Earthquake-Resistant Structures | Geology Optional Notes for UPSC PDF Download

Causes of Earthquakes

• We understand that earthquakes occur due to the release of stored seismic energy through various processes.
• The Earth's lithosphere consists of large irregular plates that are constantly moving slowly.
• These plates interact with each other as they move over the convection cells in the asthenosphere, a part of the Earth's mantle.
• Plate boundaries, where these plates meet, serve as outlets for releasing the energy stored in rocks.
• This energy is released suddenly through the generation of seismic waves by rock ruptures or movements along faults.
• Seismic waves, such as 'P' and 'S' waves, cause main shocks, while surface waves like 'L' waves lead to aftershocks.
• Earthquakes can continue for seconds to minutes, resulting in smaller aftershocks as the fault adjusts.

Other Causes of Earthquakes:

• Volcanic activity can trigger earthquakes due to magma movement and underground disturbances.
• Caldera collapses and massive landslides may also lead to localized seismic events.
• Human activities like nuclear explosions, mining blasts, and dam constructions can induce earthquakes.
• Reservoir-induced seismicity occurs when reservoir water levels increase, exerting pressure on rocks and causing them to fail.
• Increased water levels can also lead to enhanced water flow through fractures in rocks, contributing to seismic events.

Earthquake Classification and Terminologies

• Fault Zones and Earthquakes:
  • Earthquakes are caused by sliding along fault walls due to decreased friction in sheared and pulverized rocks in fault zones.
• Factors Affecting Earthquake Damage:
  • Damage from earthquakes varies based on factors like local geology, soil cover thickness and type, coastal location, terrain steepness, construction materials, and building engineering.
• Classification and Terminologies:
  • Focus, Epicentre, and Ante Centre:
    • The point where an earthquake originates due to rupture is the focus or hypocentre.
    • The epicentre is the point on the ground directly above the focus.
    • The ante centre is the point opposite the epicentre on the other side of the globe.
  • Isosingmal Lines:
    • These lines show areas of equal earthquake intensity.
  • Classification by Focal Depth:
    • Shallow Focus:
      • Originates within the top 70 km of the lithosphere, with 80% of earthquakes falling into this category.
      • 90% of shallow focus earthquakes occur within the top 30 km of the crust.
    • Medium Focus:
      • Originate between 70 to 300 km depth, accounting for 15% of all earthquakes.

Classification of Earthquakes

• Inter-plate Earthquakes:
  • Originate along the boundaries of lithospheric plates.
• Intra-plate Earthquakes:
  • Originate within the interior of lithospheric plates.
• Stable Continental Region (SCR) Earthquakes:
  • Occur in stable and older parts of continents known as cratons.

Measuring Earthquakes

• Assessment of Earthquake Damage:
  • Can be qualitative (Intensity) or quantitative (Magnitude).
• Commonly Used Scales:
  • Modified Mercalli Intensity (MMI) Scale
  • Medve Scale

Deep Focus Earthquakes

• Definition:
  • Earthquakes originating from 300 to 700 km below the surface.
• Statistics:
  • Approximately 5% of earthquakes fall into this category.
  • Deepest recorded earthquake had a focal depth of 720 km in Japan.

Modified Mercalli Intensity (MMI) Scale

• Description of Damage Levels:
  • Not Felt
  • Felt by Few Persons
  • Felt Quite Noticeably
  • Felt by Nearly Everyone
  • Felt by All
  • Noticed by Persons Driving

Earthquake Damage Intensity and MSK Scale

Medvedev-Sponheuer-Karnik (MSK) Scale

• Level I - Slight damage in specially designed structures
• Level II - Considerable damage in ordinary structures with partial collapse
• Level III - Heavy damage in poorly built structures; chimneys, factory stacks, columns, and walls may fall

Damage Grades

• Grade 1 - Slight damage
• Grade 2 - Moderate damage
• Grade 3 - Heavy damage
• Grade 4 - Destruction
• Grade 5 - Total damage

Geological Impact

• Slight damage: Seen in especially designed structures
• Considerable damage: Observed in ordinary structures with partial collapse
• Heavy damage: Poorly built structures face collapse
• Total damage: Complete destruction, with significant ground shifts and structural collapse

General Impact

• Well-designed buildings experience considerable damage
• Structures are thrown off plumb
• Buildings slide off foundations
• Complete destruction with little to no structures standing

MSK Scale Description

• Not felt: Recorded by seismographs only
• Felt by some individuals at rest, especially on upper floors
• Felt by some inside buildings; vibrations akin to a light truck passing
• Felt by all inside buildings and many outside; noticeable effects on objects and structures
• Felt by majority inside and outside buildings; significant damage to structures and surroundings

Effects of Earthquakes

• During an earthquake, various intensities of damage occur based on the type of buildings and structures present.
• People experience fear and panic, with different levels of impact depending on the building construction.
• Damage ranges from slight to severe, affecting buildings, roads, and natural landscapes.
• Examples of damage include collapsing buildings, landslides, changes in water levels, and structural deformations.

Impact on Buildings

• Type A buildings typically collapse during significant earthquakes.
• Type B buildings suffer serious damage and may also collapse.
• Type C buildings experience moderate to severe damage but are relatively more resistant.
• Structural elements like stonewalls, monuments, and columns are prone to damage or collapse.

Environmental Effects

• Earthquakes can lead to changes in natural surroundings, such as landslides, rock falls, and alterations in water bodies.
• Animals may exhibit unusual behavior, and geological formations like cracks and landslips become prominent.
• Underground infrastructure like pipes and railway tracks are also vulnerable to damage.

Geological Impact

• Severe earthquakes can cause the destruction of structures, including underground facilities.
• Topographical changes, large ground cracks, and shifts in rivers and water bodies are common outcomes.
• Measurements and analysis of earthquake intensity are crucial for understanding the impact on different areas.

Measuring Earthquakes

• Seismographs are instrumental in quantifying earthquakes by measuring seismic waves.
• Charles Richter's scale helps determine the magnitude of earthquakes based on recorded data.
• Quantification aids in assessing the level of destruction and preparing for future seismic events.

