What is an Earthquake?

• An earthquake is a shaking of the Earth’s surface caused by the sudden release of energy in the Earth’s lithosphere.
• Earthquakes create seismic waves, which are forms of energy transmitted through the surface layer of the Earth.
• Causes: It can be caused by faulting, folding, plate movement, volcanic eruptions, and human activities like dam and reservoir construction.
• Earthquakes are highly destructive and unpredictable, and major earthquakes can cause significant damage, especially in densely populated areas.
• Minor earth tremors caused by gentle vibrations within the Earth’s crust happen frequently, while major earthquakes are usually caused by movement along faults.

Plate Boundaries

• There are 3 types of plate boundaries along which earthquake occurs: convergent, divergent and transform.
• 
• When these plates shift their position, they release intense amount of energy that are nothing but earthquakes.

Terminology used in the study of earthquakes

• Focus and Epicenter:
  • The point where faulting starts during an earthquake is called the focus or hypocenter.
  • The epicenter is the point on the Earth’s surface directly above the focus.
  • The intensity of an earthquake is greatest at the epicenter and decreases with increasing distance from it.
  • 
• Richter Scale:
  • The Richter magnitude scale is used to measure the energy released by an earthquake.
  • This scale was developed by Charles F. Richter in 1935.
  • The magnitude is indicated by a number on the scale, ranging from 0 to 9.
  • An earthquake with a magnitude of 5.0 on the Richter scale has 10 times the shaking amplitude of an earthquake with a magnitude of 4.0, and releases 31.6 times more energy.
• Mercalli Scale:
  • The Mercalli intensity scale is a seismic scale used to measure the intensity of an earthquake.
  • The Mercalli scale measures the effects of an earthquake, rather than its energy release.
  • The intensity of an earthquake is indicated by a number on the scale, ranging from 1 to 12.
• Seismic Waves:
  • Seismic waves are waves of energy produced by the sudden breaking of rock within the Earth.
  • These are the energy that travels through the Earth and is recorded by seismographs.
  • There are two main types of seismic waves: body waves and surface waves.
  • Body waves:
    • Primary waves (p-waves):
      • The first type of body wave is called the P wave or primary wave.
      • It is the fastest type of seismic wave.
      • The P wave can travel through gaseous, solid rock, and fluid substances, like water or the Earth’s liquid layers.
      • It moves through rock by alternately pushing and pulling it, similar to how sound waves push and pull air.
    • Secondary waves (S-waves):
      • The second type of body wave is the S wave or secondary wave.
      • S waves are slower than P waves and can only travel through solid rock.
      • It moves rock in an up-and-down or side-to-side motion.
      • It arrives at the surface with a time delay compared to P waves.
  • Surface waves:
    • Love waves (L-waves):
      • The first kind of surface wave is called a Love wave, named after A.E.H. Love, a British mathematician.
      • It’s the fastest surface wave and moves the ground from side-to-side.
    • Rayleigh waves
      • The other kind of surface wave is the Rayleigh wave, named after Lord Rayleigh.
      • A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean.
      • Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving.
      • Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves.
  • 

Classification of Earthquakes

• On the basis of causative force:
  • Natural: volcanic, tectonic, isostatic, plutonic.
  • Artificial: hydrostatic pressure from dams, mining blasts, unchecked construction, nuclear explosion.
• On the basis of depth of focus:
  • Moderate(0-50km)
  • Intermediate(50-250km)
  • Deep focus( 250-700km)
• On the basis of human casualties:
  • Moderate (deaths<50,oo)
  • Highly hazardous(51,000-1,00,00)
  • Most hazardous(>1,00,00)

Geographical Distribition of Earthquakes

• World:
  • The distribution of earthquakes in the world closely matches the distribution of volcanoes.
  • The region with the greatest seismicity is the Circum-Pacific area, with the most frequent earthquakes and epicenters along the “Pacific Ring of Fire.”
  • Approximately 70% of earthquakes occur in the Circum-Pacific belt.
  • Another 20% of earthquakes occur in the Mediterranean-Himalayan belt, including Asia Minor, the Himalayas, and parts of northwest China.
  • The remaining earthquakes occur in the interior of tectonic plates and at spreading ridge centers.
  • 
• India:
  • Earthquakes of mild intensity occur daily, while strong earthquakes causing large-scale destruction are less frequent.
  • Earthquakes are more frequent in areas of plate boundaries, especially along convergent boundaries.
  • The region of convergence between the Indian Plate and the Eurasian Plate is more vulnerable to earthquakes, such as the Himalayan region.
  • The peninsular part of India is considered to be a stable block, but some earthquakes are occasionally felt along the margins of minor plates (e.g., the Koyna earthquake of 1967 and the Latur earthquake of 1993).
  • The experts of Indian Seismology have divided India into four seismic zones (Zone II, Zone III, Zone IV, and Zone V).
  • The highest and high-risk areas for earthquakes, Zone V and IV, include the entire Himalayan region, states of North-East India, western and northern Punjab, Haryana, Uttar Pradesh, Delhi, and parts of Gujarat.
  • The remaining parts of the northern plains and western coastal areas fall in the moderate-risk zone, while a large part of the peninsular region lies in the low-risk zone.
  • 

Consequences of Earthquakes

• Damage to human life and property:
  • The deformation of the ground surface due to the movement of the Earth’s crust causes damage and destruction to human structures.
  • The Nepal earthquake of 2015 (magnitude 7.8, depth 8.2 km) is an example of an urban disaster caused by an earthquake.
  • The heavy casualties in Nepal were due to unplanned urban construction, poor building design, and unscientific structures.
  • The urban areas of Kathmandu suffered heavy damages, resulting in a death toll of 8,000 people and an economic loss of 10 billion USD.
  • Recent example: The earthquake of Turkey in February 2023.
• Landslides and Avalanches:
  • Tremors can cause slope instability and slope failure, leading to landslides.
  • Earthquakes can cause avalanches on snow-covered peaks.
  • Nepal earthquake of 2015 caused avalanches on and around Mount Everest.
  • Sikkim earthquake of 2011 caused landslides and damage to life and property, including damage to hydel projects.
• Floods:
  • Landslides and avalanches can block river courses and cause floods.
  • Example: The 1950 Assam earthquake caused a barrier in the Dihang River and resulted in flash floods in the upstream section.
• Tsunami:
  • Tsunamis are waves caused by the displacement of a large volume of water in the ocean basin, generated by seismic waves from earthquakes.
  • The 26th December 2004 Indian Ocean Tsunami was caused by an earthquake off the coast of Sumatra. It happened because of the subduction of the Indian plate under the Burmese plate, causing 2.4 lakh deaths in the Indian Ocean Region.
  • Fukushima Nuclear Accident: The 2011 Tohoku earthquake in Japan resulted in a 10 meter Tsunami that caused the Fukushima Daiichi nuclear meltdown and radioactive fallout, raising a worldwide concern.

Earthquake Management

• Earthquake management aims to reduce the harm from earthquakes through pre-earthquake risk reduction to post-earthquake recovery.
• Risk recognition:
  • identify areas more vulnerable to earthquakes.
• Earthquake monitoring/early warning system:
  • to make more precise forecasts about earthquakes to reduce impact.
  • Example: Japan has an earthquake early warning system that uses electronic signals that reach faster than earthquake waves.
• Structural Solution:
  • 95% of lives lost in earthquakes are due to building collapses that were not earthquake resistant. 
  • Seismic strengthening can be done by prioritizing structures and having an earthquake hazard map. 
  • Cost-effective solutions are a challenge.
•