthquake first appear on the surface.

The vibrations transmitting the shock of an
earthquake are called seismic waves. These waves
travel outward in all directions, like ripples from a
stone dropped in a pond.
Seismic waves
We have many different kinds of such waves and all of
them move in varied ways. The two original kinds of
waves are surface waves and body waves. Surface
waves ripples on water and only move along the
surface of the earth, whereas body waves move
through the earth's inner layers. In both body and
surface waves, earthquakes radiate seismic energy.

Body waves:
We have two kinds of body waves P waves and S
waves. The other name of P waves is primary waves
that are first kind of body wave.
This type of body wave can move through solid rock
and molten material like water or the liquid layers of
the earth. It looks like sound waves, pulling and
pushing the rock it moves through. As the sound
waves are pulling and pushing on the windows, it is
similar to P waves. Usually animals feel the P waves of
an earthquake. Usually human can only feel the
shaking and snoring of these waves. This is the fastest
waves.

The other name of S wave is secondary wave. It is the
second wave we feel in an earthquake. This kind of
waves is slower than P wave. Secondary wave can
only move among solid rock. S wave can move rock
down and up or side-to-side. S waves move the rock
in their path up and down and side to side.

Table 2: Seismic Waves
Type Particle Motion Velocity Other Characteristics
P,
Compressio
nal,
Primary,
Longitudina
l
Alternating compressions
(“pushes”) and dilations
(“pulls”) which are directed
in the same direction as the
wave is propagating (along
the raypath); and therefore,
perpendicular to the wave
front
VP ~ 5 – 7 km/s
in typical
Earth’s crust;
>~ 8 km/s in
Earth’s mantle
and core; 1.5
km/s in water;
0.3 km/s in air
P- motion travels fastest in materials,
so the P-wave is the first-arriving
energy on a seismogram. Generally
smaller and higher frequency than the
S and Surface-waves. P waves in a
liquid or gas are pressure waves,
including sound waves.
S,
Shear,
Secondary,
Transverse
Alternating transverse
motions (perpendicular to
the direction of propagation,
and the raypath); commonly
polarized such that particle
motion is in vertical or
horizontal planes
VS ~ 3 – 4 km/s
in typical
Earth’s crust;
>~ 4.5 km/s
in Earth’s
mantle; ~ 2.5-
3.0 km/s in
(solid) inner
core
S- waves do not travel through fluids,
so do not exist in Earth’s outer core
(inferred to be primarily liquid iron)
or in air or water or molten rock
(magma). S waves travel slower than
P waves in a solid and, therefore,
arrive after the P wave.

Table 2: Seismic Waves (continues)
Type Particle Motion Velocity Other Characteristics
L,
Love,
Surface
waves, Long
waves
Transverse horizontal
motion, perpendicular
to the direction of
propagation and
generally parallel to the
Earth’s surface
VL ~ 2.0 - 4.5
km/s in the
Earth
depending on
frequency of
the propagating
wave
Love waves exist because of the Earth’s
surface. They are largest at the surface and
decrease in amplitude with depth. Love
waves are dispersive, that is, the wave
velocity is dependent on frequency, with
low frequencies normally propagating at
higher velocity. Depth of penetration of
the Love waves is also dependent on
frequency, with lower frequencies
penetrating to greater depth.
R,
Rayleigh,
Surface
waves, Long
waves,
Ground roll
Motion is both in the
direction of propagation
and perpendicular (in a
vertical plane),
and “phased” so that
the motion is generally
elliptical – either
prograde or retrograde
VR ~ 2.0 - 4.5
km/s in the
Earth
depending on
frequency of
the propagating
wave
Rayleigh waves are also dispersive and the
amplitudes generally decrease with depth
in the Earth. Appearance and particle
motion are similar to water waves.

Figure. Compression (P) wave
propagation in a slinky. A
disturbance at one end results in
a compression of the coils
followed by dilation (extension),
and then another
compression. With time
(successive times are shown by
the diagrams of the slinky at
times t1 through t6), the
disturbance propagates along
the slinky. After the energy
passes, the coils of the slinky
return to their original,
undisturbed position. The
direction of particle motion is in
the direction of propagation.

