Chapter 2
Elements of
Seismology and Seismicity
CIE 619
Chapter 2 – Seismology and Seismicity
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CONTENT
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13
13.
14.
15.
16.
17.
Introduction
Causes of Earthquakes
Theory of Plate Tectonics
Reid’s Elastic Rebound Theory
Fault Mechanisms
Definition of Seismic Waves
Location of an Earthquake
Modified Mercalli Intensity Scale
Richter Magnitude Scale
Evolution of Magnitude Scales
Relationships Between Magnitude Scales
Seismic Parameters Influencing Structural Response
Att
Attenuation
ti Relationships
R l ti hi
Elastic Seismic Wave Model
Eastern and Western North America Earthquakes
Directivity Pulse Phenomenon and Near-Field Ground Motions
References
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1. Introduction
• Earthquake design of a structure always depends on degree of regional
seismic activity
• Many seismological factors directly influence work of a structural
engineer:
–
–
–
–
–
distribution of earthquake sources affecting the construction site
fault mechanisms of various sources
seismic activity of various sources in terms of recurrence of magnitudes
ground motion intensity;
attenuation of the ground motion with distance
• Ch
Chapter
t provides
id overview
i off fundamental
f d
t l properties
ti off various
i
seismological aspects
• Acquaint the structural engineer with «language» of seismologists.
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2. Causes of Earthquakes
• Natural Earthquakes
– most natural earthquakes occur in the earth
earth’ss crust
– crust measures between 60 and 100 km in depth
– crust made of different segments that are continuously in motion
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2. Causes of Earthquakes
• Natural Earthquakes
– deformations occur in the rock, which cause a build up
of elastic energy
– rupture or slip along fault line causes a sudden release
of energy into a seismic shock (an earthquake) which,
in turn, causes propagation of seismic waves and
ground shaking
– Most earthquakes occur in two specific zones on the
planet:
• Circum Pacific Belt: South America, the California coast,
Alaska, Japan, Formosa, Philippines, New Zealand;
• Alpine Belt: the Mediterranean, North India, Indonesia.
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2. Causes of Earthquakes
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2. Causes of Earthquakes
• Earthquakes can damage the built
environment a number of ways, including:
–
–
–
–
–
–
Ground shaking
Fault rupture
Liquefaction or soil failure
Tsunami (sea) or seiche (lake)
Flooding
Fire
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2. Causes of Earthquakes
• Damage by Fault Rupture
– 1-dimensional
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2. Causes of Earthquakes
• Damage by Liquefaction or Soil Failure
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2. Causes of Earthquakes
• Damage by Tsunami
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2. Causes of Earthquakes
• Damage by Flooding
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2. Causes of Earthquakes
• Damage by Fire
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2. Causes of Earthquakes
1995 Kobe Earthquake
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2. Causes of Earthquakes
• Induced Earthquakes
– some human interventions influence amplitude and distribution of strains
in earth’s crust
– interventions such as filling of a water reservoir, mining, excavation of
huge quarries, high pressure injection of fluids to generate geothermic
energy, oil wells and underground nuclear explosions, may cause major
induced earthquakes.
– filling of water reservoirs usually causes most severe induced earthquakes
• may reach magnitude 6 on the Richter scale
• More than 70 earthquakes of this type have been recorded world wide
• H.K.
HK G
Gupta
pta (1992)* is an eexcellent
cellent reference on earthquakes
earthq akes induced
ind ced by
b the
filling of water reservoirs.
• Course limited to naturally occurring earthquakes.
*Gupta, H.K. 1992. «Reservoir-Induced Earthquake». Developments in
Geotechnical Engineering, 64. Amsterdam, Elsevier.
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3. Theory of Plate Tectonics
 The Earth is characterized by a small number of lithospheric plates that
float on a viscous underlayer called the asthenosphere.
