Earthquake: Definition, Causes, Types And Seismic Zones Of India

Earthquake Definition

An earthquake can be defined as a sudden shaking in the earth’s crust due to the movement in the plates resulting in the release of sudden energy and the formation of seismic waves. When a part of earth’s surface starts moving backwards and upwards, tremors are observed on the surface of the earth and hence called an earthquake. Earthquake meaning can be simply said as when Earth ‘quakes’. The earth’s crust is made up of different parts known as plates. In this article, we will cover everything you need to about Earthquake including the causes of an earthquake, facts, and earthquake zones in India.

Types of Earthquake

There are various types of earthquake that can be witnessed:

  • Tectonic earthquake: It is the most common form of an earthquake. It is generally caused by the movement of plates present in the earth’s crust known as tectonic plates.
  • Volcanic earthquake: This type of earthquake is less common as compared to the tectonic earthquake. These type of earthquake happen before or after the explosion of a volcano. It is generally caused due to the magma leaving the volcano which is filled by rocks being pushed to the surface.
  • Collapse earthquake: This type of earthquake occurs in underground mines. The main cause can be the pressure generated within the rocks.
  • Explosion earthquake: This type of earthquake is artificial in nature which means it is generated by the man-made activities. High-density explosion on the ground such as nuclear explosions is the primary cause of explosion earthquake.

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Cause of an Earthquake

Earth’s crust consists of several large and small tectonic plates around the earth. The major earthquake happen in the belts coinciding with the tectonic plates and the boundaries of these plates act as an epicentre. These plates form 3 types of boundaries, they move towards each other (a convergent boundary), apart from each other (a divergent boundary) or past each other (a transform boundary). The continuous movements of these tectonic plates along the boundaries build up pressure on both sides of the boundaries until the stress is great and is released in a sudden jerky movement. Hence, seismic waves created due to the energy released, travel through the surface of earth causing the shakes known as an earthquake.

Terms Related to Earthquake

  • Seismology: It is the branch of Geology dealing with the study of an earthquake.
  • Seismic Waves: These are the waves of energy caused by the earthquake that travels through the layers of earth.
  • Epicentre: It is the point on the ground surface that is closest to the focus.
  • Focus or Hypocentre: The place where the origin of seismic waves takes place beneath the earth’s surface is called the focus of an Earthquake.
  • Seismograph: The instrument on which the seismic waves are recorded is known as a seismograph.
  • Richter Scale: The instrument used to measure the magnitude of an earthquake.
  • Mercalli Scale: The instrument used to record the intensity of an earthquake.
  • Types of seismic waves: There are 3 types of seismic waves namely Primary waves, secondary waves and Surface or long waves.

Seismic Zones of India

The Indian subcontinent is divided into four seismic zones namely II, III, IV, and V based on the seismicity, earthquakes occurred in the past and tectonic setup of the region. As per the geographical data, almost 54% of India’s land is prone to earthquakes. Zone 5 covers the areas with the highest risk of getting an earthquake while Zone 2 is considerably low damage risk zone. Check out the map of India given below for the seismic zones of India based on the risk factor.


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