Transcript Lecture 2

Unit II
Earthquake Disaster Mitigation
EARTHQUAKES
Source: Kobe I EERI - Slide #43
Earthquakes are a natural phenomena, like drought, flood
and Cyclones.
Earthquakes in simple terms is a sudden trembling or shaking
movement of the earth surface.
Larger earthquakes usually preceded by tremors and some violent
shocks and followed by smaller earthquakes of diminishing size
called aftershocks.
What are Earthquakes?
A sudden release of
energy accumulated in
deformed rocks
causing the ground to
tremble or shake.
- Causes rupturing
or brittle failure of
crustal rocks.
- Energy is released.
- Movement of fault
blocks takes place
along a fault plane.
Source: www.earth.leeds.ac.uk/dynamicearth
INSIDE EARTH
Layers of the Earth
By analyzing the seismograms from many
earthquakes, scientists have discovered
that three main levels or shells exist
within the Earth:
CRUST
Source: www.thetech.org
MANTLE
The region just below
the crust and
extending all the way
down to the Earth's
core is called the
mantle. The mantle, a
dense, hot layer of
semi-solid rock
approximately 2,900
km thick.
The Earth's outermost surface is
called the crust. The crust is
relatively light and brittle. Most
earthquakes occur within the crust.
Scientists believe that below the
lithosphere is a relatively narrow,
mobile zone in the mantle called the
asthenosphere (from asthenes,
Greek for weak).
CORE Beneath the mantle is the Earth's core.
The Earth's core consists of a fluid outer core and
a solid inner core.
Local convective currents in the mantle
BASIC TERMINOLOGY
Earthquake
Hypocentre or focus
Epicentre
Focal depth
Epicentral distance
Origin time
Foreshocks and Aftershocks
Fault
Types of inter plate boundaries
Types of Faults
CAUSES OF EARTHQUAKES
• Plate Tectonics and Elastic Rebound
Theory
• Earthquakes due to volcanic activity
Major Tectonic plates on the Earth’s surface
SIZE OF EARTHQUAKES
Earthquake Intensity
• Oldest measure of earthquake size
• Qualitative description
• Depends on
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Distance of the site from epicenter
focal depth
Magnitude of earthquake
Soil condition
• The intensity is maximum near the epicentre and
decreases with increase in distance from the
epicentre.
INTENSITY
Intensity is a qualitative measure of the actual shaking at a location
during an earthquake and it is assigned in Roman Capital Letters
There are many intensity scales. Two commonly used ones are
1) Modified Mercalli Intensity (MMI) Scale.
2) MSK Scale
Both scales are quite similar and range from I(less perceptive) to
XII (most severe)
The intensity scales are based on three features of shaking
- perception by people and animals
- performance of buildings
- changes to surroundings
Basic Difference : Magnitude versus Intensity
Reducing illumination with distance from an electric bulb
Earthquake Magnitude
• It is measured on Richter Scale and is
related to the logarithm (base 10) of the
amount of energy released by an
earthquake.
• The magnitude M of an earthquake is
related to the energy released at the focus
of the earthquake, and is given by the
approximate formula
Log E ( ergs) = 11. 8 + 1.5 M
• The smallest earthquake perceptible by
human being corresponds to the
magnitude of 2
• largest and most destructive earthquake
so far known to have occurred has been
assigned a magnitude 8.7.
• The damage from an earthquake starts
from magnitude 5 and above.
• Earthquakes are often classified into
different groups based on their size
SEISMIC WAVES
Seismic waves are of two types
- Body waves
- Surface waves
Body waves consist of Primary waves (P-waves) and
Secondary
waves (S-waves)
Surface waves consist of Love waves and Rayleigh
waves
Arrival of seismic wave at a site
Types of Waves
Fastest waves
Do not travel through
liquid
Seismic Hazards
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•
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Ground shaking
Structural hazards
Liquefaction
Landslides
Retaining structures failure
Lifeline hazards
Tsuanami
Classification of earthquakes
• Based on location
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Interplate
Intraplate
• Based on epicentral distance
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Local earthquakes
Regional earthquakes
Teleseismic earthquakes
< 1º
1 - 10 º
> 10º
• Based on Focal depth
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Shallow depth
Intermediate depth
Deep earthquake
0-71 km
71-300 km
> 300km
• Based on Magnitude
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Micro earthquake
Intermediate earthquake
Moderate earthquake
Strong earthquake
Major earthquake
Great earthquake
<3
3-4.9
5-5.9
6-6.9
7-7.9
>8
Indian Subcontinent: among the world’s most disaster prone
areas
65% of land vulnerable to Earthquakes
8% of land vulnerable to Cyclones
5% of land vulnerable to Floods
> 1 million houses damaged annually + human, social, other losses
PAST EARTHQUAKES IN INDIA
Seismic Zone
Map of
India:
Year-1962
Seismic Zone
Map of
India:
-1966
Seismic Zone
Map of
India:
-1970
Seismic Zone
Map of
India:
-2002
About 65 percent of the
land area of India is
liable to seismic hazard
damage (about 26%
under MSK Intensity
VII, 18% under VIII and
12% under IX and
higher).
Earthquake Risk
•Hazard
= Probability of ground motion
•Site effects
= Soil properties, topography
presence of Reservoirs (RIS),
Mines (MIS)
•Vulnerability
= Building types, Age
•Risk
= Hazard x Site effects x Vulnerability
Earthquake Don’t Kill
People
but
Buildings Do
VULNERABILITY
• 1819 Gujarat [Kutch] 8.0 (2000 deaths)
• 2001 Gujarat [Bhuj] 6.9 (13805 deaths)
• Increased vulnerability in two centuries
EFFECT OF SEISMIC FORCES
ON STRUCTURES
• Inertia Forces in Structures
– From Newton’s First Law of Motion, even
though the base of the building moves
with the ground, the roof has a tendency to
stay in its original position
– If the roof has a mass M and experiences an acceleration a, then
from
inertia force = Ma (direction is opposite to acc.) Clearly, more
mass means higher inertia force. Therefore, lighter buildings
sustain the earthquake shaking better.
• Stiffness forces
– The inertia force experienced by the roof is
transferred to the ground via the columns, causing
forces in columns.
– During earthquake shaking, the columns undergo
relative movement between their ends
– when forced to bend, they develop internal forces
– The larger is the relative horizontal displacement u
between the top and bottom of the column, the larger
this internal force in columns.
– Also, the stiffer the columns are (i.e., bigger is the
column size), larger is this force
Flow of inertial forces to foundation
Structural elements and connections must be designed to transfer the inertial
forces through them
Twisting
Location of Building
Design aspects of Earthquake
resistant buidings
Design aspects of RCC
buildings