seismic

Seismic Risk: Seismic Risk: An IntroductionAn Introduction

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IntroductionEarthquake engineering is a super-specialisation of structural engineering

Earthquake engineering deals with understanding earthquakes, their causes, their consequences, and designing structures to withstand earthquake forces

The field of study is highly multi-disciplinary with lead taken by structural engineers

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Background• Earthquakes are one of the most

devastating forces in nature

• Earthquakes disasters have been known since ancient times

• Earthquakes have been instrumental in changing the course of history

• Some of the most significant disasters in the last hundred years have been caused by earthquakes

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• The causes of earthquakes have been guessed by different civilisations since historical times

Mongolian LegendMongolian Legend

Earthquake Background

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• The causes of earthquakes have been guessed by different civilisations since historical times

Japanese LegendJapanese Legend

Earthquake BackgroundEarthquake Background

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Risk

Vulnerability

Site Effects

Hazard

Earthquake Risk

Probability of ground motion

Amplification due to: Soil Topography

Effect on structures due to: Building type and age Population density Land use Month and time

Probability of damage and losses

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Seismic Hazard Assessment

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• Records of every major earthquake in China during the last 3000 years

• Records of major earthquakes in India up to last 2500 years

• Records of major earthquakes over 2000 years in Middle-East

• Legends about earthquakes in India and several other ancient civilisations

Historical Background

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• Modern study of seismology has been carried out over the last 45-50 years only

• Most useful data has been collected using a world-wide network of seismological stations

• Records show that earthquakes are not uniformally distributed but concentrated along well defined lines

Modern Studies

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Earthquake Sources

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Most earthquakes are concentrated along boundaries of earth’s platesSome earthquakes also occur away from plate boundariesEarthquakes in many places are also associated with volcanic activitiesIn recent times, earthquakes may have been triggered by human structures and activities (dams, mining etc.)

Earthquake Sources

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• Earth is not a rigid and motionless mass

• Cross-section of the earth can be classified into four distinct concentric layers: Inner Core (Solid) Outer Core (Liquid) Mantle (Liquid) Crust (Solid)

Structure of Earth

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Structure of Earth

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Structure of Earth

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• Motion of earth’s plates are explained using Plate Tectonics

• According to Plate Tectonics: Earth’s land-mass were earlier joined

together The land-mass have broken up and

have drifted apart Relative motion is still continuing,

relative motion at plate boundaries cause earthquakes

Plate Tectonics

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Plate Tectonics

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Earth’s Plates

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Earth’s Plates

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Drift of Indian SubcontinentDrift of Indian Subcontinent

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• Considerable evidence now exist to support Plate Tectonics

• Types of evidence: Geological and geomorphological -

similar rock formations Anthropological - similar vegetation and

animal life Geomagnetic - magnetic anomalies

support drifting away of land mass from Atlantic ridge and other places

Plate Tectonics

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Prehistoric Flora and Fauna

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Elastic Rebound Theory

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Example of Fault Rupture

Chile EarthquakeChile Earthquake

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Taiwan EarthquakeTaiwan Earthquake

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Kobe EarthquakeKobe Earthquake

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Earthquake Waves• Elastic rebound produces waves from the

point of rupture

• The rupture may be localised at a point, along a slip line or a slip surface

• Earthquake waves have clearly identifiable components Primary wave (refractory) Secondary or shear wave

(transverse) Raleigh wave (refractory) Love wave (transverse)

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Earthquake Waves

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Earthquake Magnitude• Earthquake magnitude is most commonly defined

in RichterRichter magnitude It is logarithm of the maximum displacement

(in µm) recorded on a particular type of seismograph 100 km from the epicentre

Richter magnitude is open-ended and has no maximum value

• Scientifically more useful measure is based on seismic moment and measures the total energy that is released Both magnitudes give similar value for

moderate earthquakes (M 5.0 - M7.5)

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Earthquake Intensity• Earthquake intensity is a measure of its

consequence• Most popular intensity scales are primarily

based on structure damage MMI (Defines 12 intensities) based only on

performance of buildings MSK (Defines 12 intensities) based on building

performance, geotechnical effects as well as human perception

• Most countries (including India) use MSK intensity scale or its modifications to suit local conditions

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Indian Seismicity

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Seismic Hazard

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Assessment of Site Effects

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Influence of Local ConditionsMaximum ground motion also depends on local

soil/rock properties

Maximum ground displacement in Northridge earthquake (1994)

10 km

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Mumbai Description

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Influence of Local Conditions

Anjar Town-Plan and Local Soil Conditions

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Seismic HazardChallenges in Hazard Assessment

• Location of seismogenic features• Fault size, movement rate, return

period• Inadequate historical earthquake

data• Information on local soil conditions

These require significant additional scientific studies to provide reliable information

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Seismic Vulnerability Assessment

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Seismic VulnerabilityDepends on type of structures (structure category) and their age

Depends on land use in city (space between adjacent buildings, height of buildings etc.)

Depends on month and time (buildings may be weaker during the rainy season, and residential buildings more fully occupied during nights)

Depends on population density (impact of damage of a building to number of people)

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Urban Construction Practice

Engineered Constructions Reinforced concrete buildings Brick masonry buildings with RCC roof

Non-Engineered Constructions Informal brick masonry buildings Other non-engineered buildings using

light weight materials

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Seismic Vulnerability• Seismic vulnerability can be expressed

in terms of vulnerability curves

V VI VII VIII IX X XI

50%

100%

0%

% D

AM

AG

E

EARTHQUAKE INTENSITY (MSK)

Non-engineeredMasonry

RCC Steel

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Seismic Vulnerability

Year Collapses

1993-94 236

1994-95 253

1995-96 224

1996-97 272

1997-98 259

1998-99 305

1999-00 154

2000-01 260

2001-02 273 Num

ber

of b

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colla

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in

Mum

bai –

wit

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t an

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wit

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t an

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eart

hq

uake

eart

hq

uake

• Must consider the consequences of very poor building stock – example: Mumbai

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Seismic Risk

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• Risk has the following components–Hazard–Site Effects–Vulnerability

• Inadequate understanding of earthquake hazard in large parts of India

• Vulnerability of different structure types are poorly assessed

Earthquake Risk

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Research indicates following scenario for an earthquake disaster in Mumbai (2001)

Example :: Mumbai

Time MSK VI MSK VII MSK VIII

Midnight 11,200 42,600 100,100

6 A.M. 9,000 34,000 80,000

12 Noon 6,700 25,500 60,100

Estimated number of fatalities and injuries due to building collapse

Time MSK VI MSK VII MSK VIII

Midnight 31,400 118,400 277,600

6 A.M. 25,000 94,600 222,100

12 Noon 18,800 71,000 166,500

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SummarySignificant knowledge about earthquakes and its consequences exist in scientific communityInterest in earthquakes continue since prehistoric timesEarthquakes can cause severe damage due to strong ground motions and/or deformationsSeismic risk depends on hazard, site conditions and structure vulnerability

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India is divided into 4 seismic zones (low to very high seismic hazard)Most large cities have moderate to high seismic hazardThe damage at a locality is influenced by the local soil conditionsBuilding performance depends on ground motions as well as structural characteristics

Summary

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Vulnerability is expected to be very high due to poor building stockInformation on hazard, site effect and vulnerability can be combined to assess seismic riskSeismic risk is high due to uncertainty about hazard and high vulnerability

Summary

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seismic

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