Figure. Shear (S) wave
propagation in a slinky. A
disturbance at one end results in
an up motion of the coils followed
by a down motion of the
coils. With time (successive times
are shown by the diagrams of the
slinky at times t1 through t6), the
disturbance propagates along the
slinky. After the energy passes,
the coils of the slinky return to
their original, undisturbed
position. The direction of particle
motion is perpendicular (for
example, up and down or side to
side) to the direction of
propagation.

Figure. Perspective view of Love-
wave propagation through a grid
representing a volume of elastic
material. Love waves are surface
waves. The disturbance that is
propagated is horizontal and
perpendicular to the direction of
propagation. The amplitudes of the
Love wave motion decrease with
distance away from the
surface. The material returns to its
original shape after the wave has
passed.

Figure. Perspective view of Rayleigh-
wave propagation through a grid
representing a volume of elastic
material. Rayleigh waves are
surface waves. The disturbance that
is propagated is, in general, an
elliptical motion which consists of
both vertical (shear; perpendicular
to the direction of propagation but
in the plane of the raypath) and
horizontal (compression; in the
direction of propagation) particle
motion. The amplitudes of the
Rayleigh wave motion decrease with
distance away from the surface. The
material returns to its original shape
after the wave has passed.

Figure 27. Seismograms recorded by a 3-component seismograph at Nana, Peru for an earthquake located near the coast of central Chile on
September 3, 1998. The three seismograms record motion in the horizontal (east-west and north-south) and the vertical (Z) directions.
P, S, Rayleigh and Love waves are identified on the record. The S wave arrives significantly after the P-wave because S-wave velocity in rocks is
lower than P wave velocity. Additional arrivals between the P and the S wave are P and S waves that have traveled more complicated paths
(such as the pP and PP phases and P-to-S converted phases, Figure 26) from the earthquake location to the seismograph. The surface waves
arrive after the S waves because surface wave velocities in rocks are lower than the shear wave velocity. The surface waves extend over a long
time interval because surface wave propagation is dispersive (the velocity of propagation is dependent on the frequency of the wave). This
dispersive character can easily be seen in the Rayleigh wave on the vertical (Z) component seismogram in that the earliest Rayleigh
wave energy has a longer period (lower frequency; see Figure 1) than the later arriving waves. The seismic data that are displayed here were
acquired from the IRIS website (www.iris.edu) using the WILBER program from the IRIS Data Management Center.

In comparison with rock, softer soils are particularly
prone to substantial local amplification of the seismic
waves
Note that the ground
displacement amplifies
with decrease in soil
stiffness

3) Earthquake Measuring scales
Magnitude
Magnitude is a quantitative
measure of the actual size of the
earthquake. An increase in
magnitude (M) by 1.0 implies 10
times higher waveform
amplitude and about 31 times
higher energy released
Intensity
Intensity is a qualitative
measure of the actual shaking
at a location during an
earthquake.

Basic Difference: Magnitude versus Intensity:
Magnitude of an earthquake is a measure of its size.
For instance, one can measure the size of an
earthquake by the amount of strain energy released
by the fault rupture. This means that the magnitude
of the earthquake is a single value for a given
earthquake.
On the other hand, intensity is an indicator of the
severity of shaking generated at a given location.
Clearly, the severity of shaking is much higher near
the epicenter than farther away. Thus, during the
same earthquake of a certain magnitude, different
locations experience different levels of intensity.

