Chapter 5:
Earthquakes
5.1 General.
An earthquake is the result of a sudden release of energy
in
the
Earth's crust that creates seismic
seismicity or seismic
activity of
an
area
waves.
refers
The
to
the
frequency, type and size of earthquakes experienced over a
period of time.
Earthquakes are measured using observations from
seismometers, Figure (5.1).
Figure (5.1): A picture showing a seismograph
At the Earth's surface, earthquakes manifest themselves by
shaking and sometimes displacement of the ground. When the
epicenter of a large earthquake is located offshore, the seabed
may be displaced sufficiently to cause a tsunami. Earthquakes
can also trigger landslides, and occasionally volcanic activity.
5-2 Causes of earthquake
Earthquakes are caused mostly by rupture of geological faults, but
also by other events such as volcanic activity, landslides, mine
blasts, and nuclear tests. An earthquake's point
of
initial
rupture is called its focus or hypocenter. The epicenter is the
point at ground level directly above the hypocenter, Figure (5.2).
Figure (5.2): 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.
The occurrence of earthquake is explained by different theories;
the following are the most famous ones:
5-2-1 Elastic Rebound Theory.
The elastic
rebound
theory is
an
explanation
for
how
energy is spread during earthquakes. 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
undeformed shape.
5-2-2 Volcanic earthquake theory
Earthquakes
caused
often
there,
occur
in
volcanic
regions
and
are
both by tectonic faults and the movement of
magma in volcanoes.
The Earth's surface is formed of massive slabs of rock called
plates. These plates, also called tectonic plates, are always
moving. Sometimes they just slide past one another. At other
times they actually collide with one another. Plate movement
causes the buildup of tremendous quantities of energy in the rock.
When the energy is released, it produces vibrations that travel
through the rock, leading to earthquakes. During earthquakes,
faults, or giant cracks, are produced by the pressure of the
moving rock.
Earthquakes and volcanoes occur along the edges of the plates.
Scientists have developed a theory that explains how these giant
plates move, thereby creating, destroying, and re-forming
continents and oceans over long periods of time. This theory is
called the theory of plate tectonics
5-2-3 Landslides theory
Earthquakes are a major cause of landslides. Landslides, in
turn,
are a
major contributor to the damage and causalities
associated with earthquakes. It is desirable to quantify this
association between landslides and earthquakes
5-3 Distribution of Earthquakes
Earthquakes have taken place in all parts of the world. Frequent
activity occurs along certain belts. 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 are all part of the Pacific belt.
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
Arctic, Atlantic, and Indian Oceans. Antarctica and Australia
experience the least amount of earthquake activity than any other
areas of the world, see Figure (5.3).
Figure (5.3): This map shows the distribution of earthquakes around the world
5-4 Types of seismic waves
There are many types of seismic waves, body wave, surface
waves, S waves and P waves.
5-4-1 Primary Waves.
Primary waves (P-waves) are compression waves that are
longitudinal in nature. P waves are pressure waves that travel
faster than other waves through the earth to arrive at seismograph
stations first hence the name "Primary". These waves can travel
through any type of material, including fluids, and can travel at
nearly twice the speed of S waves. In air, they take the form of
sound waves; hence they travel at the speed of sound. Speeds are
330 m/s in air, 1450 m/s in water and about 5000 m/s in granite.
5-4-2 Secondary waves (S-waves).
They are shear waves that are transverse in nature. These waves
arrive at seismograph stations after the faster moving P waves
during an earthquake and displace the ground perpendicular to the
direction of propagation. Depending on the preoperational
direction, the wave can take on different surface characteristics;
for example, in the case of horizontally polarized S waves, the
ground moves alternately to one side and then the other. S waves
can travel only through solids, as fluids (liquids and gases) do
not support shear stresses. S waves are slower than P waves, and
speeds are typically around 60% of that of P waves in any given
material.
5-4-3 Surface Waves.
They are analogous 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, Figure (5.4) and Figure (5.5).
Figure (5.4): Types of waves
Figure (5.5): Body and surface
waves
5-5 Effects of earthquakes
a) Shaking and ground rupture
Shaking and ground rupture are the main effects created by
earthquakes, principally resulting in more or less severe damage to
buildings and other rigid structures. Ground rupture is a visible
breaking and displacement of the Earth's surface along the trace of
the fault. Ground rupture is a major risk for large engineering
structures such as dams, bridges and nuclear power stations and
requires careful mapping of existing faults to identify any which
are likely to break the ground surface within the life of the
structure.
b) Landslides and avalanches
Earthquakes, along with severe storms, volcanic activity, and
coastal wave attack, and wildfires, can produce slope instability
leading to landslides, a major geological hazard.
