Delhi Experiences Second Strongest Earthquake in Three Decades

Delhi Experiences Second Strongest Earthquake in Three Decades

Sub: Geo

Sec: Geomorphology

Why in News

• Delhi experienced a 4.0-magnitude earthquake, making it the strongest earthquake in the region since 2007. The shallow depth of the earthquake resulted in significant tremors being felt across the National Capital Region (NCR), including Noida, Ghaziabad, and Gurugram.

Seismic Activity in Delhi:

• According to the National Centre for Seismology (NCS), Delhi has recorded 446 earthquakes ranging from 1.1 to 4.6 in magnitude between 1993 and 2025.
• The strongest earthquake in the region prior to this was a 4.6-magnitude quake on November 25, 2007, located about six kilometres northwest of the latest event.
• Explanation of the Earthquake: The earthquake originated at a depth of only 5 km, making it shallow.
• Shallow earthquakes tend to have a stronger impact on the surface due to reduced energy dissipation.
• In contrast, the 4.6-magnitude quake in 2007 had a depth of 10 km, which resulted in a lesser impact.
• According to India’s earthquake hazard map, Delhi is classified under Zone 4, which is the second-highest classification for earthquake-prone regions.
• Areas in this zone can experience shaking of MSK-8 intensity during an earthquake. The MSK (Medvedev-Sponheuer-Karnik) scale measures the susceptibility of an area to earthquakes, rather than the strength or energy released.
• The earthquake was caused by normal faulting, which occurs when the Earth’s crust extends and fractures.
• According to the NCS report, hydrofracturing played a role in the seismic activity.
• Hydrofracturing refers to the gradual erosion of underlying rock due to the movement of ancient water channels and aquifers beneath Delhi.
• Delhi’s Seismic Vulnerability: Delhi sits on several active fault lines, including:
  • Mahendranagar Fault
  • Sohana Fault

What is the Aravalli-Delhi Fold Belt?

• Delhi lies within the Aravalli-Delhi Fold Belt, a seismically active geological zone that stretches from Rajasthan to Delhi and Haryana. This region is characterized by deformed rock layers due to geological processes from millions of years ago. These stresses sometimes release in the form of earthquakes.
• Aravalli-Delhi Fold Belt vs. Himalayan Subduction: Earthquakes in Delhi are primarily the result of stress release in the Aravalli-Delhi Fold Belt, a much slower tectonic process.
• In contrast, earthquakes in the Himalayan region are driven by the subduction of the Indian plate under the Eurasian plate, which builds significant stress, leading to larger and more frequent earthquakes.
• Historical Earthquakes in Delhi: A strong earthquake is recorded around 1720, and another in 1803, which allegedly caused damage to the Qutub Minar.
• However, newer research suggests that the 1803 earthquake was likely centred in the Srinagar-Devprayag region in Uttarakhand, with tremors felt in Delhi.
• Why Large Earthquakes Are Unlikely in Delhi: Delhi has experienced moderate tremors over the years, but large earthquakes, such as those in the Himalayan region, are highly unlikely due to the nature of seismic activity in the area.

About Earthquake:

• 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 (sudden release of energy) in the earth’s crust (shallow-focus earthquakes) or upper mantle (some shallow-focus and all intermediate and deep-focus earthquakes).
• The point where the energy is released is called the focus or the hypocentre of an earthquake.
• The point on the surface directly above the focus is called epicentre (first surface point to experience the earthquake waves).
• A line connecting all points on the surface where the intensity is the same is called an isoseismic line.

