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

Major Terms Image
Elastic Rebound Image
  • 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
    Causes of Earthquake Image
    • 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

Plate Tectonics Image

Strike Slip Faults

Strike Slip Faults Image

Dip Slip Faults

Dip Slip Faults Image

Types of Seismic Waves

Types of Seismic Waves Image

Body & Surface Waves

Body & Surface Waves Image

P and S Waves

P and S Waves Image

Spread of P & S Waves

Spread of P & S Waves Image

Movement of Seismic Waves

Movement of Seismic Waves Image

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

Earthquake Distribution Image

Pacific Ring of Fire

Pacific Ring of Fire Image

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

Richter & Mercalli Scale Image

Working of Seismograph

Working of Seismograph Image-1
Working of Seismograph Image-2

Seismograph Recording

Seismograph Recording Image

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

Major Earthquakes Image

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

Sample Devastation Image

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