Earthquakes: Causes, Effects, Seismic Zone of India | Geology Optional Notes for UPSC PDF Download

Earthquakes

• Definition of Earthquakes: An earthquake is any sudden movement of the Earth's crust due to a natural cause that produces shaking or trembling of the ground.
• Terminology:
  • Focus or Hypocentre: The point of origin of an earthquake within the Earth.
  • Epicentre or Epifocal: The point on the Earth's surface directly above the focus of an earthquake.
  • Isoseismal Lines: Lines joining points of equal intensity on the surface where the earthquake occurs.
  • Homoseismal or Coseismal Lines: Lines drawn through points where the earthquake is recorded at the same time.
  • Pleistoseistic Region: The area around the epicentre where maximum destruction occurs.

Causes of Earthquakes

• Surface Causes: Minor earthquakes caused by events such as collapse of caves, landslides, or blasting of rocks.
• Volcanic Causes: Localized earthquakes associated with volcanic activity.
• Tectonic Causes: Major earthquakes resulting from movements along fault planes, plate tectonics, and the Elastic Rebound Theory.

a) Elastic rebound showing stages of rupture. A, B and C represent different stages of rupture

b) Schematics showing generation of an earthquake. Earthquakes are usually associated with slippage along a fault.

Types of Earthquakes

• Normal or Shallow Depth Earthquakes: Originate within a depth of 50 km.
• Intermediate Depth Earthquakes: Originate at a depth of 50 to 240 km.
• Deep-Focus Earthquakes: Originate at a depth of several hundred kilometers (240 to 725 km).

Earthquake Waves:

• Body Waves: P-waves (push waves) and S-waves (shear waves), which travel through solid and liquid and solid only, respectively.
• Surface Waves: Rayleigh waves and Love waves, which travel along the Earth's surface and have specific types of motion.
• Understanding these concepts is crucial for studying and preparing for earthquakes, as well as for the field of seismology.
• This section delves into the concepts of magnitude, intensity, and the recording of earthquakes:

Diagrammatic representation of earthquake waves  its origin, movement and the recording station.

Properties of Seismic waves

Magnitude and Intensity:

• Magnitude: Indicates the amount of energy released at the earthquake's source or epicentre. It is measured using the Richter scale, which is logarithmic. Each unit increase on the Richter scale represents a tenfold increase in amplitude of ground motion and approximately a thirtyfold increase in energy. The scale was established by Charles Richter and Beno Gutenberg in 1935.
• Intensity: Defines the extent of destruction caused by an earthquake and depends not only on magnitude but also on factors such as soil type, building materials, and population density. The Modified Mercalli Intensity Scale, commonly known as the Mercalli scale, measures earthquake severity based on observed effects. It ranges from I to XII, with increasing intensity levels correlating to escalating levels of damage and tremor effects.

The Modified Mercalli Intensity Scale

Recording of Earthquakes:

• Seismographs: Instruments used to record earthquakes. They measure the time, intensity, and direction of each individual shock. Seismographs record both horizontal and vertical components of ground motion.
• Seismograms: The records of seismic shocks produced by seismographs. They are typically presented on photosensitized paper, calibrated to coordinate with time and date. Seismograms capture the arrival of seismic waves, including P-waves (primary waves), S-waves (secondary waves), and surface waves.
• Understanding magnitude, intensity, and earthquake recording methods is crucial for accurately assessing and monitoring seismic activity, as well as for earthquake preparedness and mitigation efforts.

A sketch of Seismograph, the instrument used to record earthquakes.

A typical seismogram. The first wave to arrive at a seismometer is a P-wave, followed by the S-wave, and then by the surface waves.

Distribution of Seismic Belts

Introduction

You might be wondering why certain areas in the world experience earthquakes more frequently? These areas are known as Earthquake or Seismic Belts. Let's delve into the major seismic belts and their characteristics.

Pacific Belt

• The Pacific belt, also known as the Circum-Pacific seismic belt, accounts for about 81% of the world's largest earthquakes. This belt includes the rim of the Pacific plate and nearby plates, corresponding to the Pacific 'Ring of Fire'. Notably, eight out of ten of the largest earthquakes since 1900 have occurred along this seismic belt.

Alpine or Mediterranean Belt

• Extending from Java to Sumatra through the Himalayas, Mediterranean, and into the Atlantic, the Alpine or Mediterranean belt accounts for about 17% of the world's largest earthquakes. This belt includes some of the most destructive earthquakes in history, such as the Andaman-Sumatra earthquake in 2004.

Mid Atlantic Belt

• The Mid Atlantic belt follows the submerged Mid Atlantic Ridge in the Atlantic Ocean. While earthquakes in this belt are prominent, they have not been particularly destructive as they occur far from populated areas.

Distribution of earthquakes: the shaded areas are the zones of active epicenters.

Major Earthquakes in the Indian Subcontinent

Predictability of Earthquakes

• The predictability of earthquakes is a topic that puzzles many. While scientists worldwide are actively researching earthquake prediction methods, reliable prediction remains elusive.
• Currently, work is being done in countries like China and Japan, focusing on animal behavior as a potential indicator of impending earthquakes. However, this method has not yet proven to be highly reliable.
• It is hoped that in the near future, advancements in seismic monitoring and predictive techniques will lead to more reliable earthquake prediction methods.