Waves:

• There are several different kinds of seismic waves, and they all move in different ways. The two main types of waves are body waves and surface waves.
• Body waves can travel through the earth’s inner layers, but surface waves can only move along the surface of the planet like ripples on water
• The first kind of body wave is the P wave or primary wave. This is the fastest kind of seismic wave, and, consequently, the first to ‘arrive’ at a seismic station. The P wave can move through solid rock and fluids, like water or the liquid layers of the earth. It pushes and pulls the rock it moves through just like sound waves push and pull the air.
• The second type of body wave is the S wave or secondary wave, which is the second wave felt in an earthquake. An S wave is slower than a P wave and can only move through solid rock, not through any liquid medium. It is this property of S waves that led seismologists to conclude that the Earth’s outer core is a liquid.
• S waves move rock particles up and down, or side-to-side–perpendicular to the direction that the wave is traveling in (the direction of wave propagation)

Range:

• Earthquakes typically range from 0 (micro) to 9.0+ (mega).
  • Minor Earthquakes: Magnitude 3.0 – 3.9
  • Light Earthquakes: Magnitude 4.0 – 4.9
  • Moderate Earthquakes: Magnitude 5.0 – 5.9
  • Strong Earthquakes: Magnitude 6.0 – 6.9
  • Major Earthquakes: Magnitude 7.0 – 7.9
  • Great Earthquakes: Magnitude 8.0+

Seismic Zones in India:

• India is divided into four seismic zones:
  • Zone II: Low seismic activity.
  • Zone III: Moderate seismic activity.
  • Zone IV: High seismic activity.
  • Zone V: Very high seismic activity.
  • Zone V: Covers about 11% of India, with 18%in Zone IV and 30% in Zone III. The remaining 41% of the country is in Zone II.
  • Seismic Vulnerability: Approximately 59%of India is prone to earthquakes of varying intensities.

Measurement of Earthquake:

• Richter Scale (Magnitude): Measures the magnitudeof an earthquake, which indicates the total energy released during the event.
  • The Richter scale is logarithmic, meaning that for each whole number increase on the scale, the earthquake’s magnitude increases by a factor of 10 in amplitude and roughly 32 times more energyis released.
  • The Richter scale is less accurate for large earthquakes (magnitude above 7.0) and is less commonly used today for large events.
• Moment Magnitude Scale (Mw): A more accurate and modern way of measuring the magnitudeof large earthquakes (especially those above magnitude 7.0).

• • Like the Richter scale, it is logarithmic but considers more factors, including the fault length, fault slip, and rock properties.
  • Provides a more reliable measure of an earthquake’s energy release, especially for large-scale events.
  • Similar to the Richter scale, but preferred for measuring very large earthquakes.

• Mercalli Intensity Scale (Intensity): Measures the intensityor effects of an earthquake at specific locations, based on human perception, structural damage, and ground shaking.
  • The Mercalli scale is subjectiveand varies by location, as the effects of an earthquake can differ greatly depending on distance from the epicentre, local geology, and building structures.
  • The scale ranges from I (not felt)to XII (total destruction).
  • The Mercalli scale is influenced by subjective reports and human perception, making it less precise than the magnitude scales for comparison across different events.
• Body-Wave Magnitude (Mb): Measures the magnitude of seismic waves that travel through the Earth’s interior(body waves), such as the P-waves (primary waves) and S-waves (secondary waves).
  • Typically used for distant earthquakes, where body waves are the first to arrive and can provide early data on the earthquake’s size.
• Surface-Wave Magnitude (Ms): Measures the magnitude based on surface waves, which are the seismic waves that travel along the Earth’s surface.
  • Used mainly for larger earthquakeswith surface effects, especially those that are felt on the ground.
• The Shindo Scale (Japan): Used primarily in Japanto measure the intensity of an earthquake’s ground shaking, much like the Mercalli scale.
  • Ranges from 1 to 7:
    • 1: Not felt by people, or only a slight shaking.
    • 7: Severe shaking, causing total destruction in affected areas.
• Medvedev-Sponheuer-Karnik (MSK) Scale: The MSK Scale measures earthquake intensity, focusing on the effects experienced at the Earth’s surface rather than the energy released or magnitude.
  • It helps assess the degree of shaking felt by people, as well as the damage to buildings and infrastructure.
  • The MSK scale has a range from I (very weak) to X (extreme). The higher the level, the more severe the shaking and damage caused by the earthquake.
  • MSK-8 (Severe damage): Areas with MSK-8 intensity can experience major damage to buildings, bridges, and other infrastructure, with widespread effects on the population.
  • MSK-9 (Very Severe damage): Found in Zone 5 of India, the highest seismic risk zones, where extensive destruction is expected.