[EAS664] - Part 1_Introduction to Earthquake.pdf

Introduction to
Earthquake
EAS 664
Principle of Structural Design
SEM 1 2022/23
Assoc. Prof. Ir. Dr Fadzli Mohamed Nazri
cefmn@usm.my

• Earthquakes in Malaysia
• Primary and Secondary sources of
Earthquake Damage
• What are Earthquakes?
• Faults
• How are earthquakes recorded?
• Earthquake source Characterization
• Seismic Waves
• Seismicity
• How is an Earthquake’s Epicenter
Located?
• How to Make A Building Earthquake-
Proof
• Seismic Hazard
• Earthquake Prediction
Outline

Earthquakes in East Malaysia
Ranau area Kudat area
Sandakan area
Lahad Datu area
Tawau and Semporna area

Earthquakes in West Malaysia
Kota Tinggi Earthquake
Kuala Lumpur Fault Zone
List of Earthquakes around Kuala Lumpur
Fault zone (Shuib, 2009)
Baling Earthquake
A magnitude 3.8
earthquake shook
Baling (Kedah,
Malaysia) at
8.26am, 20th
August 2013
Tasik Kenyir Earthquake
1) 28 earthquake was recorded from
1984 to 1986
2) The area was hit by earthquakes
measuring between 2.6 to 4.6 on
the Richter scale.
Most recent: a magnitude 2.7
earthquake on 23rd February 2016
(9.25pm) but not signs of structural
damage to Kenyir Dam

Primary and Secondary sources of Earthquake Damage
The potential for a large earthquake to cause damage comes in
the first instance from the violent shaking of the ground, which
may affect an area many hundreds of kilometres in radius.
This is the primary source of damage with which
earthquake engineers must deal.
Large relative displacements across a fault which breaks up
to the surface can also be damaging but usually relatively few
unlucky structures are affected.

The seismic shaking causes direct effects on structures, due to the inertia forces set
up by the ground accelerations, but important secondary sources of damage may also
arise.
Most significantly, large soil movements
may occur due to consolidation,
liquefaction (the temporary loss of shear
strength in loose, saturated, sandy soils)
and landslides

Coastal sites may need to consider tsunamis (commonly referred to as
tidal waves) triggered by offshore earthquakes;

Other secondary effects are those due to fire following an earthquake (which has
cost many lives in the past), the collapse of one structure onto another, and the
release of noxious chemical or radioactive materials, although to date the last
possibility has never caused major problems.

What are Earthquakes?
• The shaking or trembling caused by
the sudden release of energy and
propagates in all directions as
seismic waves causing earthquakes
• Usually associated with faulting or
breaking of rocks
• Continuing adjustment of position
results in aftershocks
Northridge Earthquake of 1994
The shaking was less than 20 seconds. But it was
enough to kill at least 57 people, injure 9,000 and
cause $40 billion in damage throughout Southland
communities.

Where do Earthquakes Start?
• The starting point of an
earthquake below ground is called a
focus, or hypocenter.
• The area directly above the
hypocenter on land is called the
epicenter.
• Earthquakes are strongest at the
epicenter and become gradually
weaker farther away!

• Earthquake = Vibration of the
Earth produced by the rapid
release of energy
• Seismic waves = Energy
moving outward from the
focus of an earthquake
• Focus= location of initial slip
on the fault; where the
earthquake origins
• Epicenter= spot on Earth’s
surface directly above the
focus

Time
Causes:
accumulated
strain
leads to fault
rupture
- the elastic
rebound model

Causes: fault movement
releases energy as seismic
waves radiating from rupture
Seismic
waves

Fault Motion
Faults are formed when mutual slip of the rock beds occurs on a
certain plane.
1. DIP-SLIP FAULTS
a) Normal Fault
The block above the fault moves down relative to the block below the
fault. This fault motion is caused by tensional forces and results in
extension. [Other names: normal-slip fault, tensional fault or gravity
fault]
b) Reverse Fault
The block above the fault moves up relative to the block below the
fault. This fault motion is caused by compressional forces and results
in shortening. A reverse fault is called a thrust fault if the dip of
the fault plane is small. [Other names: thrust fault, reverse-slip
fault or compressional fault]

2. STRIKE-SLIP FAULT
The movement of blocks along a fault is horizontal. If the block on
the far side of the fault moves to the left, as shown in this
animation, the fault is called left-lateral. If the block on the far
side moves to the right, the fault is called right-lateral. The fault
motion of a strike-slip fault is caused by shearing forces. [Other
names: transcurrent fault, lateral fault, tear fault or wrench fault]
3. OBLIQUE-SLIP FAULT
Oblique-slip faulting suggests both dip-slip faulting and strike-slip
faulting. It is caused by a combination of shearing and tension of
compressional forces.