Understanding Earthquakes

• Seismometers

  Seismometers are devices used to detect and record the vibrations caused by earthquakes. Modern electronic seismometers are highly sensitive and can capture even the slightest tremors.

  Example: Electronic-computerized Seismometer

• Magnitude and Intensity

  Magnitude is a measure of the energy released by an earthquake, typically recorded on the Richter scale. Intensity, on the other hand, refers to the effects of an earthquake on the ground and structures.

  • Richter Scale

    The Richter scale uses Arabic numerals from 1 to 9.5 to quantify the magnitude of earthquakes. It is an open-ended scale, meaning it can accommodate earthquakes of various intensities.

    Example: The highest recorded magnitude was 9.5 for the 1960 Chile earthquake.

  • Intensity Scale

    Intensity scales categorize the effects of earthquakes on the surrounding area, ranging from I to XII.

    Example: Damage levels ranging from social disturbance to total destruction.

• Seismograph

  A seismograph is a device that records the vibrations of the ground caused by seismic waves. It helps in determining the influence area of an earthquake.

  • Influence Area

    The influence area of an earthquake varies based on its magnitude and energy release.

    • Classification based on Magnitude and Energy Release

      Earthquakes can be classified according to the energy they release, which correlates with their magnitude.

      Example: Classifying earthquakes based on their energy release and impact.

Earthquake Classifications

• Great Earthquake:
  • Magnitude (Richter Scale): Greater than 8
  • Intensity (MMI Scale): X to XII
  • Energy Released (Ergs): Greater than 5.8 x 10^23 J
  • Frequency (No./Yr.): 2
• Major Earthquake:
  • Magnitude (Richter Scale): 7-7.9
  • Intensity (MMI Scale): IX - XI
  • Energy Released (Ergs): 2-42 x 10^22 to 8-150 x 10^20
  • Frequency (No./Yr.): 20
• Strong Earthquake:
  • Magnitude (Richter Scale): 6-6.9
  • Intensity (MMI Scale): VIII - X
  • Energy Released (Ergs): 3-55 x 10^19
  • Frequency (No./Yr.): 100
• Moderate Earthquake:
  • Magnitude (Richter Scale): 5-5.9
  • Intensity (MMI Scale): VII-IX
  • Energy Released (Ergs): 1-20 x 10^18
  • Frequency (No./Yr.): 3000
• Light Earthquake:
  • Magnitude (Richter Scale): 4 - 4.9
  • Intensity (MMI Scale): V - VII
  • Energy Released (Ergs): 4-72 x 10^16
  • Frequency (No./Yr.): 15,000
• Minor Earthquake:
  • Magnitude (Richter Scale): 3-3.9
  • Intensity (MMI Scale): III - V
  • Energy Released (Ergs): 1-20 x 10^15
  • Frequency (No./Yr.): 6,000

Other Earthquake Related Disasters and Precautions

• Earthquake may lead to various other disasters such as:
  • Landslides
  • Natural damming of lakes and rivers due to landsliding
  • Ground settlement
  • Liquefaction
  • Fire outbreaks
• Some important consequences of earthquakes include:
  • Tsunami: Tsunami, a Japanese term for 'harbor wave', can cause inundation and flooding on landmasses. When generated in deep oceans, tsunami waves have large wavelengths but become more destructive as they approach continents.
  • Velocity: Tsunami waves can reach speeds as high as 700 km/h and may not be easily detected in open seas, posing a significant threat to coastal areas.

Causes and Types of Tsunamis

• Most tsunamis are caused by the displacement of large amounts of water along faults, resulting in the formation of oscillating tsunami waves.
• Large-scale volcanism in oceans or near the coast, leading to the eruption of a large amount of volcanic material suddenly poured out on the ocean floor, can also cause tsunamis.
• Collapse of underwater volcanoes can result in large-scale displacement of water and the formation of giant waves.
• Huge landslides plunging into the sea or large ice blocks breaking off from ice shelves and dropping into the ocean may also lead to the generation of mega waves.

Types of Tsunamis

• Local Tsunami: Local or near-field tsunamis have a very short travel time of 30 minutes or less.
• Regional Tsunami: Regional or mid-field tsunami waves have travel times ranging from 30 minutes to 2 hours.
• Tele Tsunami: Earthquake sources more than 1000 km away from the area of interest, with travel times exceeding 2 hours, are also known as distant-source or far-field tsunamis.

Pacific Ocean and Tsunami Generation

• The Pacific Ocean is notorious for generating tsunami waves due to the presence of very active subduction zones along its eastern and western margins.
• The Pacific Ocean also features the Mid-Pacific Rise, the most active mid-oceanic ridge system in terms of volcanic and seismic activity.
• The International Coordination Group for the Tsunami Warning System in the Pacific (ICG/ITSU) operates under UNESCO and is responsible for international tsunami cooperation.

Earthquake Ground Failure and Liquefaction

Ground Failure

• Earthquakes can lead to ground failure in hilly areas and settlement in alluvial plains.
• Earthquake-induced landslides and settlements not only damage buildings but also disrupt road and rail networks, hindering rescue efforts.

Liquefaction

• Liquefaction occurs when clay-rich soil loses shear strength and behaves like a fluid during seismic activity.
• Saturated well-rounded sand deposits can also lose strength due to excessive pore water pressure buildup.
• Buildings may topple or sink significantly in the ground as a result of liquefaction.
• Embankments, bridges, and underground pipelines are also at risk due to liquefaction.

Earthquake Induced Ground Shaking and Building Response

• Ground shaking during earthquakes is unpredictable in terms of duration, amplitude, direction, and time.
• Factors such as energy release at the hypocenter, fault slip nature, local geology, soil type, and ground water level influence the intensity and frequency of seismic waves.
• Building construction differences between rural and urban areas lead to varying levels of damage from earthquakes.
• Earthquake-induced ground acceleration weakens building stability by counteracting gravity and inertia forces, causing buildings to collapse.
• Peak ground acceleration (PGA) quantifies the severity of ground shaking and can be correlated with earthquake intensity and magnitude.