Magnitude (ML) of an earthquake
Relation between Magnitude (ML) of and intensity (Io) of an earthquake

Modified Mercalli Scale (intensity based) vs. Richter Scale (magnitude based)

•Shaking
•Landslides
•Liquefaction
•Tsunamis
4) Effects of Earthquake

Shaking Hazards on Structures
Most earthquake-related deaths are caused by the collapse of structures and
the construction practices play a tremendous role in the death toll of an
earthquake.
The following occurrences happen with structures during earthquake:
•The whole building including contents are shaken from the position of rest
•The earthquake motion results into vibration of the building along its all three axes
•The movement is reversible in direction. The number of cycles per second depends
on the characteristics of earthquake as well as the structure
•Inertia forces are created on the masses due to ground acceleration. These are
proportional to the mass of the system. Lighter the material, smaller will be the
earthquake force
•Additional vertical load effect is caused on beams and columns due to vertical
vibrations. Being reversible, at certain instant of time the effective load is increased,
at others it is decreased
•The supporting members, walls or columns which were carrying only vertical loads
before the earthquake, have now to carry horizontal bending and shearing effects as
well
•The dumping in the building system has the effect to reduce the effective
accelerations on the masses and higher the dumping greater is the reduction
•The dynamic and damage behavior of a building is a function of the stiffness and

Not only buildings, but also other
important structures and life lines can
be damaged:
•Dams
•Towers
•Power plants
•Water treatment plants
•Roads
•Bridges and culverts
•Electricity
•Water supply
•Gas lines
•Oil lines
•Telecommunication
Failure of retaining wall

Landslides
A landslide is a geological phenomenon which includes a wide
range of ground movement, including rock falls. Typically, the
action of gravity is the primary driving force for a landslide to occur
though in this case there was another contributing factor which
affected the original slope stability: the landslide required an
earthquake trigger before being released. Often unstable regions of
hillsides or mountains fail due to earthquake. In addition to the
obvious hazard posed by large landslides, even non lethal slides
can cause problems when they block highways they can be
inconvenient or cause problems for emergency and rescue
operations.
In 1970 an earthquake off the coast of Peru produced a landslide
than began 80 miles away from the earthquake. The slide was
large (witnesses estimated it's height at about 30 meters or 100
feet), traveled at more than one-hundred miles per hour and plowed
through part of one village and annihilated another, killing more
than 18,000 people.

Liquefaction:
Soil liquefaction describes the behavior of soils that, when loaded, suddenly go
from a solid state to a liquefied state, or having the consistency of a heavy liquid.
Liquefaction is more likely to occur in loose to moderate saturated granular soils
with poor drainage, such as silty sands or sands and gravels capped or containing
seams of impermeable sediments. During loading, usually cyclic undrained loading,
e.g. earthquake loading, loose sands tend to decrease in volume, which produces
an increase in their porewater pressures and consequently a decrease in shear
strength, i.e. reduction in effective stress.
Liquefaction can cause damage to structures in several ways. Buildings whose
foundations bear directly on sand which liquefies will experience a sudden loss of
support, which will result in drastic and irregular settlement of the building.
Liquefaction causes irregular settlements in the area liquefied, which can damage
buildings and break underground utility lines where the differential settlements are
large. Pipelines and ducts may float up through the liquefied sand. Sand boils can
erupt into buildings through utility openings, and may allow water to damage the
structure or electrical systems. Soil liquefaction can also cause slope failures. Areas
of land reclamation are often prone to liquefaction because many are reclaimed
with hydraulic fill, and are often underlain by soft soils which can amplify
earthquake shaking. Soil liquefaction was a major factor in the destruction in San
Francisco's Marina District during the 1989 Loma Prieta earthquake. Mitigating
potential damage from liquefaction is part of the field of geotechnical engineering.

Tsunamis
Sometimes a dramatic byproduct of certain types of
earthquakes is tsunami. Tsunami is a Japanese term
that means "harbor wave". Tsunamis are frequently
confused with tidal waves, but they have nothing to do
with the tides, they are the result of a sudden vertical
offset in the ocean floor caused by earthquakes,
submarine landslides, and volcanic deformation. The
speed of this wave depends on the ocean depth and is
typically about as fast as a commercial passenger jet (about
0.2 km/s or 712 km/hr).

