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Watch Video Lecture on YouTube: Earthquakes - Causes, Distribution, Impact & 4 Types of Waves

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.

• 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

• 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

• 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

Distribution of Earthquake

• Circum-Pacific Belt – 68% earthquakes

• Mid Continental Belt – 21% earthquakes

• East African Belt

• East Indian Belt

• Mid Atlantic Ridge

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

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

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

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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