Earthquake Engineering 2012 Lecture 0102 Nature of Earthquakes

EARTHQUAKE
ENGINEERING
1.2. N
Nature of
f
Earthquakes

1.2.1. Causes of Earthquakes

1.2.2. Faults

1.2.3. Earthquake Effects


Tectonic Earthquakes
are caused by the sudden dislocation of large rock masses along
geological faults within the earth's crust. The Earth is formed of
several l
l layers that h
h have very diffdifferent physical and chemical
h i l d h i l
properties. The outer layer, which averages about 70 kilometers
in thickness, consists of about a dozen large, irregularly shaped
plates that slide over under and past each other on top of the
over,
partly molten inner layer .Most earthquakes occur at the
boundaries where the plates meet .

Plate Tectonic Interactions
Flash movie

Dr. Osman Shalan Dr. Tharwat Sakr


Tectonic Earthquakes



Volcanic Earthquakes
Earthquakes that can be
reasonably associated with
volcanoes are relatively rare
and fall into three categories:

(i) volcanic explosions,

(ii) shallow earthquakes from
magma movements, and

(iii) sympathetic tectonic
earthquakes




Dr. Osman Shaalan Dr. Tharwat Sakr


Explosions

Earthquakes may be produced
q y p
by the underground
detonation of chemical or
nuclear devices When a
devices.
nuclear device is detonated in
a borehole underground,
enormous nuclear energy is
released .



Collapse Earthquakes

Collapse earthquakes are small earthquakes occur in
regions of underground caverns and mines. The immediate
cause of ground shaking is the sudden collapse of the roof
g g p
of the mine or cavern.



Large Reservoir-induced Earthquakes
Reservoir-

The idea that earthquakes might be triggered by impounding
q g gg y p g
surface water is not new. The first detailed evidence of such an
effect came with the filling of Lake Mead behind Hoover Dam
(height 221 m. After impounding began , reports of local shaking
became prevalent.
b l t



Tsunami
Underwater earthquakes,
volcanoes, or landslides
can produce a tsunami or
tidal wave. This wave can
travel very rapidly
thousands of miles across
the ocean. In deep water
the tsunami may only raise
the ocean level by a few
centimeters,
centimeters hardly enough
to notice. But as it
approaches land, the
shallower water causes the
wave to build in height to
as much as 10-20 meters
or more and suddenly
flood coastal areas

1.2.2. Faults

Definition
A fault is a fracture within some
particular rocky mass within the
earth's crust. Th d h and
h' The depth d
length of faults vary greatly.
Faults may range in length from
few meters to many kilometers
and are drawn on a geological
map as continuous or broken
lines. Earthquakes are caused by
q y
active faults, that is, faults along
faults
which the two sides of the
fracture move with respect to
each other. S an earthquake i
h th So, th k is
caused by the sudden movement
of the two sides of a fault with
respect to another .


1.2.2. Faults

Active Faults

Prof.Dr. Osman Shaalan Dr. Tharwat Sakr

1.2.2. Faults

Tectonic Plates


1.2.2. Faults

Types of Faults
a) Normal faults
These occur in response to pulling
or tension: the overlying bl k
i h l i block
moves down the dip of the fault
plane. movie
b) Thrust (reverse) faults
These occur in response to
squeezing or compression: the
overlying block moves up the dip
y g p p
of the fault plane. movie
c) Strike‐slip (lateral) faults
Strike‐
These occur in response to either
type of stress: th bl k move
t f t the blocks
horizontally past one another .
movie

Prof.Dr. Osman Shaalan Earthquake Engineering Dr. Tharwat Sakr

1.2.2. Faults

Earthquake Focus
The point on the fault where
rupture initiates is referred to
as the f
h focus or h hypocentert
of an earthquake.

The hypocenter of an
earthquake is described by
q y
its depth in kilometers, its
map location in latitude and The term epicenter is the
longitude, its date and time point on the earth’s surface
earth s
of occurrence, and i
f d its directly above the
magnitude hypocenter

Prof. Dr. Osman Shaalan Earthquake Engineering Dr. Tharwat Sak

How do Earthquakes affect Structures?
q

1‐ By inertial forces
generated b severe
d by
ground shaking

Overturning collapse of a high-rise building
Chi Chi
Chi-Chi earthquake, Taiwan, September 20,
1999, Magnitude 7.6

q

2. By earthquake‐
induced fires:
i d d fi

Damage caused by the earthquake induced fires
Hanshin (Kobe) earthquake, January 17, 1995, Japan,
Magnitude 6.9


q
3. By changing the physical properties of the foundation soils such as
Liquefaction:

Overturning due to liquefaction
g q
Damage due to liquefaction
Kocaeli earthquake, Turkey, 1999, Magnitude 7.4
1964 Niigata, Japan, earthquake.

q


q
4. By direct fault displacement at the site of a structure:


q
5. By landslides, or other movement:


q
6. By seismic sea waves (tsunamis) or fluid motions in
reservoirs and lakes (seiches):


Earthquake Engineering 2012 Lecture 0102 Nature of Earthquakes

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Earthquake Engineering 2012 Lecture 0102 Nature of Earthquakes