engineering seismology basics

8/18/2019 Engineering Seismology Basics

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FUNDAMETNALS OF EARTHQUAKE

ENGINEERING BY: DR. MUKESH KUMAR


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PERSONAL PROFILE

Bachelors of Civil Engineering, 2002

NED University of Engineering & Technology, Karachi

Masters in Engineering Seismology by Research, 2006

NED University of Engineering & Technology, KarachiMasters in Earthquake Engineering, 2007

University of Patras, Greece

ROSE School, University of Pavia, Italy

Doctor of Philosophy, 2012

Imperial College of London, UK


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WHAT WILL YOU LEARN IN

THIS COURSE

How the Earthquakes are Produced?

How to Quantify the Earthquakes?

What is the Response of Buildings to the Earthquakes?

How to Design the Buildings Accordingly?


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FUNDAMENTALS OF

EARTHQUAKE ENGINEERING

ENGINEERINGSEISMOLOGY

EARTHQUAKEENGINEERING


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RECOMMENDED BOOKS

Geotechnical Earthquake Engineering, By Steven L.

Kramer

Fundamentals Of Earthquake Engineering, By Amr S.

Elnashai , Luigi Di Sarno Displacement Based Seismic Design Of Structures, By

M. J. Nigel Priestley, Gian Michele Calvi , Mervyn J.

Kowalsky

Dynamics Of Structures Theory And Applications ToEarthquake Engineering, By Anil K. Chopra


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SOFTWARES

SeismoStruct (www.seismosoft.com)

SeismoSignal (www.seismosoft.com)

SAP/ETABS

Response 2000

(http://www.ecf.utoronto.ca/~bentz/r2k.htm)


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INTERNAL STRUCTURE OF

THE EARTHThe diameter of earth is approximately 12,700 km.

Crust

It is upper most layer of the earth. The thickness of the

crust is about 25 to 40 km beneath continents, 60 to 70

km beneath the mountains and very thin, about 5 km,

below the oceans.

Mantle

It is about 2850 km thick and can be divided in upper

mantle (650 km) and lower mantle.

Core

It is also divided in two portions: outer core and inner

core. Outer core or liquid core is 2260 km thick. The

rest is inner core.


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INTERNAL STRUCTURE OF

THE EARTH


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SEISMIC WAVES

Two major categories of waves: Body Waves

As the name indicates, the body waves have capability to

travel through a medium.

Primary Waves (P-waves)These waves are also known as compressional waves

or longitudinal waves, and involve successive

compression and rarefaction of the materials through

which they pass. The motion of the particles in P-waves

is parallel to the direction of the travel of wave. Likesound waves, they can travel through solids and fluids.

Since the geologic materials are stiffest in compression

the P-waves travel faster than the other waves and

therefore are the first waves to arrive at the site.


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SEISMIC WAVES

Two major categories of waves:

Secondary Waves (S-waves)

These waves are also known as shear waves or

transverse waves, and cause shearing deformation

of the materials through which they pass. The

motion of the particles is perpendicular to the

direction of the travel of wave. Therefore, it can be

divided two further categories: SV (Vertical Plane

Movement) and SH (Horizontal Plane Movement).

The shear waves cannot travel through fluids withno shearing stiffness.


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SEISMIC WAVES

Primary Waves (S-waves)


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SEISMIC WAVES

Two major categories of waves: Surface Waves

These waves travel along layers of the earth, and are

produced with an interaction of P- and S- waves. Since

these are formed by the interaction of the two types of body

waves, surface waves are generally generated at greater

distances from the earthquake source. There are two types

of surface waves:

Rayleigh Waves

These are produced as a result of interaction of P- andSV- waves, and involve both vertical and horizontal

component.


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SEISMIC WAVES

Love WavesThese are produced as a result of interaction of P- and SH-

waves, and does not involve any vertical component.


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SEISMIC WAVES THROUGH

EARTH


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CONTINENTAL DRIFT AND

PLATETECTONICS

According to this theory, the earth’s crust consists of a

large number of plates.

These plates continue to move towards or away from each

other, possibly to attain thermomechanical equilibrium ofearth’s material.

Based on this theory, it is believed that all the plates, which

are currently separated, used to be a single plate about

300 million years ago. With the passage of time that singlecontinent broke, around 200 million years ago, in various

pieces to form various plates.


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PLATETECTONICS


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PLATETECTONICS


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PLATETECTONICS


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PLATETECTONICS


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PLATETECTONICS


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PLATE BOUNDARIES

Spreading Ridge Boundary

Subduction Zone Boundary

Transform Plate Boundary


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PLATE BOUNDARIES

Spreading Ridge BoundarySpreading ridges or spreading rifts are the plate boundaries,

which move apart from each other. The gap left by the

extensional movement of the plates is filled by the molten

rock from the underlying mantle and becomes the part ofcrust after cooling.


