Earthquakes
Dr. SaMeH Saadeldin Ahmed
Associate Prof. Mining and Environmental Engineering
Email: s.mohamed@mu.edu.sa
Engineering Geology CE101
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Contents
1. Causes of Earthquakes
a) Elastic rebound Theory
b) Volcanic Theory
c) Landslides
2. Distribution of Earthquakes
3. Types of Waves
4. Focus and Epicentre
5. Size of Earthquakes
6. Earthquakes in Arab world
7. Constitution of the Earth
Engineering Geology CE101
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An earthquake is the result of a sudden release of
energy in the Earth’s crust that creates seismic
waves.
Some of these vibrations are severe and destructive,
others are too low to be recorded unless very
sensitive instruments are used for this purpose
(Seismograph).
Earthquakes are measured using observations
from seismometers. The moment magnitude is
the most common scale on which earthquakes
larger than approximately 5 are reported for the
entire globe.
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1. Causes of Earthquakes
a)
Elastic rebound theory
The elastic rebound theory is an explanation for how energy is
spread.
As rocks on opposite sides of a fault are subjected to force and
shift, they accumulate energy and slowly deform until their
internal strength is exceeded. At that time, a sudden
movement occurs along the fault, releasing the
accumulated energy, and the rocks snap back to their
original unreformed shape during earthquakes.
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b) Volcanic theory
One of the most causes of earthquakes is volcanoes. As
volcanoes occurs due to great explosives or sudden
movements of magma inside the earth, it reflects
movements in the earth’s crust that causes earthquake.
c) Landslides
Earthquakes are a major cause of landslides. Landslides,
in turn, are a major contributor to the damage and
causalities associated with earthquakes.
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2. Distribution of Earthquakes
• Earthquakes take place or have taken place in all
parts of the world. 80% of all seismic energy is
generated from a belt that is found at the border of
the Pacific Ocean. A great deal of volcanoes is also
found there, and volcanoes set off many
earthquakes. Japan, the Philippine Islands, New
Guinea, and New Zealand.
• A second seismic belt produces 15% of seismic
activity. It goes through southern Asia to the region
of the Mediterranean Sea. The final 5% of seismic
energy comes from parts of the Atlantic, and Indian
Oceans.
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This map shows the distribution of earthquakes around the world
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3. Types of Waves
a) Primary waves (P)
•
•
•
•
Primary waves (P-waves) are compression
waves that are longitudinal in nature.
It has the highest velocity and is therefore the
first to be recorded by seismographs.
These waves can travel through any type of material,
including fluids, and can travel at nearly twice the
speed of S waves.
Typical speeds are 330 m/s in air, 1450 m/s in
water and about 5000 m/s in granite.
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b) Secondary waves (S)
•
•
•
Secondary waves (S-waves) are shear waves
that are transverse in nature.
These waves arrive at seismograph stations
after the faster moving P waves.
S -waves are slower than P waves, and
speeds are typically around 60% of that of P
waves in any given material.
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c) Surface or Longitudinal waves (L)
•
•
•
•
Surface waves (L-waves) are similar to
water waves and travel along the Earth's
surface.
They travel slower than body waves.
Because of their low frequency, long
duration, and large amplitude.
They can be the most destructive type of
seismic wave.
They are called surface waves because they
diminish as they get further from the
surface.
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Body waves
Surface waves
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‫‪4. Focus of Earthquake‬‬
‫‪An earthquake's point of initial rupture is called‬‬
‫‪its focus or hypocenter.‬‬
‫وتصنف الزالزل اعتمادا على عمقها البؤري إلى‪:‬‬
‫–‬
‫–‬
‫–‬
‫‪13‬‬
‫الزالزل الضحلة‪ :‬وهي الزالزل التي ال يزيد عمق‬
‫بؤرتها عن ‪ 50‬كيلومترا‪.‬‬
‫الزالزل المتوسطة‪ :‬وهي الزالزل التي يقع عمق‬
‫بؤرتها بين ‪ 250 – 50‬كيلومترا‪.‬‬
‫الزالزل العميقة‪ :‬وهي الزالزل التي يقع عمقها البؤري‬
‫بين ‪ 700 – 250‬كيلومترا‪.‬‬
‫‪Engineering Geology CE101‬‬
5. Epicentre
The epicenter is the point at ground level directly
above the hypocenter
Epicentre
Focus
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Hypocenter (Focus) and epicenter of an earthquake
An earthquake's focus is the position where the strain energy
stored in the rock is first released, marking the point where
the fault begins to rupture. This occurs at the focal depth
below the epicenter.