Table 5: Relationship of Peak Ground Acceleration with Intensity and Magnitude

• Magnitude M3 - Intensity VI - PGA (g): 0.06 - 0.07
• Magnitude M4 - Intensity VII - PGA (g): 0.10 - 0.15
• Magnitude M5 - Intensity VIII - PGA (g): 0.25 - 0.30
• Magnitude M6 - Intensity IX - PGA (g): 0.50 - 0.55
• Magnitude M7 - Intensity X - PGA (g): > 0.60

Understanding Earthquake Resistance Structures

• Impact of Seismic Forces on Buildings
  • Horizontal PGA value of 0.6g implies that shaking can exert a horizontal force on a structure equal to 60% of its weight.
  • Building failure is often caused by shaking and shifting due to ground vibrations, starting at the base.
  • Upper parts of a building tend to remain in place due to inertia (Newton's First Law), while walls and columns try to pull the structure.
  • Differential stresses occur in various parts of the building, foundation, and soil due to the interaction of inertia and acceleration, leading to failure.
• Effects of Ground Shaking
  • Ground shaking can occur in three orthogonal directions: North-South, East-West, and vertical.
  • All structures are primarily designed to support gravity loads equal to M x g (M = Mass of the building, g = Acceleration due to gravity).
  • Horizontal shaking can cause twist, swings, and torsion forces, especially when safety provisions for horizontal movements are insufficient.
• Geological Considerations for Earthquake Resistance
  • Varying geological conditions in different regions have a significant impact on earthquake vulnerability.
  • A seismic zone map is essential to classify regions based on geological factors and earthquake history.
  • India's seismic zone map has evolved over time, with the country currently having four seismic zones (Zones II to IV).
  • Zone V is identified as the most vulnerable, while Zone I is the least vulnerable.

Geological Considerations for Construction of Earthquake Resistance Structure

• Seismic Zoning in India
  • India's geographic zones like Zone I, Zone V, etc., indicate seismic activity levels.
  • The seismic zoning map of India helps in planning earthquake-resistant structures.
• Indian Subcontinent's Seismic History
  • ISC has a rich history of earthquakes dating back to 1250 B.C.
  • Several devastating earthquakes have impacted the region over the years.
  • Recent decades have seen a resurgence in significant earthquake events in India.
• Geological Vulnerabilities
  • India's susceptibility to earthquakes is due to its unique geotectonic setup.
  • The presence of active ocean ridge systems and plate boundaries influence seismic activity.
  • Elastic strain buildup and release contribute to seismic events in the region.
• Plate Boundaries and Fault Zones
  • Ocean ridge systems like the Carlsberg Ridge impact seismic activity in the Indian subcontinent.
  • The Owen Fracture Zone in the Arabian Sea serves as a significant plate boundary.
  • Translational plate boundaries, like the Chaman Fault, play a role in seismic movements.
• Engineering Implications
  • Understanding geological factors is crucial for constructing earthquake-resistant structures.
  • Structural designs must account for seismic risks based on regional seismicity.
  • Examples include using flexible foundations and shock-absorbing materials in construction.

Geological Considerations for Earthquake Resistance Structures

• Geotectonic Units in India:
  • Craton: Areas like Dharwar, Singhbhum, Aravalli, and Bhandara characterized by very low seismic potential due to geological age.
  • Shield: Regions such as Bastar and Vindhyan with a moderate seismicity potential.
  • Rifts: Examples include Son-Narmada-Tapti, Koyna, Damodar, Mahanadi, Krishna rifts, and Cambay, exhibiting high seismicity potential.
  • Fold Belt: The Himalayan fold belt is an area with extremely high seismic potential.
  • Islands: Notably the Andaman-Nicobar islands, displaying very high seismic potential.
  • Basins: Indo-Gangetic basin, Bay of Bengal, and Arabian Sea basins pose medium to high seismicity potential.
• Geological Features and Seismic Activity:
  • Older geological formations like cratons and shields have low seismic potential but can experience earthquake activity if faults within them are reactivated by external forces.
  • Younger geotectonic elements such as rifts, fold belts, and basins are more vulnerable to seismic activity due to their geological characteristics.
• Seismic Zones in India:
  • India is prone to seismic activity due to its location near plate boundaries and geological features that contribute to stress buildup and release.
  • Earthquakes in India can be attributed to faults along the ocean ridge systems in the south and the presence of younger geotectonic elements.

Seismicity in India

• Mainland India has numerous epicenters located in the Himalayan belt.
• Seismic activity is primarily due to movement along the Main Boundary Thrust (MBT) and Main Central Thrust (MCT).
• The high seismicity in the northwestern part of the Indian Shield, like the Bhuj area in Gujarat, is a result of slip along the Owen-Chaman Fracture Zone.
• Seismic activity is also influenced by the meeting of Narmada-Tapti and Cambay rifts, as well as east-west (EW) and north-south (NS) trending faults.
• The fault-ridden south-central part of peninsular India shows seismic activity along NNW-SSE and NE-SW trending faults and fractures.
• The Indo-Gangetic basin, formed due to the buckling down of the Indian Plate beneath the Eurasian Plate, has not experienced strong earthquakes but remains vulnerable.
• Basement structures like the Moradabad Fault and Great Boundary Fault, along with basement highs such as Delhi-Hardwar and Faizabad Ridge, contribute to the seismic potential of the region.

Tsunamis in the Indian Subcontinent

• Indian coasts have not faced significant tsunami disasters, except for a notable event on Nov. 27, 1945, near Mumbai with a run-up height of 12 meters.
• The devastating tsunami on Dec. 26, 2004, resulted in loss of life and economic damage in countries like Indonesia, Thailand, Sri Lanka, Andaman and Nicobar Islands, and the eastern coast of India.
• This tsunami originated near the western coast of Indonesia following a massive earthquake, affecting regions from the Maldives to Chennai.
• The earthquake causing this tsunami was one of the largest in the instrumental era, with significant impact on coastal areas.

