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Seismic Hazard map "EMPOWER IAS"

Why is this in News?

  • Recently, scientists have found the first geological evidence of an earthquake on the border of Assam and Arunachal Pradesh, documented as the Sadiya earthquake in history. This finding could contribute to a seismic hazard map of the eastern Himalaya, which can facilitate construction and planning in the region.
  • This site is near the Tuting-Tidding Suture Zone- a major part of the Eastern Himalaya, where the Himalaya takes a sharp southward bend and connects with the Indo-Burma Range.

 

Findings :

  • Scientists from Wadia Institute of Himalayan Geology (WIHG) carried out a mega trench excavation at Himebasti village, Arunachal Pradesh where the most recent event records the imprints of the 1697 Sadiya earthquake and analysed it with modern geological techniques.
  • Scientists found large tree trunks embedded in the youngest flood deposits at the exit of the Subansiri River (Sadiya town is located roughly 145 km southeast of Subansiri river), suggesting the post-seismic aggradation of the river following an array of aftershocks till six months.
  • Aggradation is the term used in geology for the increase in land elevation due to deposition of sediment.
  • Aftershocks are earthquakes that follow the largest shock of an earthquake sequence. They are smaller than the mainshock and within 1-2 rupture lengths distance from the mainshock.

 

What is the Significance of finding :

  • Studying the past earthquakes help to determine the seismic potential of the region. This helps in mapping the earthquake hazard of the region and enables syncing the development activities accordingly.
  • Arunachal Pradesh, being near the Indo-China Border, and sometimes centre of dispute regarding its ownership, is strategically important.
  • A number of development initiatives like constructing roads, bridges and hydropower projects are being undertaken, therefore there is an urgent need for understanding the pattern of seismicity in this region.

 

Seismic Waves

  • Vibrations from an earthquake are categorised as P or S waves. They travel through the Earth in different ways and at different speeds. They can be detected and analysed.
    • P-waves (P stands for primary)
      • These are the first waves detected by seismograph.
      • These are longitudinal waves i.e.they vibrates along the same direction as it travels.
      • Other examples of longitudinal waves include sound waves and waves in a stretched spring.
    • S-waves (S stands for secondary)
      • These waves arrive at the detector after primary waves.
      • These are transverse waves i.e. they vibrates at a right angle to the direction in which they travel.
      • Other examples of transverse waves include light waves and water waves.
  • Both types of seismic waves can be detected near the earthquake centre but only P-waves can be detected on the other side of the Earth.
    • P-waves can travel through solids and liquids (since they are longitudinal waves) whereas S-waves can only travel through solids (as they are transverse waves). This means the liquid part of the core blocks the passage of S-waves.
  • The earthquake events are scaled either according to the magnitude or intensity of the shock.
    • The magnitude scale is known as the Richter scale. The magnitude relates to the energy released during the earthquake which is expressed in absolute numbers, 0-10.
    • The intensity scale or Mercalli scale takes into account the visible damage caused by the event. The range of intensity scale is from 1-12.

 

Seismic Hazard Map of India:

  • India is one of the highly earthquake affected countries because of the presence of tectonically active young fold mountains, Himalayas.
  • India has been divided into four seismic zones (II, III, IV, and V) based on scientific inputs relating to seismicity, earthquakes occurred in the past and tectonic setup of the region.
  • Previously, earthquake zones were divided into five zones with respect to the severity of the earthquakes but the Bureau of Indian Standards (BIS) grouped the country into four seismic zones by unifying the first two zones.
  • BIS is the official agency for publishing the seismic hazard maps and codes.

 

List of Earthquake (Seismic) Zones in India

  • Based on the past seismic history, Bureau of Indian Standards grouped the country into four seismic zones namely Zone-II, Zone-III, Zone-IV and Zone-V. Of all these four zones, Zone-V is the most seismic active region whereas Zone-II is the least.

 

Regions that fall under the Earthquake (seismic) Zones in India

  • Zone-V covers entire northeastern India, some parts of Jammu and Kashmir, some parts of Ladakh, Himachal Pradesh, Uttarakhand, Rann of Kutch in Gujarat, some parts of North Bihar and Andaman & Nicobar Islands.
  • Zone-IV covers remaining parts of Jammu & Kashmir, Ladakh and Himachal Pradesh, Union Territory of Delhi, Sikkim, northern parts of Uttar Pradesh, Bihar and West Bengal, parts of Gujarat and small portions of Maharashtra near the west coast and Rajasthan.
  • Zone-III comprises of Kerala, Goa, Lakshadweep islands, remaining parts of Uttar Pradesh, Gujarat and West Bengal, parts of Punjab, Rajasthan, Madhya Pradesh, Bihar, Jharkhand, Chhattisgarh, Maharashtra, Odisha, Andhra Pradesh, Tamil Nadu and Karnataka.
  • Zone-II covers remaining parts of the country.
  • Seismic zoning map of India helps in identifying the lowest, moderate as well as the highest hazardous or earthquake-prone areas in India. Also, such maps are used or looked before the construction of high rise building so as to check the level of seismology in any particular area. In the long run, this helps in saving lives.

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Earthquake Prone Zones in India

  • Earthquake is the violent shaking of the ground due to the release of accumulated stress of the moving lithospheric or crustal plates.
  • Over 59 % of India’s land area is under threat of moderate to severe earthquakes.
  • Bureau of Indian Standards, based on the past seismic history, divided the country into four seismic zones, viz. Zone II, III, IV and V.
  • The regions away from the Himalayas and other inter-plate boundaries were considered to be relatively safe from damaging earthquakes
  • However, occurrence of the Killari earthquake in Maharashtra (1993) resulted in revision of the seismic zoning map in which the low hazard zone or Seismic Zone I was merged with Seismic Zone II. Thus Zone I does not appear in mapping.
  • Zone V is the most seismically active region, while zone II is the least active region.
  • The zones are divided on the basis of Modified Mercalli (MM) intensity, which measures the impact of earthquakes
  • The area covered under different seismic zones include,
     
    • Zone II:
      • It falls under low intensity zone.
      • It covers 40.93% area of the country.
      • It comprises of major parts of peninsular region and Karnataka Plateau.
    • Zone III :
      • It falls under moderate intensity zone.
      • It covers 30.79% area of the country.
      • It comprises of Kerala, Goa, Lakshadweep islands, remaining parts of Uttar Pradesh, Gujarat and West Bengal, Parts of Punjab, Rajasthan, Madhya Pradesh, Bihar, Jharkhand, Chhattisgarh, Maharashtra, Odisha, Andhra Pradesh, Tamil Nadu and Karnataka.
    • Zone IV :
      • It falls under severe intensity zone.
      • It covers 17.49% area of the country.
      • It comprises of remaining parts of Jammu and Kashmir, Himachal Pradesh, National Capital Territory (NCT) of Delhi, Sikkim, Northern Parts of Uttar Pradesh, Bihar, West Bengal, parts of Gujarat, small portions of Maharashtra near the west coast and Rajasthan.
    • Zone V :
      • It falls under very severe intensity zone.
      • It covers 10.79% area of the country.
      • It comprises of the entire northeastern India, parts of Jammu and Kashmir, Himachal Pradesh, Uttarakhand, Rann of Kutch in Gujarat, part of North Bihar and Andaman & Nicobar Islands.