1) In 1883 the volcanic eruption of Krakatoa resulted in the
collapse of a caldera that initiated a tsunami which killed
36,000 people on nearby islands.
2) On June 25, 1896 an earthquake off the Japanese coast
generated a tsunami that hit the shore with wave heights
ranging from 10 to 100 feet. As the fishing fleets returned to
shore following an overnight trip they found their villages
destroyed and 22,000 people dead. In the last century more
than 50,000 people have died as a result of tsunamis.
3) A devastating tsunami hit Indonesia after a 9.1 earthquake
in 2004, which claimed the lives of 230,000 and that tsunami
did not also affect Bangladesh.
Examples

Tsunami at Indonesia 2004- LOCATION

BEFORE DECEMBER 26, 2004
EARTHQUAKE-TSUNAMI

AFTER DECEMBER 26, 2004
EARTHQUAKE-TSUNAMI

Tsunami Warning and vulnerable area of Bangladesh
The unique tectonic plate structure (about 4000 km in
length) of the Indian Ocean that passes through
Bangladesh-Myanmar-India boundary zone. It is the
zone of tsunamigenic sources that could affect the
coastlines of the Bangladesh, Indian and Myanmar.
Geologists have identified structural changes along
the tectonic boundary, severe deformation of
Sumatra Island in south-west part. Many geologists
expect some bathymetric changes in seafloor of Bay
Bengal. They have also enabled to identify possible
source locations that would be potential for
generation of local tsunami.

Considering the state of tsunami vulnerability and potential seismic
sources the coastal belt of Bangladesh can be divided into three
Tsunami Vulnerability Coastal Belts:
1. Tsunami Vulnerability Coastal Belt – I of Chittagong-Teknaf coastline
– Most vulnerable.
The intra-deltaic coastline is very close to the tectonic interface of Indian
and Burmese plates. The active Andaman-Nicobar fault system is often
capable of generating tsunami waves.
2. Tsunami Vulnerability Coastal Belt – II of Sundarban-Barisal
coastline – Moderately vulnerable.
This old deltaic belt is extremely vulnerable to local tsunamis due to
presence of Swatch of No Ground.
3. Tsunami Vulnerability Coastal Belt – III of Barisal-Sandwip estuarine
coastline – Low vulnerability.
The estuarine coastal belt considered to be less vulnerable due to
presence numerous islets and shoals in the upper regime of the
continental shelf.

Why Indonesia Tsunami did not affect Bangladesh:
According to the Earthquake Observatory Centre of Dhaka University, the magnitude
of the tremor was 8.9 on the Richter scale at its epicenter in North Sumatra of
Indonesia, 2,370 kilometers away from Bangladesh.
The tsunami route is East-West. Bangladesh is situated in north of Indonesia. So,
Bangladesh remained safe from tsunami.
Bangladesh is must be thanking their lucky stars and may still be wondering how they
came out the tsunami calamity relatively unscathed. While tens of thousands have
died in neighboring India, Sri Lanka and Thailand, only two Bangladeshi children
drowned when their boat capsized in the high waves during tsunami.
Geologists attribute Bangladesh's good luck to a natural process of sedimentation,
making the sea bed shallow along the coast. Billions of tons of sediment, which the
country's numerous rivers carry into the sea, have created a natural barrier against
the tsunami. Known as the continental shelf, the barrier helped slow down the sea
surges before they hit the coast. The barrier has helped keep the sea floor shallow -
the coast water in Bangladesh is up to 66 feet deep - and "absorbed the impact of the
tsunami."

As in BANGLADESH, it is a hope of matter that Bangladesh
escaped the unpredictable devastation during 26
December 2004 Indian Ocean Tsunami though 700 km
long coastal front of the country was hit by tsunami after 3
hours of the Earthquake with only 25-30 cm of wave
height. This is because of the escape path-
1. Long distance from the Epicenter
2. Long Continental Shelf (about 200 km) at the front of
Ganges- Brahmaputra active Detla System
3. Thick sedimentation in Bay of Bengal
4. High density of sea water in Bay of Bengal around /
along the coast (suspended load).
5. Anticlockwise oceanic current at Bay of Bengal (winter
time).

Assignment
• Discuss on any public health related issue in
disaster. Discuss based on your personal and
professional context.
• Maximum number of participants = 02/ Assignment
Presentation and Submission of soft copy or a hard copy should
be made to the course teacher before the final term.

Lecture earthquake-1

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