Context: Recently, the Wadia Institute of Himalayan Geology (WIHG) has observed that the recent series of tremors in the National Capital Region-Delhi (Delhi-NCR) are not unusual and are indicative of strain energy build up in the region.
Key Points

• They have said that since the seismic network is quite good, present micro to minor earthquakes in and around Delhi-NCR could be recorded.
• Though our understanding, in terms of when, where and with how much energy (or magnitude) an earthquake can occur, is not clear, but the vulnerability of a region can be understood from the past seismicity, calculation of strain budget, mapping of active faults etc.
• The Delhi-NCR has been identified as the second highest seismic hazard zone (Zone IV).
• Sometimes, a vulnerable zone remains quiet, experiences small magnitude earthquakes that do not indicate any bigger earthquake, or receives a sudden jolt by a big earthquake without any call.
• Out of 14 small magnitude earthquakes in the Delhi-NCR, the 29th May Rohtak earthquake had the magnitude of 4.6.

Why earthquakes happen in Delhi-NCR?

• All the earthquakes in Delhi-NCR are due to the release of strain energy, which have been accumulated as a result of northward movement of Indian plate and its collision with the Eurasian plate, through the fault or weak zones.
• 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.
• These 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.

Seismic Zones in India

• 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: 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 of MM scale.
• Seismic Zone IV: Major damage corresponding to intensity VII and higher of MM scale.
• Seismic Zone V: 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.

Measurement Devices 

• 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.

Precautions

• The subsurface structures, geometry, and disposition of faults and ridges are to be investigated thoroughly using Geo-scientific 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.

Road Ahead

• Earthquakes are not predictable but there lies a probability of a large to 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 minimise 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.
• Therefore People’s participation, cooperation and awareness are the key to success.