Summary: Tsunamis and Earthquake Preparedness

Tsunamis Since 1900

• Indian coast experienced a devastating tsunami in December 2004, affecting over 2260 km of mainland coastline and the Andaman & Nicobar Islands.
• The maximum height of the tsunami wave struck various regions:
  • Marmugao: 1.0
  • Kerala: 0.6
  • Kochi
  • Allapuzha
  • Kollam
  • Kanyakumari
  • Tuticorin
  • New Delhi
  • Andhra Pradesh
  • Tamil Nadu
  • Mypadu
  • Chennai
  • Visakhapatnam: 3.0
  • Pondicherry
  • Cuddalore
  • Nagapattinam: 8.0
  • Hut Bay: 5.0
  • Malacca
  • Port Blair
  • Campbell Bay

Preparedness and Mitigation

• Despite advancements in earthquake understanding, predicting earthquakes remains uncertain.
• Scientists are developing models to study earthquake nucleation, recurrence, and fault interactions to identify potential seismic source zones.
• Key developments in earthquake research over the last century:
  • Establishment of a seismic network of 120 stations by the US Government in 1960.

Geological Considerations for Construction of Earthquake Resistance Structure

Key Technological Advancements in Earthquake Research

• World Wide Standard Seismographic Network (WWSSN) established in sixty countries.
• Theory of Plate Tectonics providing insights into the dynamics of the earth and earthquakes.
• Global Positioning System (GPS) for improved temporal and spatial control of earth processes.
• Computing Technology enabling enhanced data acquisition, processing, and interpretation.

Present-day Investigations in Tectonics

• Active tectonics: Movements expected in the future, studied through direct measurements and regional positioning techniques.
• Neotectonics: Study of tectonic events spanning up to 500,000 years, including ongoing processes post major tectonic events.

Tools and Techniques

• Direct measurements of geodetic and near-field geodesy.
• Regional position determination using satellites and GPS technology.
• Geochronology for understanding spatial and tectonic activities.
• Concurrent measurements of plate movements and fault activities.

Importance in Tectonic Hazard Studies

• Seismic source modeling.
• Wave propagation and attenuation studies.
• Geological and geophysical investigations of fault zones.
• Simulation of kinematic deformation and tectonic stresses.
• Understanding induced seismicity and paleo-seismology.
• Utilization of Geographic Information System for seismic hazard information.

Seismic Codes in India

• Introduction to Seismic Codes

  Currently, there has been a global shift towards prioritizing resilience over prediction, especially in the construction sector. The focus is on building earthquake-resistant structures and retrofitting existing ones to withstand seismic activities. In India, engineers and scientists have developed seismic codes to guide the planning, designing, and construction of structures to minimize the impact of earthquakes.

• IS 1893, 1962

  This was the first seismic code introduced in India, providing the initial Seismic Zone Map of the country. The fundamental principle behind these codes is to ensure that buildings can withstand moderate-intensity earthquakes without structural damage and avoid total collapse during high-intensity seismic events.

• Indian Standard Criteria for Earthquake Resistant Design

  IS 1893 (Part 1 to 5), 2002 outlines the specifications for seismic design force, which relies on factors like the structure's mass, seismic coefficient, Seismic Zone, building importance, stiffness, ductility, and foundation soil. Part 1 focuses on general buildings, with other parts under development for retaining walls, industrial structures, and dams.

• IS 4326, 1993

  This code of practice covers the general principles and guidelines for earthquake-resistant design and construction of buildings, including material selection and special features like timber and prefabricated elements for roofing and flooring.

• Indian Standard Guidelines for Improving Earthquake Resistance

  • Earthen Buildings (IS 13827)

    These guidelines focus on enhancing earthquake resistance in rural earthen houses.

  • Low Strength Masonry Buildings (IS 13828)

    The guidelines aim to improve earthquake resistance in non-engineered low-strength constructions using materials like clay burnt bricks and stone masonry in mud and weak cement sand mortar.

• IS 13920, 1993

  This code of practice emphasizes ductile detailing for reinforced concrete structures subjected to seismic forces, enhancing the design and detailing of RCC buildings for improved ductility. Following the Bhuj earthquake in 2001, this code became mandatory for all buildings in Seismic Zone III, IV, and V.

Indian Standard Guidelines for Repair and Seismic Strengthening of Buildings

• India has comprehensive seismic codes for earthquake-resistant construction, but the key lies in enforcing and implementing these codes rigorously.
• For tsunami preparedness, understanding coastal morphology and elevation from sea level is crucial.
• Settlements close to the coast and lack of public awareness about tsunamis pose significant challenges.
• Some modern structures near shorelines are at risk from tsunamis due to their proximity.

Geological Considerations

• A proper warning system is essential for disaster awareness and evacuation.
• Effective evacuation plans must be well-rehearsed.
• Post-disaster measures should include rescue, search, medical assistance, shelter, and basic necessities.

Earthquake Design Concepts

• Historical earthquakes have exposed the vulnerability of traditional buildings to seismic forces.
• Simple and cost-effective measures, along with good construction practices, can enhance a building's earthquake resistance.
• Implementing basic strategies can reduce damage and prevent complete building collapse during earthquakes.

Proactive Measures for Disaster Mitigation

• Civil engineering interventions like constructing wave breakers, sea walls, and shelters can enhance coastal resilience.
• Designing buildings on stilts and away from historical tsunami run-up zones can reduce vulnerability.
• Coastal areas with ports and harbors are particularly at risk, necessitating structures like groynes and dikes for protection.

The Fundamentals of Earthquake Resistance Design

• Understanding Earthquake Occurrences

  If we analyze earthquake data, we find that earthquakes occur with varying frequencies and magnitudes. Minor earthquakes are frequent, moderate ones occur occasionally, and strong earthquakes are rare.

• Cost-Effective Seismic Design

  When considering earthquake-resistant construction, it's crucial to weigh the cost against the probability of occurrence. Is it wise to invest heavily in seismic design for a site that may experience a moderate to strong earthquake every 500 to 1000 years?