 Geological evidence shows that plates undergo constant, gradual
change. Magma is continually upwelling at the mid-oceanic ridges and rises
as the seafloor spreads apart.
apart
 In some areas, large sections of plates are forced to move beneath other
plates (surface layers of rocks are absorbed into the earth’s interior). These
areas are called subduction zones.
 A plate being subducted beneath another
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Chapter 2 – Seismology and Seismicity
3. Theory of Plate Tectonics
• Generally accepted as explanation for occurrence of most earthquakes
• Proposed in the 1960's
• Earth’s crust composed of several large plates that float on a viscous
medium
• Continents and oceans are supported by those plates
• Continents originally all linked together, but started drifting apart 200
million years ago
• Plates move from 1 to 15 cm every year.
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3. Theory of Plate Tectonics
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3. Theory of Plate Tectonics
95% of earthquakes occur along the edges of the interacting plates
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Chapter 2 – Seismology and Seismicity
Source: Earthquakes by Bruce A. Bolt
9
World’s Largest Magnitude Earthquakes
Earthquake
Magnitude
Year
Approx. casualties
1. Chile
9.5
1960
>2000
2. Prince William
Sound, Alaska
9.2
1964
 125
3. Andreanof
Islands, Alaska
9.1
1957
Not reported
4. Kamchatka
Peninsula
9.0
1952
Not reported
5. Sumatra
9.0
2004
>283,100
(>173,000 in Indonesia)
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Chapter 2 – Seismology and Seismicity
Source: United States Geological Survey (USGS)
3. Theory of Plate Tectonics
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3. Theory of Plate Tectonics
• Three types of plate motion can be
distinguished:
– transform motion : the plates slide past each
other
– diverge motion : the plates diverge from each
other
h fforming
i ocean ridges
id
– subducted motion : the plates converge on each
other causing the subduction of one plate under
an other.
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3. Theory of Plate Tectonics
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3. Theory of Plate Tectonics
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3. Theory of Plate Tectonics
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3. Theory of Plate Tectonics
• According to theory of plate tectonics, earthquakes arise at the
boundaries of adjacent plates
• Earthquakes occur when resistance of rock is exceeded
• Conditions create a fracture
• Energy build up between two plates (a fault) can be estimated but still
unable to predict when energy will be released
• Science of earthquake prediction still in infancy
• Besides movement of faults on surface, other factors investigated to
predict seismic events
–
–
–
–
–
variations of water levels in wells
change of wave propagation velocities
change of electric resistivity and magnetic distortion
gas emission
abnormal animal behaviour, etc.
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3. Theory of Plate Tectonics
• For than 10 years, Japanese researchers studied catfish
behaviour in relation to occurrence of earthquakes
• Research concluded that catfish had quaint ability to
predict occurrence of earthquakes
• Unfortunately, their behaviour remained the same
regardless of earthquake intensity
• Example illustrates great complexity of earthquake
prediction
• Scientists not only faced with prediction of time of
occurrence, but also with prediction of intensity of ground
motion
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3. Theory of Plate Tectonics
• In 1975,, Chinese ggovernment evacuated close to three
million people in many cities of Manchuria
• Decision came after elaborated study undertaken and an
actual earthquake of magnitude 4.8 had occurred
• Several days after evacuation, a 7.3 magnitude earthquake
destroyed 90% of the cities, but only several hundred
people lost their lives
p from the cityy of
• However,, in 1976,, evacuated ppeople
Kuantung, close to Canton, waited two months, but the
predicted earthquake never came
• Same year, Tangshan earthquake was never predicted but
an estimated 250 000 people died
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4. Reid’s Elastic Rebound Theory
•
•
•
•
•
•
Following 1906 San Francisco
earthquake, Prof. H.F. Reid from John
Hopkins University proposed an
explanation for immediate cause of an
earthquake
According to Reid’s elastic rebound
theory, a fault is incapable of movement
until strain has built up in the rock on
either side
Strain is accumulated by gradual
shifting of earth’s crust
Rock becomes distorted but holds its
original position
When accumulated stress finally
overcome resistance of rock,
rock earth
snaps back into an unstrained position
releasing a large quantity of energy
Energy release produces waves which
travel through the earth in every
directions causing what is known as
earthquakes.
