Why so many Earthquakes in the Indian Subcontinent?

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Context: The Delhi-NCR region has felt at least 14 tremors in the last two months. But these are not the only earthquakes to have hit India in the past few weeks. Even a cursory look at the record maintained by the National Centre for Seismology tells us that earthquakes have originated in Jammu and Kashmir, Ladakh, Himachal Pradesh, Punjab, Uttarakhand, Haryana, Delhi, Uttar Pradesh, Rajasthan, Gujarat, Madhya Pradesh, Sikkim, Arunachal Pradesh, Meghalaya, Mizoram, Manipur and also Andaman and Nicobar Islands. The latest earthquake to have hit India, according to the National Centre for Seismology, was in Cherrapunjee in Meghalaya ( known for constant rains).

Relevance:
Prelims: Current events of national and international importance
Mains:

  • GS I- geographical features and their location- changes in critical geographical features (including water bodies and ice-caps) and in flora and fauna and the effects of such changes.
What are the discussions?
  • These recent earthquakes have triggered discussions on the possibility of increased seismicity around Delhi and India.
  • They have also triggered fears of an impending big earthquake soon.
  • None of these apprehensions has any scientific basis.
  • Tremors have been felt almost in the entire country at some point of time in the recent past. This has led many to ask, why India is witnessing so many earthquakes.
Earthquakes
  • An earthquake is shaking or trembling of the earth’s surface, caused by the seismic waves or earthquake waves that are generated due to a sudden movement in the earth’s crust or upper mantle, due to the sudden release of energy.
  • The instrument used to detect and record earthquakes is called a seismograph or seismometer.
Mechanism of the earthquake in India
  • India is situated on a tectonic plate that holds the landmass of the Indian subcontinent and the Indian Ocean.
  • This place is ramming the Eurasian plate which is moving south. These two plates have been colliding for a few million years. Their collision raised the tallest mountain structure on the earth, the Himalayas.
  • The Indic plate is heavier and hence is sinking below the Eurasian plate. The collision is still happening and the Indic plate is still submerging beneath the Himalayas at the rate of 5-6 cm every year. True! India is losing its 5-6 cm of land to its northern neighbor every year.
  • The Indic plate is huge. To move such a big body, immense energy is needed. This energy is being generated beneath the crust (topmost 30-50 km surface of the earth) in the molten mantle. It acts like a furnace. Energy moves upward and jets out in different directions. This illustration explains the mechanism.
  • The same thing is happening with the Indian landmass. This is causing strain on the rest of the shield. The shield has too many faults or linear zones of weaknesses. These faults crack or shake when there is excessive energy flow or strain.

Past earthquakes scenario In Delhi-NCR:
  • The historical earthquake catalog shows that there were strong earthquakes of ~ 6.5 magnitude at Delhi in 1720; 6.8 at Mathura in 1803; 5.5 near Mathura in 1842; 6.7 near Bulandshahar in 1956; 6.0 near Faridabad in 1960; 5.8 near Moradabad in 1966 in the Delhi-NCR.
Why do earthquakes happen in Delhi-NCR?
  • Release of Energy:
    • Release of strain energy through the weak zones or faults which accumulates as a result of northward movement of the Indian plate and its collision with the Eurasian plate.
  • Movement of Plates:
    • The Himalayan seismic belt is the region where the Indian plate collided with the Eurasian plate and under trusted beneath the Himalayan wedge.
  • Proximity to the Himalayas:
    • Delhi-NCR is not very far from the entire north-west and north-east Himalayan belt lies in the highest seismic potential zone V and IV, where major to great earthquakes can take place.
  • There are so many weak zones and faults in the Delhi-NCR:
    • Delhi-Haridwar ridge, Mahendragarh-Dehradun subsurface fault, Moradabad fault, Sohna fault, Great boundary fault, Delhi-Sargodha ridge, Yamuna river lineament, Ganga river lineament, etc.
    • We must understand that the Himalayan seismic belt, where the Indian plate collided with the Eurasian plate and underthrusted beneath the Himalayan wedge, accumulates strain energy at the plate boundary due to relative movement of plates against each other causing crustal shortening and deformation of rocks.
    • This energy can be released through the weak zones and faults in the form of earthquakes ranging from micro (<3.0), minor (3.0-3.9), light (4.0-4.9), moderate (5.0-5.9), strong (6.0-6.9), major (7.0-7.9) or great (>8.0) earthquake, defined as per the amount of energy released.
Impact of Earthquakes in the Himalaya to Delhi-NCR:
  • The Isoseismals of the 1905 Kangra (7.8), 1934 Bihar-Nepal (8.0), 1950 Assam (8.6), 2005 Muzaffarabad (6.7) and 2015 Nepal (7.8) earthquakes in the Himalayan arc are bounded by the Main Central Thrust (MCT) to the north and the Himalayan Frontal Thrust (HFT) to the south.
  • These earthquakes are the result of slip on a décollement surface i.e. the contact between the underthrusting Indian plate and overlying Himalayan wedge, which extends southward from 16-27 km depth beneath the MCT to its surficial expression as the HFT at a distance of 50-100 km from MCT.
  • The rupture areas due to large earthquakes show gaps along the Himalayan arc, which have not experienced great earthquakes for a long time and are identified as the future potential zones for great earthquakes.
  • Three main seismic gaps have been identified in the Himalaya:
    • The Assam Gap between the 1950 Assam earthquake and the 1934 Bihar-Nepal earthquake; the Kashmir Gap between the 1905 Kangra earthquake and the 1975 Kinnaur earthquake; and the ~700 km long Central Gap between the 1905 Kangra earthquake and the 1934 Bihar-Nepal earthquake.
    • The entire NW-NE Himalayan belt lies in the highest seismic potential zone V and IV, where major to great earthquakes can take place.