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PLATE BOUNDARIES

Subduction Zone BoundaryIt is the boundary between two plates where the relative

movement of the plates is towards each other. At the point of

contact, one plate subducts under another plate. Such plate

boundaries give rise to large mountains. Take for example,Himalayas in the north of Pakistan.


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PLATE BOUNDARIES

Transform Fault BoundaryThe plate boundary where one plate is neither subducting

beneath another plate nor extending away from another plate,

it a boundary where one plate is passing by another plate.

The San Andreas Fault is an example of transform faultboundary.


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FAULTS

Fault Geometry (Strike and Dip)

The strike of a fault is the horizontal line produced by the

intersection of the fault plane and a horizontal plane. The

azimuth of the strike is used to describe the orientation of thefault with respect to due north.

The downward slope of the fault plane is described by the dip

angle, which is the angle between the fault plane andhorizontal plane measured perpendicular to the strike.


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FAULTS

Fault Geometry (Strike and Dip)


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FAULTS

Fault MovementDip-Slip Fault Movement

Normal Fault

Reverse Fault

Thrust Fault

Strike-Slip Fault Movement

Right Lateral Strike-Slip Fault

Left Lateral Strike-Slip Fault

Oblique Fault Movement


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FAULTS

Dip-Slip Fault MovementNormal Fault


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FAULTS

Dip-Slip Fault MovementReverse Fault


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FAULTS

Dip-Slip Fault MovementThrust Fault


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FAULTS

Strike-Slip Fault MovementRight Lateral Fault


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FAULTS

Strike-Slip Fault MovementLeft Lateral Fault


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FAULTS

Strike-Slip Fault MovementLeft Lateral Fault


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ELASTIC REBOUND THEORY

This theory aims to explain how the earthquakes areproduced. As per this theory, the earthquakes are produced

when the Elastic Strain Energy, stored in the materials near

the boundary as shear stresses, reaches the shear strength

of the rock.

If the rock along the fault is weak and ductile than the strain

energy stored will be released relatively slowly. On the other

hand, if the rock is strong and brittle, the failure will be rapid.

Rupture of the rock will release the stored energy explosively.

The theory of elastic rebound describes this process of thesuccessive buildup and release of strain energy in the rock

adjacent to faults.


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Seismic Gaps

http://cires.colorado.edu/~bilham/


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Seismic Gaps


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Relationship to Tectonic Environment

Based on this theory, one can explain or estimate the energy

release in a given earthquake. For instance, in case

spreading ridge plate boundary earthquakes the thickness ofthe plates involved is very small, since these boundaries are

typically found in oceans. Moreover, the movement of faults is

extensional and the rock is relatively warm hence ductile. In

such case there is relatively small release of energy hencevery large earthquakes are not experienced at such locations.


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On the other hand, for Subduction zones the crusts involved

have cooled down and the plates move towards each other.

Thus, significant amount of energy is accumulated before therupture. Consequently, very large earthquakes are

experienced at the Subduction zone boundaries.


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For the transform fault, since the huge compressive stresses

are not involved and typically there is lack of significant strain

energy, very large earthquakes do not occur.


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Seismic MomentThe elastic rebound theory is used to develop a simplerelationship between the rupture strength, μ, rupture area, A

and average amount of slip , as follows:

=


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Example Problem 2.7: An earthquake causes an average of 2.5 m strike-slip

displacement over an 80 km long, 23 km deep portion of a

transform fault. Assuming that the rock along the fault had an

average rupture strength of 175 kPa, estimate the seismicmoment and moment magnitude.

=


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Geometric Notation


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Geometric Notation


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Location of an Earthquake


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Size of Earthquake

Earthquake MagnitudeIt is an objective measure of the size of an earthquake using

ground motion records.

Richter Local Magnitude

Surface Wave Magnitude Body Wave Magnitude

Moment Magnitude


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Size of Earthquake

Richter Local Magnitude (ML)In 1935, Charles Richter defined a magnitude scale for

shallow, local earthquakes (with epicentral distance less than

about 600 km (375 miles)). It is defined as the logarithm of

the maximum trace amplitude in micrometers recorded on aWood-Anderson seismometer located 100 km from the

epicentre of the earthquake.


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Size of Earthquake

Surface Wave Magnitude(MS) At large epicentral distances, body waves have usually

attenuated and scattered sufficiently that the resulting motion

is dominated by surface waves. The surface wave magnitude

is a worldwide magnitude scale based on the amplitude ofRayleigh waves with a period of about 20 sec. The surface

wave magnitude is most commonly used to describe the size

of shallow earthquakes with focal depth less than 70 km and

distances farther than 1000 km.


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Size of Earthquake

Body Wave Magnitude (Mb)It is a scale based on the amplitude of the first few cycles of

p-waves which are not strongly influenced by the focal depth.