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14
5. Fault Mechanisms
• There are three types of fault mechanisms
– Strike-slip fault
– Normal fault
– Underthrust fault
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5. Fault Mechanisms
• Strike-slip
Strike slip fault
– Caused by transform motion (lateral motion) of plates
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5. Fault Mechanisms
•
•
Best-known strike-slip fault in the US:
S A
San
Andreas
d
F
Fault
lt iin C
California.
lif i
Information on the fault and others in the
United States is available at a number of
web sites including:
North American
Plate
– Southern California Earthquake Center
(SCEC) www.scec.org ,
– California Geological Survey
www.consrv.ca.gov
– United States Geological Survey
(USGS) www.usgs.gov .
•
San A
S
Andreas
d
Fault
l composed
d off many
segments or combinations of segments:
14 by the count of USGS, with various
slip rates, maximum magnitudes and
return periods.
Pacific
Plate
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5. Fault Mechanisms
• Normal fault
– Caused by diverge motion of plates
– Top wall of fault plane slides downward
– Fault plane generally inclined, slides on an inclination
angle
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5. Fault Mechanisms
• Underthrust fault
– Caused by subducted motion of plates
– Top wall of fault plane is pushed upward
– Particular type of underthrust fault is called thrust fault
characterized by a small inclination (< 45o) of fault
plane
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6. Definition of Seismic Waves
• Focal point (focus) or
hypocentre
– where earthquake occurs
– where fault line originates
– located at depth varying from a
few kilometres to 100 km
• Epicentre
– Vertical projection of focal
point on ground surface
• Focal depth
– Vertical distance between focal
point and epicentre.
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6. Definition of Seismic Waves
• Distance from the site of a
building or recording station
to the fault or fault projection
is described by a number of
terms:
–
–
–
–
SA = epicentral distance;
SB = distance to fault rupture;
SD = hypocentral distance;
SC = shortest distance to fault
rupture.
Plan View
Elevation View
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6. Definition of Seismic Waves
• Various kinds of waves produced during an
earthquake
• Waves travelling within solid earth called
“body waves”
• Waves travelling near ground surface are
called “surface waves”
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6. Definition of Seismic Waves
• Two kinds of body waves
– Primary waves (P-waves)
• Horizontal tension and
compression waves, which travel
in direction of wave front
• High frequency
• First waves to reach a structure
– Secondary waves (S-waves)
• Shear waves
waves, which travel
perpendicularly to wave front
• Lower frequency
• Greater amplitude
• Most destructive vibrations.
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6. Definition of Seismic Waves
• Two kinds of surface waves
– Love waves
– Rayleigh waves
• Vertical waves travelling on the
ground surface
• Horizontal waves travelling on
the ground
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6. Definition of Seismic Waves
• First arrival of seismic waves can be
id ifi d ffrom seismograph
identified
i
h (x)
( )
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7. Location of an Earthquake
• At least three geological stations
required to find location of epicentre
• For each station i, apply differential
equations between distance, velocity
and time, to primary and secondary
waves
Ri =
t s - t p 
R1
R2
1 1
 - 
 vs v p 
• ts – tp = time difference, taken from the
earthquake record at the station,
station
between the arrival of the P and Swaves
vp = velocity of the primary waves
vs = velocity of the secondary waves
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7. Location of an Earthquake
• From theory of propagation of elastic
seismic waves (sect. 14), velocities of
propagation of P and S-waves given by:
vp =
=
t s - t p 
1 1
 - 
 vs v p 
(  + 2G )

vs =
where:
Ri =
G

E
= Lame's constant
(1 + ) (1 - 2 )
G = shear modulus
E = Young' s modulus
 = Poisson' s ratio
 = rock density (mass/volume)
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