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Earthquake
• Catastrophic release of strain energy stored in the rocks around a fault.
• Where does the energy come from?
– Moving plates which are driven by gravity and heat from Earth’s interior.
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About Earthquakes
• Earthquakes generate waves that travel through the earth
• Earthquakes occur when rocks slip along faults
• Faults are classified by the kinds of movement that occur along them
• Earthquakes don’t kill people, buildings kill people
• Magnitude and Intensity
• Seismic waves are used to map the earth’s interior
• Predicting earthquakes is not yet possible
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Major Terms
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Confusing Terms!
• Isoseismic Lines: Join points of same intensity or equal damage due to earthquakes.
• Homoseismal or Coseismal: Join places where shocks from earthquake arrive at same time
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Why Earthquakes Occur? Stress
• Stress is a force per unit area : =
�� ����,
where, � = � × ��� � �
• Tectonic plates move past each other causing stress. Stress causes the rock to deform
– Plastic Deformation: Does not cause earthquakes – Elastic Deformation: Rock stretches then reaches a
breaking point, releasing energy – Flow: It is viscous behavior
– Fracture: Brittle behavior seen in solids
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Elastic Rebound
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Why Earthquakes Occur? Strain
• Deformation in a solid induced by applied stress & is dimensionless
• Example: If 5 cm long rubber band is stretched, it becomes 6 cm long the strain is:
1 cm / 5 cm = 0.20 or 20%
• Strain is due to stress but it can also lead to new stress which again causes strain.
– stress -> strain -> stress -> strain … – chicken -> egg -> chicken -> egg …
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Elastic Energy
• On straining an elastic material, it stores the energy that deforms it.
• When given an opportunity, elastic material can release the stored energy
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Causes of Earthquake
• Plate Tectonics
– Divergent Plate Boundaries: Mid Oceanic Ridges – Convergent Plate Boundaries: San Andreas Fault – Slip (Transform) Boundaries: Mid Continental
Belt
• Volcanic Activities: Krakatoa Volcano
• Anthropogenic Factors: Greece, Bhatsa Dam (near Shahapura, Maharashtra, India)
• Crustal Contraction
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Plate Tectonics
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Strike Slip Faults
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Left Lateral Fault Right Lateral Fault
Dip Slip Faults
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Normal Fault Reverse Fault
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Types of Seismic Waves
Body Waves (Push-Pull)
P Wave
Longitudinal
Shadow Zone (105-143O)
Highest Velocity
S Wave (Transverse)
SH: Horizontal – side by side
SV: Vertical – up & down
Surface Waves (Slowest)
Rayleigh Wave L Wave
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Body & Surface Waves
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Compressional Waves Surface Waves
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P and S Waves
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Spread of P & S Waves
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Movement of Seismic Waves
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Distribution of Earthquake
• Circum-Pacific Belt – 68% earthquakes
• Mid Continental Belt – 21% earthquakes – East African Belt
– East Indian Belt
• Mid Atlantic Ridge
• Gulf of Aden
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Earthquake Distribution
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Pacific Ring of Fire
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Measure Strength of Earthquake
• Modified Mercalli Scale: Ranges from I to XII
• Richter Scale: Logarithmic Scale (Increases 10 times in amplitude & 32 times in wave energy)
• Moment Magnitude Scale: Based on seismic moment. �
�= log � – .7 where � is magnitude of seismic moment in dyne centimeters
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Richter & Mercalli Scale
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Richter Magnitude
Mercalli Intensity
Description
2 I Usually not felt & can be detected only by instruments.
II Felt by very people residing on upper floors, suspended objects swing
3 III Felt by many, mainly those indoors, standing automobiles can tumble.
IV Felt by people indoors, vibration of doors take place.
4 V Felt by nearly everyone, walls crack & trees tumble.
5 VI Felt by all & even those outdoors. Slight damage occurs.
VII Everyone runs outdoors, poorly built infrastructure damages.
6 VIII Everyone runs outdoors, moderate to major damage with collapsing chimneys & walls.
7 IX All buildings suffer major damage. Cracking of ground, breaking of pipes & shifting foundations occur.
X Major damages & landslides occur & destruction of structures occur
8 XI Infrastructure falls, bridges wreck, rails bent & wide cracks in ground
XII Total destruction, visible ground surface waves & objects thrown into air
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Working of Seismograph
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Seismograph Recording
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Magnitude & Intensity
Intensity
• How Strong Earthquake Feels to Observer Magnitude
• Related to Energy Release
• Determined from Seismic Records
• Rough correlation between the two for shallow earthquakes
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Intensity of Earthquake
• Distance from epicenter
• Type of ground material (e.g. soil, rock)
• Amount of energy at epicenter
• Depth of earthquake focus
• Distance from epicenter
• Type of rocks & degree of consolidation
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Major Earthquakes
Location Date (Month-Year) Magnitude
Valdivia, Chile (Kingdom of Chile) 12 - 1575 8.5
Lima, Peru (Viceroyalty of Peru) 10 - 1687 8.5
Pacific Ocean, Shikoku region, Japan 10 - 1707 8.6
Valparaiso, Chile 07 - 1730 8.7-9.0
Concepcion, Chile (Kingdom of Chile) 05 - 1751 8.5
Atlantic Ocean, Lisbon, Portugal 11 - 1755 8.7
Lisbon, Portugal 11 - 1755 8.5-9.0
Sumatra, Indonesia 11 - 1833 8.8-9.2
Arica, Chile (then Peru) 08 - 1868 8.5-9.0
Pacific Ocean, Tohoku region, Japan 07 - 1869 8.6
Atacama Region, Chile 11 - 1922 8.5
Kamchatka, Russia (USSR) 02 - 1923 8.5
Banda Sea, Indonesia (Dutch East Indies) 02 - 1938 8.5
Assam, India-Tibet, China 08 - 1950 8.6
Kamchatka, Russia 11 - 1952 9
Andreanof Islands, Alaska, USA 03 - 1957 8.6
Valdivia, Chile 05 - 1960 9.5
Kuril Islands, Russia (USSR) 10 - 1963 8.5
Prince William Sound, Alaska, USA 03 - 1964 9.2
Rat Islands, Alaska, USA 02 - 1965 8.7
Indian Ocean, Sumatra, Indonesia 12 – 2004 9.1-9.3
Maule, Chile 02 – 2010 8.8
Pacific Ocean, Tohoku region, Japan 03 – 2011 9
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Geographical Impacts of Earthquake
• Landsides e.g., Lathur in India
• Damming of rivers & floods
• Raising or lowering sea floor
• Raising or lowering coastal regions, e.g., in 1899 Alaska coast was lowered by 16 m
• Fault formation
• Change in surface drainage & underground circulation of water
• Tsunamis & consequent damage
• Vertical & lateral displacement of crust
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Economic Impacts of Earthquake
• Damage to oil pipelines, electric wires &
consequent fires
• Devastation of cities, fires & diseases
• Fall of buildings
• Loss of life & property
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Sample Devastation
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Earthquake Management
• Early warning seismic stations
• Long term predictions
• Earthquake resistant buildings
• Abnormal changes in animal behavior
• Expertise training
• 3 R’s (Rescue, Relief, Rehabilitation)
• Coordination among authorities
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