Neighboring faults and ridges
  • There are so many faults, ridges, and lineaments transverse to the Himalayan arc, large sediment thickness in the Ganga Alluvium Plains to the north of Delhi-NCR.
  • Again, the Delhi-NCR is ~200 km away from the Himalayan arc.
  • Therefore, a major earthquake in the Himalayan seismic belt may also be a threat to Delhi-NCR.
  • The Garhwal Himalaya, lying in the Central Seismic Gap and north of Delhi-NCR, has experienced the 1991 Uttarkashi earthquake (6.8), 1999 Chamoli earthquake (6.6) and 2017 Rudraprayag earthquake (5.7), and is due for a major to a great earthquake. Such a scenario can make a pronounced impact on north India and Delhi-NCR.
Seismic Zones in India
  • About
    • There are four seismic zones (II, III, IV, and V) in India 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.
  • Seismic Zone II:
    • The area with minor damage earthquakes corresponding to intensities V to VI of MM scale (MM-Modified Mercalli Intensity scale).
  • Seismic Zone III:
    • Moderate damage corresponding to intensity VII or MM scale.
  • Seismic Zone IV:
    • Major damage corresponding to intensity VII and higher of MM scale.
  • Seismic Zone V:
    • The area determined by pro seismically of certain major fault systems and is seismically the most active region.
    • Earthquake zone V is the most vulnerable to earthquakes, where historically some of the country’s most powerful shocks have occurred.
    • Earthquakes with magnitudes in excess of 7.0 have occured in these areas, and have had intensities higher than IX.

Seismic Waves, Richter Scale and Mercalli scale
  • Seismic waves are the vibrations from earthquakes that travel through the Earth and are recorded on instruments called seismographs.
  • Seismographs record a zigzag trace that shows the varying amplitude of ground oscillations beneath the instrument.
  • 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.

 

Is it unusual?
  • Scientists are unequivocal in asserting that no unusual seismic activity is taking place around Delhi in the last few months.
  • According to the earthquake catalogue, Delhi and its surrounding areas usually experience between two and three earthquakes of magnitude 2.5 and above every month. But there are monthly and annual variations as well. 
  • Geological and seismological processes are not very smooth. So sometimes you would expect to see a higher number of earthquakes as well. 
  • Detection of earthquakes depends on the number of seismic recorders installed in that area. The area around Delhi has the densest concentration of seismometers anywhere in the country, even more than the Himalayan region which is seismically much more active. 
Do these small earthquakes foretell a bigger one?
  • Earthquakes of magnitude four or below hardly cause any damage anywhere and are mostly inconsequential for practical purposes. 
  • Thousands of such earthquakes are recorded around the world every year, and most of them are uneventful. 
  • And, they certainly do not signal any big upcoming event.
  • The concept of foreshocks tells that when a big event happens, all the smaller earthquakes that have occurred in that region in the near past are classified as foreshocks. 
  • So all this talk of these being foreshocks of a big earthquake in Delhi has no basis at all. However, a big earthquake might still occur, no one can rule it out. But they cannot be predicted. 
Signal to an upcoming earthquake:
  • Scientists have been working for years to identify “precursors” to an earthquake, but have so far met with no success. 
  • Predicting earthquakes in a region like Delhi is all the more difficult because the place does not lie on any fault lines.
  • Some special earthquakes, the ones that are triggered by volcanic activity, can be predicted to some extent.
Chances of a big earthquake coming to the region
  • Scientists say that the Himalayan region is due for a big earthquake, of magnitude 8 or even higher. That is because they have been able to measure the energy that is getting trapped under the surface as a result of one tectonic plate trying to move beneath the other one. 
  • But even here, scientists have no idea when this big earthquake will occur. The prediction about the big one is based only on the estimate of the energy that is ready to be released.
  • The argument that the smaller quakes are helping release the energy bit by bit so that a big one would not be necessary also does not hold. 
Precautions
  • The subsurface structures, geometry, and disposition of faults and ridges are to be investigated thoroughly using Geoscientific studies in and around Delhi and NCR.
  • Since the soft soils do not support the structures’ foundations, structures anchored to bedrock or stiff soils in earthquake-prone areas suffer less damage.
  • Thus, soil liquefaction studies are to be carried out to know the thickness of soft soils.
  • Active faults are to be delineated, and lifeline structures or other infrastructures are to be avoided from nearby active faults and to be constructed as per the guiding principles of the Bureau of Indian Standard (BIS).
  • The outcome of recent micro zonation studies for Delhi-NCR by IMD should be considered for important construction.
Way Forward
  • Earthquakes are not predictable but there lies a probability of a large to a great earthquake with magnitude 6 and more in the highest seismic potential zone V and IV, which fall in the entire Himalaya and Delhi-NCR.
  • The only solution to minimize the loss of lives and properties is the effective preparedness against the earthquake. Countries like Japan have proved this, where earthquakes are a common phenomenon yet the losses are negligible.
  • People’s participation, cooperation, and awareness are the keys to success.



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