c) Fires
Earthquakes can cause fires by damaging electrical power or
gas lines. In the event of water mains rupturing and a loss of
pressure, it may also become difficult to stop the spread of a fire
once it has started. For example, more deaths in the 1906 San
Francisco earthquake were caused by fire than by the earthquake
itself.
d) Soil liquefaction
Soil
liquefaction
shaking,
occurs
when,
because
of
the
water- saturated granular material (such as sand)
temporarily loses its strength and transforms from a solid to a
liquid. Soil liquefaction may cause rigid structures, like buildings
and bridges, to tilt or sink into the liquefied deposits. This can be a
devastating effect of earthquakes. For example, in the 1964
Alaska earthquake, soil liquefaction caused many buildings to
sink into the ground, eventually collapsing upon themselves.
e) Tsunami
Tsunamis are long-wavelength, long-period sea waves produced
by the sudden or abrupt movement of large volumes of water. In
the open ocean the distance between wave crests can surpass 100
kilometers (62 mi), and the wave periods can vary from five
minutes to one hour. Such tsunamis travel 600-800 kilometers per
hour (373–497 miles per hour), depending on water depth. Most
destructive tsunamis are caused by earthquakes of magnitude 7.5
or more.
f) Floods
A flood is an overflow of any amount of water that reaches land.
Floods occur usually when the volume of water within a body of
water, such as a river or lake, exceeds the total capacity of the
formation, and as a result some of the water flows or sits outside
of the normal perimeter of the body. However, floods may be
secondary effects of earthquakes, if dams are damaged.
g) Human impacts
An earthquake may cause injury and loss of life, road and
bridge
damage, general property damage, and collapse or
destabilization of buildings.
5-6 Size of Earthquakes
The earthquakes are measured by two ways: its intensity and
magnitude.
5-6-1 Mercalli intensity scale
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). Values depend upon the
distance to the earthquake, with the highest intensities being
around the epicentral area. Data gathered from people who
have experienced the quake are used to determine an intensity
value for their location.
Table (5.1): Mercalli Earthquake Intensity Scale.
Intensity
Description
1. Instrumental
Generally not felt by people unless in favorable conditions.
2. Weak
Felt only by a few people at rest, especially on the upper floors of
buildings. Delicately suspended objects (including chandeliers)
may swing slightly.
3. Slight
Felt quite noticeably by people indoors, especially on the upper
floors of buildings. Many do not recognize it as an earthquake.
Standing automobiles may rock slightly. Vibration similar to the
passing of a truck. Duration can be
estimated. Indoor objects (including chandeliers) may shake
4. Moderate
Felt indoors by many to all people, and outdoors by few
people. Some awakened. People can report it as strong intensity.
Dishes, windows, and doors disturbed, and walls make cracking
sounds. Chandeliers and indoor objects shake noticeably. The
sensation is more like a heavy truck striking building. Standing
automobiles rock noticeably. Dishes and windows rattle
alarmingly. Damage none to minimal/very light
5. Rather Strong
Felt inside by most, may not be felt by some outside in nonfavorable conditions. Dishes and windows may break and bells
will ring. Vibrations are more like a large train passing close to a
house. Possible slight damage to buildings. Liquids may spill out
of glasses or open containers. A few people
are frightened and run outdoors
6. Strong
Felt by everyone, outside or inside; many frightened and run
outdoors, walk unsteadily. Windows, dishes, glassware broken;
books fall off shelves; some heavy furniture moved or overturned;
a few instances of fallen plaster. Damage slight to moderate to
poorly designed buildings, all others receive
none to slight damage
7. Very Strong
Difficult to stand. Furniture broken. Damage light in building of
good design and
construction;
ordinarily
structures, considerable damage in poorly
built
slight
built or badly designed structures; some
to
moderate
in
chimneys broken. Noticed by people driving automobiles.
8. Destructive
Damage slight in structures of good design, considerable in
normal buildings with a possible partial collapse. Damage great in
poorly built structures. Brick buildings easily receive moderate to
extremely heavy damage. Possible fall of chimneys, factory
stacks, columns, monuments, walls, etc. Heavy furniture moved.
9. Violent
General panic. Damage slight to moderate in well-designed
structures. Well- designed structures thrown out of plumb.
Damage moderate to great
in substantial buildings, with a
possible partial collapse. Some buildings may be shifted off
foundations. Walls can fall down or collapse
10. Intense
Many well-built structures destroyed, collapsed, or moderately
to severely damaged. Most other structures destroyed, possibly
shifted off foundation.
Large landslides
11. Extreme
Few, if any structures remain standing. Numerous landslides,
cracks and deformation of the ground
12. Catastrophic
Total destruction – everything is destroyed. Lines of sight and
level distorted. Objects thrown into the air. The ground moves in
waves or ripples. Large amounts of rock move position.
Landscape altered, or leveled by several meters. Even the routes
of rivers can be changed