Seismic Deformation
1989 Loma Prieta
Earthquake

How are earthquakes recorded?
Seismographs record
earthquake events

Giuseppe Mercalli
(1850-1914)
John Milne
(1850-1913)
Chang Heng ‘Seismometer’AD132
Sassa Seismometer (~1935), Abuyama, Kyoto Univ.

Earthquake source Characterization
• Magnitude
• Fault Dimensions
• Slip Distribution
• Fourier Transform Refresher
• Point source representation
- Spectral shape
- Corner frequency
- Stress parameter

Earthquake Magnitude
• Earthquake magnitude scales originated because of
- The desire for an objective measure of earthquake size
• In the 1930’s, Wadati in Japan
and Richter in California noticed
that although the peak
amplitudes on seismograms from
different events differed, the
peak amplitudes decreased with
distance in a similar manner for
different quakes.

Richter’s Local Magnitude, ML
- Richter used these observations to
construct the first magnitude scale, ML
(Richter’s Local Magnitude for Southern
California).
- Defined for specific attenuation
conditions valid for southern California
- Only valid for one specific type of
seismometer
- Not often used now, although it IS a
measure of ground shaking at frequencies
of engineering interest

Richter’s Local Magnitude, ML
In contrast to the general magnitude formula, ML considers only the maximum
displacement amplitudes but not their periods. Reason: instruments are short-period and
their traditional analog recorders had a limited paper speed. Proper reading of the period
of high-frequency waves from local events was rather difficult.
The smallest events recorded in local microearthquake
studies have negative values of ML while the largest ML is
about 7 , i.e., the ML scale also suffers saturation. Despite
these limitations, ML estimates of earthquake size are
relevant for earthquake engineers and risk assessment since
they are closely related to earthquake damage. The main
reason is that many structures have natural periods close to
that of the WA seismometer (0.8s) or are within the range
of its pass-band (about 0.1 - 1 s).

Modern Seismic Magnitudes
• Today seismologists use different seismic waves to
compute magnitudes
• These waves generally have lower frequencies than those
used by Richter
• These waves are generally recorded at distances of 1000s
of kilometers instead of the 100s of kilometers for the
Richter scale (this is important because most earthquakes
occur in remote places, such as under the oceans, without
instruments within 100s of kilometers)

Seismic Waves
Four main types of seismic wave:
Body wave, mb
Surface wave, Ms
1. Compressional/Primary wave (P),
2. Shear/Secondary wave (S),
3. Rayleigh wave (R), and
4. Love wave (L).
mb and Ms are commonly used in modern magnitude scales

Particle motion consists of alternating compression and dilation.
Particle motion is parallel to the direction of propagation
(longitudinal).

Particle motion consists of alternating transverse motion.
Particle motion is perpendicular to the direction of propagation
(transverse).

Particle motion consists of elliptical motions (generally
retrograde elliptical) in the vertical plane and parallel to the
direction of propagation. Amplitude decreases with depth.

Particle motion consists of alternating transverse motions.
Particle motion is horizontal and perpendicular to the direction
of propagation (transverse). Amplitude decreases with depth.

Types of Seismic Waves
• P waves are seismic waves that compress and expand the
ground like an accordion. S waves are seismic waves that
vibrate from side to side as well as up and down.
- Earthquakes and Seismic Waves

Types of Seismic Waves
Surface waves move more slowly than Body waves, but they
produce the most severe ground movements.

