GS Notes

1.5 - Geography Notes 16

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Elaborate Notes

Earthquakes: Causes and Study

An earthquake is the shaking of the Earth’s surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. The scientific study of earthquakes and their propagating waves is known as seismology. The foundations of modern seismology were laid by scientists like John Milne, an English geologist who, in the late 19th century, invented the first modern seismograph and established a global network of seismic stations.

Natural Causes of Earthquakes

  • Plate Movements: This is the most significant cause of earthquakes. The theory of Plate Tectonics, building upon Alfred Wegener’s theory of Continental Drift (1912) and later developed by figures like Harry Hess and J. Tuzo Wilson in the mid-20th century, posits that the Earth’s lithosphere is divided into several rigid plates. These plates are in constant motion, and earthquakes occur primarily along their boundaries (fault lines).
    • Convergent Boundaries: Where plates collide, immense stress builds up. Examples include the collision of the Indian Plate with the Eurasian Plate, forming the Himalayas, a seismically active region. The 2015 Nepal earthquake (Magnitude 7.8) is a recent example.
    • Divergent Boundaries: Where plates move apart, such as at the Mid-Atlantic Ridge, tensional stress leads to frequent but generally less powerful, shallow-focus earthquakes.
    • Transform Boundaries: Where plates slide past each other horizontally, shear stress accumulates and is released as an earthquake. The San Andreas Fault in California is a classic example, responsible for events like the 1906 San Francisco earthquake.
  • Volcanic Eruptions: Earthquakes can be triggered by the movement of magma beneath a volcano. As magma rises, it fractures rock, causing continuous, small earthquakes known as volcanic tremors. The 1883 eruption of Krakatoa was preceded and accompanied by intense seismic activity.
  • Landslides: The sudden movement of large masses of rock or debris down a slope can generate seismic waves, though these are typically localized and of low magnitude.
  • Land Subsidence: The collapse of underground caverns, particularly in karst landscapes (limestone regions), or the collapse of large mines can cause minor, surface-level tremors.
  • Meteoritic Impact: The impact of a large meteorite or asteroid can generate immense energy, equivalent to a major earthquake. The Chicxulub impact 66 million years ago, linked to the extinction of dinosaurs, would have generated an earthquake of an estimated magnitude greater than 11.

Anthropogenic Causes of Earthquakes

  • Mining and Blasting: Large-scale mining operations, especially those involving the removal of vast quantities of rock (e.g., mountaintop removal) or controlled explosions (blasting), can induce minor local seismicity by altering stress patterns in the crust.
  • Nuclear Explosions: Underground nuclear tests release enormous amounts of energy, creating seismic waves indistinguishable from natural earthquakes on a seismogram. This phenomenon is used by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) to monitor for illicit tests.
  • Reservoir-Induced Seismicity (RIS): The construction of large dams and the impoundment of vast water reservoirs can trigger earthquakes. The weight of the water increases the stress on underlying faults, and water seeping into the ground can lubricate fault planes, making them more likely to slip. The most cited example is the 1967 Koyna earthquake (Magnitude 6.6) in Maharashtra, India, which is widely believed to have been induced by the Koyna Dam reservoir.

Earthquake Waves

When an earthquake occurs, the stored elastic strain energy is released. The point within the Earth where the fault rupture begins is the focus or hypocentre. The point on the Earth’s surface directly above the focus is the epicentre, which is where the strongest shaking is usually felt. The released energy propagates outwards in the form of seismic waves.

Body Waves

These waves travel through the Earth’s interior from the focus. Their study provided the first concrete evidence for the Earth’s layered structure.

  • P-waves (Primary Waves):

    • They are the fastest seismic waves and arrive first at seismograph stations.
    • They are longitudinal or compressional waves, meaning the particle motion is a push-pull (compression and rarefaction) parallel to the direction of wave propagation, similar to sound waves.
    • Their velocity is dependent on the density and, more importantly, the compressibility and rigidity of the medium.
    • Crucially, P-waves can travel through solids, liquids, and gases. They are refracted (bent) when they pass from one medium to another, for instance, at the core-mantle boundary.