• Philosophy of Earthquake Engineering

  Earthquake engineering aims for buildings that are not "Earthquake Proof" but rather "Earthquake Resistant." The goal is to ensure structures can withstand seismic activity to a reasonable extent.

• Basic Concepts of Earthquake Resistant Construction

  • Behavior under Different Earthquake Intensities

    Structural elements like columns and beams should remain undamaged during minor earthquakes but may sustain repairable damage. Under stronger earthquakes, some parts may require replacement.

  • Damage Mitigation and Building Safety

    Main structural components should withstand severe shaking without collapsing. Lifeline structures such as hospitals and dams require higher levels of earthquake protection due to their critical roles.

• Effects of Earthquakes on Buildings

  During earthquakes, buildings sway and experience inertia, potentially leading to collapse. Proper foundations and structural design are essential to prevent catastrophic damage.

• Importance of Earthquake-Resistant Design

  Post-earthquake rescue and rehabilitation efforts heavily rely on earthquake-resistant structures. Buildings must be able to withstand seismic forces to ensure safety and functionality.

Geological Considerations for Earthquake-Resistant Structures

• Building Design

  • Buildings should have a regular and symmetrical plan to enhance stability.
  • Complex shapes with irregularities in geometry, size, and height should be avoided.
  • Simple geometric plans are preferable as they perform better during earthquakes.
  • Structures with excessive height relative to their base, large one-dimensional lengths, or expansive plan areas are risky.

• Foundation and Soil

  • Foundations must be on stable soil with a bearing capacity exceeding 10 t/m².
  • Compaction of weak soils is necessary to enhance their load-bearing capacity.
  • Avoid building on cohesionless soils and ensure proper footing on slopes or rocky terrain.
  • The thickness of soil cover is critical; thin soil covers can be damaging for taller buildings.

• Seismic Vibration

  • Vibrations during earthquakes can be longitudinal, transverse, or vertical.
  • Structures should be designed to withstand resultant inertia forces and base movements.
  • Illustration: Consider a building swaying back and forth during an earthquake to understand transverse vibrations.

Geological Considerations for Earthquake-Resistant Structures

Introduction

• Building height influences soil cover; taller buildings require deeper foundations.
• Non-engineered structures often use materials like adobe, burnt clay bricks, and stones with mud, lime, or cement mortar.

Load-Bearing Elements

• Adobe and brick buildings can handle compressive loads but are weak in tension.
• Brick masonry buildings are heavy, leading to significant lateral forces and numerous cracks during earthquakes.

Earthquake Effects

• Horizontal and diagonal cracks indicate structural weaknesses.
• Cracks can occur due to bending or shear forces.

Structural Recommendations

• Buildings should incorporate RCC or wood/bamboo in key areas like foundations, plinths, sills, and roof bands.
• Inertia forces at the roof must be properly transferred to the walls through effective tying of all structural elements.
• Optimal sizing of doors and windows enhances the structural integrity of the building.

Materials and Construction

• Stone masonry, especially in rocky regions, is common but requires proper mortar application.
• Stone buildings are typically thick, with two exterior walls (Wythes) filled with rubble, leading to structural deficiencies.

Conclusion

• Proper geological considerations are crucial for constructing earthquake-resistant structures.
• Correct material usage and structural design can significantly reduce damage during seismic events.

Earthquake Resistance Structure Construction

Damage Minimization Techniques

• Construct walls as unitary structures
• Ensure wall thickness is less than 450 mm
• Dress rock pieces as cuboids with sharp corners
• Use a cement-sand mortar binder richer than 1:6
• Include a single through keystone or pair of overlapping bond stones at specified intervals

Common Damage Patterns

• Bulging and delamination of walls
• Separation of walls at T-junctions and corners
• Separation of roof from walls
• Disaggregation and collapse of walls

Key Stone and Bond Elements

• Key stones and bond elements play a crucial role in wall stability
• If bond stones are unavailable, alternative support like wood pieces or steel bars can be provided

Structural Design Considerations

• Ductility is essential for earthquake-resistant buildings
• Steel reinforcement at critical junctions enhances ductility
• Framed structures with columns and beams are commonly used to withstand seismic loads
• Concrete requires reinforcement with steel bars for added ductility

Role of Steel in Reinforced Cement Concrete (RCC)

• Steel bars provide ductility to RCC structures
• Proper design of steel placement ensures ductile failure

Modern Construction Trends

• Reinforced cement concrete buildings are prevalent in urban India
• Many structures include lower ground stories or basements for parking

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Earthquake Resistant Buildings

• Buildings on Stilts:
  • Large flexibility at ground story, reducing at higher stories.
  • Shear walls necessary at proper locations for seismic performance.
  • Use of interlocking and 'L' shaped dowel bars at wall joints.
  • RCC roof, gable, lintel, and plinth bands for strength.
  • Vertical reinforcement bars in wall corners tied to foundation for bending prevention.
• Quality Control:
  • Utilize high-grade burnt clay bricks with low porosity.
  • Ensure brick strength is greater than cement sand mortar.
• Architectural Design Considerations:
  • Restrict overhanging and projecting elements in building design.

Fire Hazards during Earthquakes

• Fire Outbreaks:
  • Can start from household gas leaks or short circuits.
  • Loss of water supply and blocked routes can exacerbate fire spread.

Summary

• Earthquake Impact:
  • Most unpredictable and destructive natural disaster.
  • Notable earthquake incidents in India and globally.
• Causes and Effects:
  • Generated by seismic energy release due to faulting.
  • Different factors influencing earthquake damage severity.

Geological Considerations for Earthquake Resistant Structures

• Geological Impact:
  • Significance of geological factors in construction.

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Earthquake Classification

• The Richter scale is a scale based on seismographic readings to measure the magnitude of earthquakes.
• Earthquakes can be classified based on their depth of focus as shallow, medium, and deep, depending on their origin in the lithosphere.
• Shallow focus earthquakes constitute eight percent of all earthquakes, with ninety percent of them originating in the top 30 km of the Earth's crust, causing significant damage.