Seismicity
• The severity of an earthquake can be expressed in several ways:
1. The magnitude of an earthquake (Richter Scale) is a measure of the
amplitude of the seismic waves.
2. The moment magnitude of an earthquake is a measure of the amount of
energy released - an amount that can be estimated from seismograph
readings.
3. The intensity (Modified Mercalli Scale) is a subjective measure that
describes how strong a shock was felt at a particular location.
• The Peak Ground Acceleration (PGA) is one of the most important
characteristics of an earthquake.
• PGA is given in units g’s, i.e. as a fraction of gravitational acceleration: a
(m/sec2) / 9.81

Why use moment magnitude?
• It is the best single measure of overall earthquake size
• It does not saturate
• It can be estimated from geological observations
• It can be estimated from paleoseismology studies
• It can be tied to plate motions and recurrence relations

Magnitude Discrepancies
Ideally, you want the same value of magnitude for any
one earthquake from each scale you develop, i.e.
MS = mb = ML = M
But this does not always happen:
San Francisco 1906: MS = 8.3, M = 7.8
Chile 1960: MS = 8.3, M = 9.6

Why Don’t Magnitude Scales Agree?
• Simplest Answer:
– Earthquakes are complicated physical phenomena that
are not well described by a single number.
– Can a thunderstorm be well described by one number ?
(No. It takes wind speed, rainfall, lightning strikes,
spatial area, etc.)

• More Complicated Answers:
• The distance correction for amplitudes depends on geology.
• Deep earthquakes do not generate large surface waves - MS is
biased low for deep earthquakes.
• Some earthquakes last longer than others, even though the peak
amplitude is the same.
• Variations in stress release along fault, for same moment.
• Not all earthquakes are self similar (that is, the relative radiation
at different frequencies can differ--- examples: 1999 Chi-Chi
compared to “standard” California earthquake).

• Most complicated reason:
– Magnitude scales saturate
– This means there is an upper limit to magnitude
no matter how “large” the earthquake is
– For instance Ms (surface wave magnitude)
seldom gets above 8.2-8.3

Example: mb “Saturation”
mb seldom gives values
above 6.7 - it “saturates”.
mb must be measured in
the first 5 seconds - that’s
the (old) rule.

What Causes Saturation?
The rupture process.
 Small earthquakes rupture small areas and are relatively
depleted in long-period signals.
 Large earthquakes rupture large areas and are rich in long-
period motions

Are mb and Ms still useful?
• YES!
• Many (most) earthquakes are small enough that saturation does not
occur
• Empirical relations between energy release and mb and Ms exist
• The ratio of mb to Ms can indicate whether a given seismogram is
from an earthquake or a nuclear explosion (verification seismology)

Magnitude Summary
Magnitude Symbol Wave Period
Local (Richter) ML S or Surface Wave* 0.8 s
Body-Wave mb P 1 s
Surface-Wave Ms Rayleigh 20 s
Moment Mw, M Rupture Area, Slip 100’s-1000’s
• Magnitude is a measure of ground shaking amplitude.
• More than one magnitude scale is used to study earthquakes.
• All magnitude scales have the same logarithmic form.
• Since different scales use different waves and different period
vibrations, they do not always give the same value.

Intensity Map
•An earthquake has one
magnitude, but many
intensities
•Not directly related to
earthquake source
•Damage obviously distance
dependent
•Needs population to report
damage
•Affected by site response

How is an Earthquake’s Epicenter Located?
Seismic wave behavior
• P waves arrive first, then S waves, then L and R
• Average speeds for all these waves is known
• After an earthquake, the difference in arrival times at a seismograph
station can be used to calculate the distance from the seismograph to
the epicenter.

The Richter scale
Steps:
1. Measure the interval (in seconds) between the
arrival of the first P and S waves.
2. Measure the amplitude of the largest S waves.
3. Use nomogram to estimate distance from
earthquake (S-P interval) and magnitude (join points
on S-P interval scale and S amplitude scale).
4. Use seismograms from at least three geographic
locations to locate epicentre
by triangulation.
The Ritcher Scale Nomogram

Locating the epicentre:
X, Y and Z are seismograph stations
Z
Y
X
220 km
epicentre
280 km
150 km
Triangle of Uncertainty
• The area where the 3 circles (from the 3 cities
reporting a quake) meet isn’t perfect. They do not
completely overlap each other at the exact location of
the epicenter.
• The space of overlap is the TRIANGLE OF UNCERTAINTY.
The earthquake started in this area!
• Modern Seismic Networks usually use hundreds of
stations.
Geologists use seismic waves to
locate an earthquake’s epicenter.

