Earthquake Notes

Earthquake

Definition

  • An earthquake is a motion of the ground surface, ranging from a faint tremor to a wild motion.
  • It can shake buildings apart and cause fissures to open in the ground.
  • It's a major demonstration of tectonic forces caused by the Earth's internal thermal conditions.
  • An earthquake is a form of energy transmitted as wave motion through the Earth's surface layer.
  • These waves spread in widening circles from the focus, which is the point of sudden energy release.

Focus (Hypocenter)

  • The place of origin of an earthquake is called the focus or hypocenter.
  • It is always hidden inside the earth.
  • Its depth varies from place to place:
    • Deepest: 700700 km
    • Shallowest: 20−3020-30 km

Epicenter

  • The place on the ground surface perpendicular to the buried focus.
  • It is the location that records seismic waves for the first time.

Seismic Waves

  • Waves of energy generated by an earthquake or other earth vibration.
  • They travel within the earth or along its surface.

Seismograph (Seismometer)

  • An instrument used to detect and record earthquakes.

Seismology

  • The study of earthquakes and seismic waves.

Causes of Earthquakes

  • Earthquakes are caused by disequilibrium in the Earth's crust.
  • Several causes can lead to this disequilibrium:
    • Volcanic eruption
    • Gaseous expansion
    • Contraction inside the earth
    • Plate Movement
    • Faulting and folding
    • Up-warping and down-warping
    • Hydrostatic pressure of man-made water bodies (reservoirs & lakes)
Volcanic Eruption
  • Volcanic activity is considered a major cause of earthquakes.
  • Volcanic eruption and earthquake are cause and effect for each other.
  • Each volcanic eruption is often followed by an earthquake, and severe earthquakes can trigger volcanic eruptions.
  • Earth tremors can be precursors of volcanic eruptions.
  • The escape of magma and ash from a volcano can result in an earthquake.
  • Molten rock movement can cause nearby plates to move, resulting in an earthquake.
  • Explosive gases push the crustal surface from below, causing earth tremors.
  • If gases break the crustal surface, a violent explosion can cause a devastating earthquake.
  • The magnitude of the earthquake depends on the intensity of the volcanic eruption.
Plate Movements
  • The lithosphere is the solid, outer part of the Earth, including the brittle upper mantle and the crust.
  • Rigid lithospheric slabs are called plates.
  • Plate tectonics refers to the evolution, nature, and motion of these plates.
  • Lithospheric plates glide slowly over the asthenosphere.
  • Tectonic earthquakes occur at plate tectonic boundaries.
  • Friction between plates can cause them to lock together, leading to increased pressure.
  • When the locked section succumbs to the pressure, plates move rapidly, causing a tectonic earthquake.
  • Energy waves move through the Earth's crust.
  • Plate tectonic theory is based on:
    • Continental drift
    • Seafloor spreading
  • Six major and twenty minor plates have been identified.
Major Plates

  • Eurasian Plate

  • Indian-Australian Plate

  • American Plate

  • Pacific Plate

  • African Plate

  • Antarctic Plate

  • Plate boundaries or margins are most important for tectonic activity.

Plate Margins
  • Constructive Plate Margin (Divergent/Accreting)
    • Plates pull away from each other.
    • Magma pushes up from the mantle to create new crust.
    • Continuous upwelling of molten material.
    • Example: Himalayan Mountain Range, Mid-Atlantic Ridge
  • Destructive Plate Margin (Convergent/Consuming)
    • Two plates move towards each other.
    • One plate overrides the other.
    • The overridden plate is subducted into the mantle.
    • Example: The Marianas Trench
  • Conservative Plate Boundary (Shear/Transform)
    • The relative motion of the plates is horizontal.
    • Plates slide past one another along transform faults.
    • Crust is neither created nor destroyed.
    • Example: San Andreas Fault, California
Faulting and Elastic Rebound Theory
  • Horizontal and vertical movements from endogenic forces result in folds and faults.
  • This causes isostatic disequilibrium in the crustal rocks, leading to earthquakes.
  • Sudden dislocation of rock blocks triggers immediate earth tremors.
  • Elastic rebound theory:
    • Proposed after the 1906 San Francisco earthquake by geologist Henry Fielding Reid.
    • Explains deformation caused by earthquakes.
    • Stress buildup in rocks on either side of a fault results in gradual deformation.
    • When stress exceeds friction, a sudden slip occurs along the fault.
    • This releases accumulated stress, and rocks return to their original shape but are offset.
  • Subsiding/sinking/down warping and uplifting/up warping are vertical earth movements causing raised cliffs, tilt blocks, rift valleys, plateaus, basins, etc. Earthquakes might happen during these processes.
Hydrostatic Pressure and Anthropogenic Pressure
  • Earthquakes are natural phenomena caused by endogenic forces.
  • However, certain human activities can also cause earthquakes:
    • Pumping of ground water and oil
    • Deep underground mining
    • Blasting of rocks by dynamites for constructional purposes
    • Nuclear explosion
    • Storage of huge water volumes in big reservoirs
  • The construction of large dams and impounding of water in reservoirs can cause disequilibrium or augment fragile structures.