Assessment of Earthquake Damage

• Earthquake damage can be assessed qualitatively through observations or quantitatively using instruments.
• Commonly used scales for assessing earthquake damage include the Modified Mercalli Intensity (MMI) Scale and the Medvedev-Sponheuer-Karnic (MSK) Scale.
• The MSK Scale considers different types of buildings such as type "A" for rural constructions, type "B" for ordinary masonry constructions, and type "C" for well-built structures.
• Damage severity is categorized into grades, ranging from Grade 1 for slight damage to Grade 5 for total destruction.

Magnitude and Effects of Earthquakes

• Earthquake magnitude is measured using scales that range from 1 M to 9.5 M, with the scale being open-ended.
• Notable earthquakes, like the 1960 earthquake in Chile, have reached a magnitude of 9.5M.
• Associated natural disasters with earthquakes include tsunamis, landslides, and liquefaction.

Effects of Earthquakes

• Tsunamis are caused by sudden changes in water levels due to vertical movement along earthquake-generating faults on the seafloor.
• Landslides can be triggered by earthquakes, destabilizing slopes during ground shaking.
• Liquefaction occurs in saturated sandy soil, leading to the loss of shear strength during seismic waves, causing structures to sink.

Geological Considerations for Earthquake-Resistant Structures

• Modern seismic zoning focuses on 'Active' and 'Neotectonics' aspects of tectonics.
• Three main techniques are utilized to study geological considerations for constructing earthquake-resistant structures.

Understanding Earthquake Resistant Structures

Overview:

• Earthquake-resistant structures are designed to withstand seismic events by implementing specific construction techniques and materials.

Key Concepts:

• Neotectonics: Focuses on studying tectonic events occurring within the past 500,000 years, aiding in seismic modeling and stress analysis.
• Global Positioning System (GPS): Utilized for precise distance measurements between geographic features using satellite technology.

Transition in Approach:

• Global Shift: Current emphasis on resilience involves constructing earthquake-resistant buildings and retrofitting existing structures.

Seismic Codes:

• IS 1893 (Parts 1 to 5), 2002: Indian standards for earthquake-resistant design covering various types of structures.
• IS 4326, 1993: Code of practice for earthquake-resistant design and construction of buildings in India.
• IS 13827, 1993: Guidelines for enhancing earthquake resistance of earthen buildings.
• IS 13828, 1993: Guidelines for improving earthquake resistance of low strength masonry buildings.

Significance:

• Seismic codes aid in planning, constructing, and detailing structures to mitigate earthquake effects, crucial for regions prone to seismic activity.

Application:

• Engineers and scientists utilize seismic codes to ensure the structural integrity of buildings, bridges, dams, and other infrastructure.

Indian Standards for Seismic Resistant Construction

• IS 13920, 1993: Indian Standard Code of practice for ductile detailing of Reinforced Concrete Structures Subjected to Seismic Forces.
• IS 13935, 1993: Indian Standard Guidelines for Repair and seismic strengthening of buildings.

Key Points

• India has comprehensive seismic codes for earthquake-resistant construction.
• Enforcement and implementation of these codes are crucial for safety in construction.
• Strict quality control of construction materials is essential.

Tsunami Preparedness

• Coastal morphology and height from sea level are vital considerations.
• Proper warning systems are necessary for public awareness and evacuation.
• Well-rehearsed evacuation plans are essential.
• Post-disaster assistance like medical aid, shelter, and water supply is critical.

Engineering Measures

• Engineering geological studies aid in proactive measures.
• Construction of wave breakers and sea walls.
• Designing buildings on stilts away from historical tsunami run-up zones.

Vulnerability of Coastal Areas

• Ports and harbors are particularly vulnerable.
• Construction of groynes and dikes is crucial for wave control.

Earthquake Resistance

• Common buildings lack earthquake resistance.
• Simple and cost-effective measures can enhance earthquake resistance.
• Peak Ground Acceleration (PGA) measures ground shaking severity.
• Building failure is often due to shaking and shifting responses to ground vibrations.

Earthquake Resistant Construction Concepts

• Basic Principles of Earthquake Resistant Construction:
  • Structural Integrity: Columns and beams should withstand vertical and horizontal loads during minor earthquakes without damage.
  • Damage Tolerance: Parts not bearing loads may sustain repairable damage, ensuring overall structural safety.
  • Moderate Earthquakes: During occasional shaking, main structural elements may require repair, while non-essential parts may need replacement.
  • Strong Earthquakes: In rare severe shaking, main components might suffer damage, but the building should not collapse.
• Special Structures:
  • Lifeline Structures: Critical facilities like administration buildings, communication centers, etc., must have a higher level of earthquake protection for post-earthquake operations.
  • Dams: Due to the risk of flooding from dam failures, these structures need enhanced earthquake resistance measures.
• Design Considerations:
  • Symmetry and Regularity: Building plans should be symmetrical to avoid torsion effects, with regular shapes being favored over complex geometries.
  • Dimensional Considerations: Tall buildings with high base-to-height ratios and large plan areas should be avoided due to increased seismic vulnerability.
  • Sectional Separation: If extensions are necessary, separate building sections should be employed to enhance overall structural stability.
  • Geometric Simplicity: Simple geometric plans are more resilient to strong earthquakes compared to complex designs with multiple corners and angles.

