Earthquakes YouTube Lecture Handouts

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Earthquakes - Causes, Distribution, Impact & 4 Types of Waves

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.

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

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

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

Elastic Rebound

Illustration: Elastic Rebound
Illustration: Elastic Rebound
  • 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 …

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

Causes of Earthquake

  • Plate Tectonics
    Illustration: Causes of Earthquake
    • 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

Plate Tectonics

Illustration: Plate Tectonics

Strike Slip Faults

Illustration: Strike Slip Faults

Dip Slip Faults

Illustration: Dip Slip Faults

Types of Seismic Waves

Illustration: Types of Seismic Waves

Body & Surface Waves

Illustration: Body & Surface Waves

P and S Waves

Illustration: P and S Waves

Spread of P & S Waves

Illustration: Spread of P & S Waves

Movement of Seismic Waves

Illustration: Movement of Seismic Waves

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

Earthquake Distribution

Illustration: Earthquake Distribution

Pacific Ring of Fire

Illustration: Pacific Ring of Fire

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. where is magnitude of seismic moment in dyne centimeters

Richter & Mercalli Scale

Illustration: Richter & Mercalli Scale

Working of Seismograph

Illustration: Working of Seismograph
Illustration: Working of Seismograph

Seismograph Recording

Illustration: Seismograph Recording

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

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

Major Earthquakes

Illustration: Major Earthquakes

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

Economic Impacts of Earthquake

  • Damage to oil pipelines, electric wires & consequent fires
  • Devastation of cities, fires & diseases
  • Fall of buildings
  • Loss of life & property

Sample Devastation

Illustration: Sample Devastation

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

Manishika