  • S-waves (Secondary Waves):

    • They arrive after P-waves at seismograph stations.
    • They are transverse or shear waves, meaning the particle motion is perpendicular (up-and-down or side-to-side) to the direction of wave propagation.
    • Their velocity depends on the density and rigidity (shear strength) of the material.
    • Since liquids and gases have no shear strength, S-waves cannot travel through them. This critical property allowed seismologist Richard Dixon Oldham in 1906 to deduce the existence of the Earth’s liquid outer core, as S-waves were not observed to pass through it.

Surface Waves

These waves are generated when body waves reach the surface. They are confined to the near-surface layers of the Earth and are responsible for most of the structural damage during an earthquake.

  • Love Waves: Named after British mathematician A. E. H. Love who described them mathematically in 1911. They have a horizontal, side-to-side shearing motion perpendicular to the direction of propagation. They are particularly damaging to the foundations of buildings.
  • Rayleigh Waves: Named after Lord Rayleigh, who predicted their existence in 1885. They produce a rolling motion, similar to ocean waves, with particles moving in an elliptical, retrograde path in the vertical plane. This combined up-and-down and back-and-forth motion is extremely destructive.

Shadow Zones

The study of how seismic waves are reflected and refracted within the Earth leads to the identification of ‘shadow zones’—areas on the surface where seismographs do not detect waves from a particular earthquake.

  • S-Wave Shadow Zone: S-waves are stopped entirely by the liquid outer core. Therefore, any seismograph located at an angular distance greater than 105° from the epicentre will not record any direct S-waves. This creates a vast S-wave shadow zone covering the entire portion of the Earth beyond 105°.
  • P-Wave Shadow Zone: P-waves are refracted downwards as they enter the dense liquid outer core. This sharp bending creates a “shadow zone” between 105° and 145° from the epicentre where no direct P-waves are received. The existence and location of this zone allowed seismologist Beno Gutenberg in 1914 to accurately determine the depth of the core-mantle boundary (now called the Gutenberg Discontinuity) at approximately 2900 km.
  • Inferring the Inner Core: In 1936, Danish seismologist Inge Lehmann observed weak P-waves arriving within the supposed shadow zone. She correctly hypothesized that these were P-waves that had been refracted by a solid inner core within the liquid outer core, thus discovering the Earth’s innermost layer.

Types of Earthquakes by Focal Depth

The depth of the focus is a key characteristic of an earthquake. Earthquakes originating in subduction zones occur along a dipping plane of seismicity known as the Wadati-Benioff Zone, named after seismologists Kiyoo Wadati and Hugo Benioff.

  • Shallow-focus: Occurs at a depth of 0–70 km. These are the most common and often the most destructive, as the energy is released closer to the surface. They occur at all types of plate boundaries.
  • Intermediate-focus: Occurs at a depth of 70–300 km (some classifications use 350 km). These are typically associated with subduction zones.
  • Deep-focus: Occurs at depths greater than 300 km (or 350 km), reaching down to about 700 km. These are almost exclusively found in subduction zones where a cold, dense oceanic plate is being forced deep into the mantle. Below this depth, the rock is too hot and ductile to accumulate strain and rupture.

Distribution of Earthquakes

Earthquakes are not randomly distributed but are concentrated in specific seismic belts that coincide with plate boundaries.

  • Circum-Pacific Belt (The “Ring of Fire”): This is the world’s most intense earthquake belt, accounting for about 81% of the world’s largest earthquakes. It traces the margins of the Pacific Ocean, where the Pacific Plate subducts beneath continental plates (e.g., along the west coast of South America, causing the Andes) and other oceanic plates (e.g., near Japan, the Philippines).
  • Alpine-Himalayan (or Mediterranean-Trans-Asiatic) Belt: This belt extends from the Mediterranean region eastward through Turkey, Iran, and Northern India into Southeast Asia. It is a zone of continent-continent collision, most notably between the Eurasian Plate and the African, Arabian, and Indian Plates. It accounts for about 17% of the world’s largest earthquakes.
  • Mid-Oceanic Ridges: This is a global system of divergent boundaries where new oceanic crust is formed. Earthquakes here are frequent but are shallow-focus and of relatively low magnitude. The Mid-Atlantic Ridge is the most prominent example.
  • Intraplate Earthquakes: Though rare, significant earthquakes can occur within tectonic plates, far from their boundaries. These are often associated with ancient, pre-existing fault zones. The 1993 Latur earthquake (Magnitude 6.2) in India is an example of an intraplate earthquake on the Deccan Plateau.