Key Considerations for Constructing Earthquake-Resistant Structures

• Foundation Stability:
  • Ensure the ground is stable with a foundation on firm soil having a bearing capacity greater than 10 t/m².
  • Compacting weak soils can enhance their bearing capacity.
  • Avoid using cohesion-less soils for the foundation.
  • For slopes and rocky grounds, maintain the foundation footing at the same level.
  • The thickness of the soil is crucial, with thinner soil covers (< 50 m) being less suitable for up to five-story buildings, while thicker soil covers (> 100 m) are more suitable for taller buildings (> 10 stories).
  • Extra caution is necessary if the water table is high.
• Building Materials:
  • Most non-engineered buildings in rural and urban areas utilize materials like adobe, burnt clay bricks, and stones with mud, lime, and cement mortar as load-bearing elements.
  • These materials can withstand compressive loads but are weak in tension.
  • Brick masonry buildings, due to their mass, are susceptible to large lateral forces and multiple cracks during earthquakes.
• Structural Integrity:
  • Buildings should have foundations, sill bands, and roof bands of reinforced concrete (RCC) or wood/bamboo for added stability.
  • Inertia forces at the roof should be effectively transferred to the walls by properly connecting the roof, walls, and foundation.
  • Optimal sizing and placement of openings like doors and windows can enhance the resistance offered by walls.
• Stone Masonry Construction:
  • In hilly regions, stone masonry with mud is commonly used, especially in rocky areas.
  • Rock boulders or blocks are typically set randomly in mud or lime mortar, unlike bricks laid in a specific course with sand-cement mortar.
  • These buildings, typically thick (>100 cm), consist of two exteriors (Wythes) with an internal gap filled with loose rubble, making them prone to significant damage.
  • Common damages include bulging, delamination of walls, separation at junctions and corners, and roof detachment.

Construction of Earthquake Resistant Structures

• Structural Design

  • Use a mortar binder richer than 1:6.
  • Introduce steel at critical junctions for ductility.
  • Utilize framed structures involving columns and beams to withstand gravity and seismic loads.

• Reinforced Cement Concrete Buildings

  • Common in urban India with lower ground stories or basements as parking spaces.
  • Ground stories have flexibility that decreases in higher stories.
  • Shear walls and 'L' shaped dowel bars at proper locations enhance seismic resistance.
  • Employ RCC bands and reinforcement bars strategically in walls for bending prevention.

• Quality Control

  • Use high-grade burnt clay bricks with low porosity and high strength.
  • Avoid overhanging and projecting elements in architectural design.

Earthquakes and Fire Hazards

Overview

• Occurrences of fire during earthquakes can exacerbate the challenges faced by people.
• Fires may originate from various sources within households, such as gas leaks or short circuits, and can quickly spread over large areas.
• Due to disruptions in water supply and blocked routes, efforts to extinguish fires using fire trucks can be hindered.
• Regions with underground gas and petroleum reserves are particularly susceptible to such fires, as evidenced by historical earthquakes like the San Francisco Earthquake of 1906 and the Kanto Earthquake of 1923 in Japan.

Frequently Asked Questions

Q1. Define and Classify Earthquakes

• Earthquakes are among the most devastating and unpredictable natural disasters.
• The primary cause of earthquakes is the accumulation of energy within the Earth's brittle crust, primarily due to tectonic plate movements.
• When rocks cannot withstand the accumulated stress, they fracture along faults, releasing stored energy in the form of seismic waves.
• These seismic waves include body waves ('P' and 'S' waves) and surface waves ('L' waves) that trigger the mainshock and aftershocks.
• Earthquakes can last from several seconds to minutes, resulting in additional smaller earthquakes due to fault adjustments.

Geological Considerations

• Earthquakes can also stem from volcanic activities, magma movements, and underground disturbances, as well as sudden caldera collapses.
• Localized earthquakes may be induced by massive landslides.
• Human activities like nuclear explosions, mining blasts, and the construction of large dams and reservoirs can also trigger seismic events.
• Reservoir-induced seismicity has been observed in certain dam projects where rising water levels contribute to increased seismic activity.

Earthquake Formation and Classification

• Origin of Earthquakes:
  • Focus and Epicentre:

    The place where an earthquake originates due to rupture is known as the focus or hypocentre. Directly above the focus on the ground is the epicentre. The antipodal point, opposite to the epicentre on the other side of the globe, is termed as the antecentre.

  • Classification based on Depth:
    • Shallow Focus: Earthquakes that occur within the top 70 km of the lithosphere, constituting about 80% of all earthquakes. Among shallow focus earthquakes, 90% originate in the top 30 km of the crust.
    • Medium Focus: Earthquakes originating between 70 to 300 km depth, accounting for about 15% of all earthquakes.
    • Deep Focus: Earthquakes originating from 300 to 700 km depth, making up approximately 5% of earthquakes. The deepest recorded earthquake had a focal depth of 720 km.
  • Classification based on Plate Boundaries:

    Earthquakes can also be classified based on their origin with respect to lithospheric plate boundaries. Earthquakes occurring within the interior of lithospheric plates are known as intra-plate earthquakes. Those happening in stable and older continental regions are termed as stable continental region (SCR) earthquakes.

• Measuring Earthquake Power:

  Methods to measure the power of an earthquake can be categorized into:

  • Qualitative Assessment (Intensity Scale):

    This method involves observing the damage caused by the earthquake. Common intensity scales include the Modified Mercalli Intensity (MMI) Scale and the Medvedev-Sponheuer-Karnic (MSK) Scale. These scales provide a qualitative measure of the destruction on a scale from I to XII.

  • Quantitative Assessment (Magnitude Scale):

    Quantitative assessment involves measuring the vibrations caused by the earthquake using instruments. This method gives a numerical value to the earthquake's power. The destruction caused reduces as we move outward from the epicentre.

Earthquake Scale and Measurement Overview

• Medvedev-Sponheuer-Karnik (MSK) Scale

  • The MSK Scale is valuable for Civil Engineering as it assesses buildings of various types.
  • It categorizes buildings into types A, B, and C based on their construction quality.
  • Damage is classified into Grade 1 to Grade 5, indicating the severity of the impact.
  • Damage percentages are classified as Few (5%), Many (50%), and Most (75%).

• Seismographs and Richter Scale

  • Seismographs are instruments used to measure earthquakes, enabling quantification.
  • Charles Richter developed a scale based on the logarithm of seismic wave amplitudes.
  • Richter scale measures earthquake magnitudes using Arabic numerals, with 9.5 being the highest recorded magnitude.
  • There is a correlation between Magnitude and Intensity scales.