Earthquakes
• Causes - tectonics and faults
• Magnitude - energy and intensity
• Earthquake geography
• Seismic hazards - shaking, etc.
• Recurrence - frequency and regularity
• Prediction?
• Mitigation and preparedness

How are the Size and Strength of an Earthquake Measured?
Modified Mercalli Intensity Map
– 1994 Northridge, CA earthquake, magnitude
6.7
• Intensity
• subjective measure of the
kind of damage done and
people’s reactions to it

Intensity: Mercalli Scale:
• What did you feel?
• Assigns an intensity or rating to
measure an earthquake at a
particular location (qualitative)
• I (not felt) to XII (buildings
nearly destroyed)
• Measures the destructive effect
• Intensity is a function of:
• Energy released by fault
• Geology of the location
• Surface substrate: can
magnify shock waves e.g.
Mexico City (1985) and San
Francisco (1989)

Frequency of Occurrence of Earthquakes
Descriptor Magnitude Average Annually
Great 8 and higher 1 ¹
Major 7 - 7.9 17 ²
Strong 6 - 6.9 134 ²
Moderate 5 - 5.9 1319 ²
Light 4 - 4.9
13,000
(estimated)
Minor 3 - 3.9
130,000
(estimated)
Very Minor 2 - 2.9
1,300,000
(estimated)
¹ Based on observations since 1900.
² Based on observations since 1990.
Some 272 people were confirmed killed by the
7.2 magnitude earthquake which struck around
lunchtime on Sunday in the Van province.

How to Make A Building Earthquake-Proof
1. Create a Flexible Foundation

2. Counter Forces with Damping
Vibrational Control Devices
The first method involves placing dampers at each level of a building between a column and
beam. Each damper consists of piston heads inside a cylinder filled with silicone oil. When
an earthquake occurs, the building transfers the vibration energy into the pistons, pushes
against the oil. The energy is transformed into heat, dissipating the force of the
vibrations.

Pendulum Power
Another damping method is pendulum power, used primarily in skyscrapers.
Engineers suspend a large ball with steel cables with a system of hydraulics at the
top of the building. When the building begins the sway, the ball acts as a pendulum
and moves in the opposite direction to stabilize the direction. Like damping, these
features are tuned to match and counteract the building’s frequency in the event of
an earthquake

3. Shield Buildings from Vibrations
Instead of just counteracting forces, researchers are experimenting with ways
buildings can deflect and reroute the energy from earthquakes altogether. Dubbed the
“seismic invisibility cloak”, this innovation involves creating a cloak of 100 concentric
plastic and concrete rings in and burying it at least three feet beneath the foundation
of the building.
As seismic waves enter the rings, they are forced to move through to the outer rings
for easier travel. As a result, they are essentially channeled away from the building and
dissipated into the plates in the ground.

4. Reinforce the Building’s Structure
To withstand collapse, buildings need to redistribute the forces that travel through
them during a seismic event. Shear walls, cross braces, diaphragms, and moment-
resisting frames are central to reinforcing a building.

Earthquake-Resistant Materials
While shock absorbers, pendulums, and “invisibility cloaks” may help dispel the energy to an
extent, the materials used in a building are equally responsible for its stability

Can Earthquakes be Predicted?
Earthquake Prediction Programs
• include laboratory and field studies of rocks before, during, and after
earthquakes
• monitor activity along major faults
• produce risk assessments

Earthquake risk and prediction
• Long-term methods
1) seismic hazard maps
2) probability analysis based
on:
- historical EQ records
- geologic EQ records
- slip-rate on active faults
- frequency and magnitude
of recent EQ's
Real-time 24 Hour
Forecast

Short-term predictions
Precursor phenomena (<1 year to days)
1. Foreshocks: usually increase in magnitude
2. Ground deformation
3. Fluctuations in water well levels
4. Changes in local radio wave characteristics
5. Anomalous animal behavior???

Impacts of Earthquake Prediction

[EAS664] - Part 1_Introduction to Earthquake.pdf

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