Classification of Earthquakes

Based on Causative Factors
  • Natural Earthquake: Caused by natural processes (endogenic forces)
    • Volcanic: Due to volcanic eruption
    • Tectonic: Due to dislocation of rock blocks during faulting activity.
    • Isostatic: Due to sudden disturbance in isostatic balance
    • Plutonic: Deep-focused earthquake (hypocenter: >300km)
  • Artificial: Due to human activities
Based on Focus
  • Shallow Earthquake: 0−500-50 km
  • Intermediate Earthquake: 50−25050-250 km
  • Deep-focus Earthquake: 250−700250-700 km
Based on Casualties
  • Moderate Hazardous: <50,00050,000 human casualties
  • Highly Hazardous: 51,000−1,00,00051,000-1,00,000 human casualties
  • Most Hazardous: > 1,00,0001,00,000 human casualties

Earthquake Measurement

  • Magnitude scales (e.g., moment magnitude) measure the size of the earthquake at its source.
  • An earthquake has one magnitude.
  • The magnitude does not depend on where the measurement is made.
  • Intensity scales (e.g., Modified Mercalli Scale and the Rossi-Forel scale) measure the amount of shaking at a particular location.
  • An earthquake causes many different intensities of shaking in the area of the epicenter.
  • Intensity varies depending on location.
Richter Scale
  • 0-1: Feels like 0.6−200.6-20 kilograms of dynamite; We cannot feel these.
  • 2: Feels like 600 kilograms of dynamite; Smallest quake people can normally feel.
  • 3: Feels like 20,000 kilograms of dynamite; People near the epicenter feel this quake.
  • 4: Feels like 60,000 kilograms of dynamite; This will cause damage around the epicenter.
  • 5: Feels like 20,000,000 kilograms of dynamite; It is the same as a small fission bomb.
  • 6: Feels like 60,000,000 kilograms of dynamite; Damage done to weak buildings in the area of the epicenter.
  • 7: Feels like 20 billion kilograms of dynamite; Can cause great damage around the epicenter.
  • 8: Feels like 60 billion kilograms of dynamite; Creates enough energy to heat New York City for one year. Can be detected all over the world. Causes serious damage.
  • 9: Feels like 20 trillion kilograms of dynamite; Causes death and major destruction. Destroyed San Francisco in 1906.
  • 10: Feels like 20 trillion kilograms of dynamite; Rare, but would cause unbelievable damage!
Modified Mercalli Scale
  • I: Detected only by sensitive instruments; Richter Magnitude 1.5
  • II: Felt by few persons at rest, especially on upper floors; delicately suspended objects may swing; Richter Magnitude 2
  • III: Felt noticeably indoors, but not always recognized as earthquake; standing autos rock slightly, vibration like passing truck; Richter Magnitude 2.5
  • IV: Felt indoors by many, outdoors by few, at night some may awaken; dishes, windows, doors disturbed; autos rock noticeably; Richter Magnitude 3
  • V: Felt by most people; some breakage of dishes, windows, and plaster; disturbance of tall objects; Richter Magnitude 3.5
  • VI: Felt by all, many frightened and run outdoors; falling plaster and chimneys, damage small; Richter Magnitude 4.5
  • VII: Everybody runs outdoors; damage to buildings varies depending on quality of construction; noticed by drivers of autos; Richter Magnitude 5
  • VIII: Panel walls thrown out of frames; fall of walls, monuments, chimneys; sand and mud ejected; drivers of autos disturbed; Richter Magnitude 5.5
  • IX: Buildings shifted off foundations, cracked, thrown out of plumb; ground cracked; underground pipes broken; Richter Magnitude 6
  • X: Most masonry and frame structures destroyed; ground cracked, rails bent, landslides; Richter Magnitude 6.5
  • XI: Few structures remain standing; bridges destroyed, fissures in ground, pipes broken, landslides, rails bent; Richter Magnitude 7
  • XII: Damage total; waves seen on ground surface, lines of sight and level distorted, objects thrown up in air; Richter Magnitude 7.5