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Size of Earthquake

Comparison among Various Magnitude Scales


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Size of Earthquake

Earthquake Energy

The total seismic energy released during an earthquake is

often estimated from the relationship:

log = 11.8 1.5

E is expressed in Ergs.

This relationship can be used with moment magnitude as

well. The above equation implies that a unit change inmagnitude corresponds to 101.5 or 32-fold increase in seismic

energy. In other words, a magnitude 5 earthquake would

release only about 0.001 (1000 times lesser) times the

energy of a magnitude 7 earthquake.


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Size of Earthquake

Earthquake Energy

Energy released in Hiroshima bomb is equal to magnitude 6

earthquake.

Energy released in Mw 9.5 Chile earthquake in 1960 is equal

to 178000 such bombs.


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Size of Earthquake

Comparison of Energy Release in Various Events


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Size of Earthquake

Earthquake Intensity

It is the oldest measure of the size of an earthquake.

It is a quantitative measure of the effects of the earthquake

at a particular location, based on the observed damageand human reactions at that location.

Since this measure has been used for a long period of

history, it is generally used to estimate the locations and

sizes of earthquakes that occurred prior to thedevelopment of modern seismic instruments.

These subjective intensities can be correlated roughly with

the instrumental intensity measures.


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Size of Earthquake


The intensity in this method is typically scaled with whole

numbers ranging from 1 to any maximum value depending

on number.

For instance, Rossi-Forel (RF) scale of intensity, developed

in 1880s, ranges from I to X.

Modified Mercalli Intensity (MMI) scale, originally

developed by Italian seismologist Mercalli, was modified in

1931. It is the most commonly used intensity scale allaround the world.

Other intensity scales include Japanese Meteorological

Agency (JMA) and Medvedev-Spoonheuer-Karnik (MSK)

scales used in Japan and Eastern Europe respectively.


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Size of Earthquake


To document the intensities physical surveys are

conducted at various locations effected by the earthquake.

Using the intensities isoseismal maps can be developed.


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Size of Earthquake

MMI ScaleScale Observation

I. Instrumental Generally not felt by people unless in

favourable conditions.

II. Weak Felt only by a few people at rest,

especially on the upper floors of

buildings. Delicately suspended objects(including chandeliers) may swing

slightly.

III. Slight Felt quite noticeably by people indoors,

especially on the upper floors of

buildings. Many do not recognize it as an

earthquake. Standing automobiles mayrock slightly. Vibration similar to the

passing of a truck. Duration can be

estimated. Indoor objects (including

chandeliers) may shake.


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Size of Earthquake

MMI ScaleScale Observation

IV. Moderate Felt indoors by many to all people, and

outdoors by few people. Some awakened.

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.

V. Rather Strong Felt inside by most or all, and outside.

Dishes and windows may break and bells

will ring. Vibrations are more like a largetrain passing close to a house. Possible

slight damage to buildings. Liquids may spill

out of glasses or open containers. None to a

few people are frightened and run outdoors.

Size of Earthquake


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Size of Earthquake

MMI Scale

Scale Observation

VI. 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.

VII. Very Strong Difficult to stand. Furniture broken. Damage

light in building of good design and

construction; slight to moderate in ordinarily

built structures; considerable damage in

poorly built or badly designed structures;some chimneys broken or heavily damaged.

Noticed by people driving automobiles.

Size of Earthquake


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Size of Earthquake

MMI Scale

Scale Observation

VIII. 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.

IX. Violent General panic. Damage slight to moderate(possibly heavy) in well-designed structures.

Damage moderate to great in substantial

buildings, with a possible partial collapse.

Some buildings may be shifted offfoundations. Walls can fall down or collapse.

Size of Earthquake


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Size of Earthquake

MMI Scale

Scale Observation

X. Intense Many well-built structures destroyed,collapsed, or moderately to severely

damaged. Most other structures destroyed,

possibly shifted off foundation. Large

landslides.

XI. Extreme Few, if any structures remain standing.Numerous landslides, cracks and

deformation of the ground.

XII. Catastrophic Total destruction – everything is destroyed.Objects thrown into the air. The ground

moves in waves or ripples. Large amounts

of rock move position. Landscape altered, orlevelled by several meters. Even the routes

of rivers can be changed.


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FREE VIBRATION

• Undamped Free Vibration

• = 0

• = 0 =

• = 0 =

• =


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FREE VIBRATION

• Damped Free Vibration

• = 0

• = 0 =

• = 0 =

• = − +

• = 1 ζ2


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FREE VIBRATION

• Free Vibration Test

• ζ =

2

• ζ = 2


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HARMONIC VIBRATION

• Undamped Harmonic Free Vibration

• =

• = 0 =

• = 0 =

• =

−

− sinω

• = 1 ζ2


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Response Spectrum

• Undamped Harmonic Free Vibration

• =

• = 0 =

• = 0 =

• =

−

− sinω

• = 1 ζ2


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engineering seismology basics

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