• Magnitude and Intensity Equivalents

  • Explanation of the relationship between Magnitude and Intensity scales:
  • 
    
    • Magnitude 1 to 3 corresponds to Intensity I to IV.
    • Magnitude 4 corresponds to Intensity V to VI.
    • Magnitude 5 corresponds to Intensity VI to VII.
    • Magnitude 6 corresponds to Intensity VIII to IX.
    • Magnitude 7 corresponds to Intensity IX to X.
    • Magnitude 8 corresponds to Intensity X to XI.
    • Magnitude 9 corresponds to Intensity XI to XII.

• Impact Assessment

  • Damage at the Epicentre:
  • 
    
    • Categories ranging from Social disturbance to Total destruction, including effects on structures.
  • Influence Area:
  • 
    
    • Varies from Limited to 1000 km, indicating the geographical reach of the earthquake impact.

• Conclusion

  • Understanding earthquake scales and measurement is crucial for assessing seismic events and their impact on structures and society.

Earthquake Classification

• Earthquakes are categorized based on their magnitude and energy release:
• 
  
  • Great Earthquake: Magnitude > 8 on the Richter Scale, energy released > 5.8 x 10^23 Ergs
  • Major Earthquake: Magnitude 7-7.9, energy released 2-42 x 10^22 Ergs
  • Strong Earthquake: Magnitude 6-6.9, energy released 8-150 x 10^20 Ergs
  • Moderate Earthquake: Magnitude 5-5.9, energy released 19-3-55 x 10^19 Ergs
  • Light Earthquake: Magnitude 4-4.9, energy released 1-20 x 10^18 Ergs
  • Minor Earthquake: Magnitude 3-3.9, energy released 4-72 x 10^16 Ergs

Hazards Associated with Earthquakes

• Earthquakes can lead to various other disasters, including:
• 
  
  • Tsunami: Tsunamis are large ocean waves triggered by underwater earthquakes. They can cause inundation and flooding on coastlines.
  • Landslides: Earthquakes can destabilize slopes, leading to landslides that can damage structures and block roads.
  • Liquefaction: Soil liquefaction can occur during earthquakes, causing the ground to behave like a liquid, leading to building damage.
  • Fire: Earthquakes can rupture gas lines or ignite fires, posing a significant risk to affected areas.

Additional Effects of Earthquakes

• Earthquakes can also result in the following consequences:
• 
  
  • Natural Damming: Earthquakes may alter the landscape, creating natural dams that impact water flow in rivers and lakes.
  • Structural Failures: Infrastructure such as bridges, flyovers, and tunnels can collapse during earthquakes, leading to casualties.
  • Loss of Life: The combined effects of earthquakes and associated hazards can result in significant loss of life.

Summary: Geological Considerations for Construction of Earthquake-Resistant Structures

• The Pacific Ocean and Tsunami Waves:
  • The Pacific Ocean is notorious for generating tsunami waves due to active subduction zones along its margins and the presence of the Mid-Pacific Rise.
• Ground Failure:
  • Earthquakes can induce failures in hilly terrains and settlement of ground in alluvial plains, leading to landslides and destruction of infrastructure.
• Liquefaction:
  • Liquefaction involves the loss of soil strength during seismic activity, causing buildings to topple or sink into the ground, impacting structures like embankments and pipelines.
• Geological Methods for Studying Earthquakes:
  • Seismic zoning focuses on 'Active' and 'Neotectonics' aspects, utilizing techniques like direct measurements, geodetic networks, satellite triangulation, and GPS for studying tectonic motions.
  • Neotectonics examines phenomena over the past 500,000 years, aiding in seismic source modeling, wave propagation studies, and understanding induced seismicity.

Examples:

• For instance, the Pacific Ocean's unique geological features make it prone to generating powerful tsunami waves, impacting coastal regions.
• During an earthquake-induced liquefaction event, buildings in areas with clay-rich soil may collapse or sink due to the loss of soil strength.
• By studying ongoing tectonic motions using advanced techniques like GPS and satellite triangulation, scientists can better understand earthquake dynamics and mitigate risks in construction.

Test Question:

Enumerate and explain different geological methods used to study earthquakes and reduce their impact.

Geological Considerations for Earthquake-Resistant Construction

• Importance of Seismic Zoning

  • Seismic zoning is crucial for understanding the vulnerability of regions to earthquakes based on geology and geotectonic factors.
  • It helps in determining the likelihood of earthquakes in terms of their recurrence and magnitude.
  • Seismic zoning undergoes revisions with advancements in research and studies.

• Evolution of Seismic Zoning in India

  • India initially had five seismic zones, with Zone I being the least vulnerable and Zone V the most vulnerable.
  • The seismic zoning map was first introduced in 1962 and has been revised several times since then.
  • After the Bhuj earthquake in 2001, Zone I was merged with Zone II, resulting in the current four seismic zones in India (Zones II to IV).

• Civil Engineering Solutions for Earthquake Resistance

  • Civil engineers focus on constructing buildings that can withstand seismic activity rather than predicting earthquakes.
  • Codes and standards have been developed to guide the construction and retrofitting of structures to make them earthquake-resistant.
  • Important seismic codes in India include IS 1893 (1962), IS 4326 (1993), IS 13827 (1993), IS 13828 (1993), IS 13920 (1993), and IS 13935 (1993).
  • These codes cover various aspects such as general buildings, liquid retaining structures, bridges, industrial buildings, dams, earthen buildings, masonry buildings, and reinforced concrete structures.
  • The codes provide guidelines for design, construction, ductile detailing, repair, and seismic strengthening of buildings.

Key Concepts of Earthquake Resistant Construction

• Geology

  • Columns and beams should withstand vertical and horizontal loads during minor earthquakes.
  • Structural parts may undergo repairable damage under moderate shaking.
  • In case of strong shaking, the main members may suffer damage, but the building should not collapse.

• Lifeline Structures

  • Hospitals, administration buildings, and other critical structures require higher earthquake protection.
  • Dams need a higher level of protection to prevent secondary disasters like flooding.

• Building Design

  • Buildings should have a regular and symmetrical plan to avoid torsion.
  • Avoid buildings with excessive height, length, or width.
  • Complex geometries should be minimized; simple geometric plans perform better in earthquakes.
  • Ground stability is crucial; foundations should be on firm soil with proper bearing capacity.