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6. Size of Earthquakes
The earthquakes are measured by two ways:
its intensity and magnitude
a) Intensity (Mercalli)
The Mercalli intensity scale is a seismic
scale used for measuring the intensity of
an earthquake. The scale quantifies the effects
of an earthquake on the Earth's surface,
humans, objects of nature, and man-made
structures on a scale from I (not felt) to XII
(total destruction).
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b) Magnitude (Richter magnitudes)
The Richter magnitude of an earthquake is
determined
from
the
logarithm
of
the amplitude of waves recorded by
seismograph.
The scale is a base-10 logarithmic scale. The
magnitude is defined as the logarithm of the ratio
of the
amplitude
of waves measured by
a seismograph to an arbitrary small amplitude. An
earthquake that measures 5.0 on the Richter scale
has a shaking amplitude 10 times larger than one
that measures 4.0, and corresponds to an energy
release of √1000 ≈ 31.6 times greater.
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Magnitude
Description
Mercalli
intensity
Average earthquake effects
Micro earthquakes, not felt, or felt rarely
by sensitive people. Recorded by
seismographs.
Less than 2.0
Micro
I to II
2.0–2.9
Minor
I to III
Generally felt by few to many people up
to several miles/kilometers from the
epicenter. Weak shaking in the felt area.
Recorded by seismographs.
II to V
Often felt in the area by at least many
people, but very rarely causes damage.
Can be felt tens of miles/kilometers from
the epicenter, but at weak intensity
III to VII
Noticeable shaking of indoor objects and
rattling noises. Many people to everyone
feel it with slight to strong intensity.
Slightly felt outside. Generally causes
none to slight damage. Moderate, heavy,
major, or significant damage unlikely.
Some falling of objects.
3.0–3.9
4.0–4.9
Light
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Magnitude
Description
Mercalli
intensity
IV to VIII
Average earthquake effects
Can cause moderate to major damage to
poorly constructed buildings. At most,
none to slight damage to well-designed
buildings. Can be felt hundreds of
miles/kilometers from the epicenter at
low/lower intensity..
5.0–5.9
Moderate
6.0–6.9
Strong
VI to X
Can be damaging/destructive in populated
areas. Damage to many to all buildings;
poorly designed structures. Most likely
felt hundreds of miles/kilometers from the
epicenter. Can be damaging further from
the epicenter; up to tens of
miles/kilometers away. Death toll between
none and 25,000.
7.0–7.9
Major
VII to XII
Can cause great(er) damage over larger
areas. Damage to most or all buildings.
Most likely will be felt several hundred
miles/kilometers away. Death toll is
usually between none and 250,000.
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Magnitude
Description
Mercalli
intensity
Can cause major damage across very
wide, large areas. Many to all buildings in
epicentral area severely damaged. Very
strong shaking up to a few hundred
kilometers away. Death toll is usually
between 100 and one million;.
8.0–8.9
Great
9.0–9.9
10.0+
Average earthquake effects
VIII to XII
Massive/Epic IX to XII
Destructive to very devastating in
extremely large areas. Many to all
buildings severely damaged or completely
destroyed up to tens of miles from the
epicenter. Effects will go for a long
time. Ground changes. Death toll usually
between 1,000 and over one million.
Heavy, widespread, colossal
damage/devastation across enormous
areas. Will destroy buildings fairly easily
and quickly. Death toll most likely will be
over 25,000 people. Large ground
changes. Effects will last for an extremely
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long time. It has never been recorded.
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‫‪7. Constitution of the Earth‬‬
‫األرض تتكون من قشرة ‪ Crust‬ال يزيد سمكها عن‬
‫حوالي ‪ 35‬كيلومترا‪ ،‬تقع على رداء ‪ Mantle‬يمتد‬
‫حتى عمق ‪ 2900‬كيلومترا‪.‬‬
‫وتوجد تحت القارات صخور رسوبية وجرانيتية‬
‫ومتحـولة ‪ Sial‬وهي غنية جدا بمادة السيليكــا‬
‫(‪ % 70‬تقريبا) واأللومينا وتقع فوق صخور البازلت‪.‬‬
‫ويوجد تحت المحيطات قشرة صخرية تتكون من‬
‫الصخور البازلتية ‪ Sima‬وهي تمثل النطاق‬
‫الداخلي من القشرة األرضية حيث تقل فيها‬
‫نسبة السيليكون وتزيد نسبة المغنسيوم‪.‬‬
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‫‪Engineering Geology CE101‬‬
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Quiz (4)
Write your name, ID, and you have 3
minutes to answer online the
following question
A) Landslides is one of the theories explaining
the earthquakes, what are the other famous
two theories:
1) ……………………………
2) …………………………
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