Measurement of Earthquakes

  • Instrumentation: An instrument that detects and records seismic waves is a seismograph. The graphical record it produces is a seismogram.

  • Magnitude vs. Intensity: It is crucial to distinguish between these two measures.

MagnitudeIntensity
It is a quantitative measure of the total energy released at the earthquake’s source (focus).It is a qualitative measure of the effects and observable damage caused by the earthquake at a specific location.
It is calculated from seismograms and is a single value for a given earthquake.It is determined from observations of the earthquake’s effects on people, buildings, and the environment. It varies with distance from the epicentre.
It is an objective, instrument-based measurement.It is a subjective, observation-based assessment.
Measured on the Richter Scale (developed by Charles Richter and Beno Gutenberg in 1935) or more commonly today, the Moment Magnitude Scale (MMS). The Richter scale is logarithmic: a one-unit increase corresponds to a 10-fold increase in wave amplitude and a ~32-fold increase in energy release.Measured on the Modified Mercalli Intensity (MMI) Scale. This scale uses Roman numerals from I (not felt) to XII (catastrophic destruction).
  • Notable Earthquakes:
    • Great Chilean Earthquake (1960): Magnitude 9.5, the most powerful earthquake ever recorded.
    • Indian Ocean Earthquake (2004): Magnitude 9.1-9.3, originating off the coast of Sumatra, Indonesia; triggered a devastating tsunami.
    • Tōhoku Earthquake, Japan (2011): Magnitude 9.1, also caused a massive tsunami.
    • Bhuj Earthquake, India (2001): Magnitude 7.7, a devastating intraplate earthquake.
    • Kashmir Earthquake (2005): Magnitude 7.6, affecting parts of India and Pakistan.

Tsunami

A tsunami (Japanese for “harbour wave”) is a series of large ocean waves caused by the abrupt displacement of a large volume of water. They are often incorrectly called “tidal waves”.

  • Generation: The most common cause is a large, submarine earthquake associated with vertical displacement of the seafloor, typically along a subduction zone (a megathrust earthquake). Other causes include:

    • Underwater volcanic eruptions (e.g., the 1883 Krakatoa eruption).
    • Underwater landslides.
    • Meteorite impacts.
    • Underwater nuclear tests.

  • Propagation in Deep Water: In the open ocean, a tsunami has a very long wavelength (hundreds of kilometres) and a low amplitude (often less than a metre). It travels at very high speeds, comparable to a jet aircraft (up to 800 km/h), making it almost unnoticeable to ships.

  • Propagation in Shallow Water (Shoaling Effect): As the tsunami approaches a coastline and the water depth decreases, its velocity slows down. Due to the conservation of energy, the wave’s amplitude (height) increases dramatically. This process is known as shoaling.

  • Landfall (Inundation): When the tsunami reaches the shore, it appears not as a normal breaking wave but as a rapidly rising tide or a wall of water (a bore) that floods coastal areas, causing immense destruction. The area flooded is known as the inundation zone.

Prelims Pointers

  • The scientific study of earthquakes is called seismology.
  • The point of energy release below the surface is the focus or hypocentre.
  • The point on the surface directly above the focus is the epicentre.
  • P-waves (Primary waves) are longitudinal, the fastest, and can travel through solids, liquids, and gases.
  • S-waves (Secondary waves) are transverse, slower than P-waves, and can only travel through solids.
  • Surface waves (Love and Rayleigh waves) travel along the Earth’s surface and are the most destructive.
  • S-wave shadow zone exists beyond 105° from the epicentre because S-waves cannot pass through the liquid outer core.
  • P-wave shadow zone exists between 105° and 145° from the epicentre due to refraction at the core-mantle boundary.
  • The existence of the liquid outer core was inferred from the S-wave shadow zone.
  • The existence of the solid inner core was inferred by Inge Lehmann from weak P-wave arrivals inside the shadow zone.
  • Earthquake Depths:
    1. Shallow-focus: 0-70 km
    2. Intermediate-focus: 70-300 km
    3. Deep-focus: > 300 km
  • The Wadati-Benioff Zone is a dipping plane of seismicity at subduction zones.
  • Major Earthquake Belts:
    • Circum-Pacific Belt (“Ring of Fire”) - Most active.
    • Alpine-Himalayan Belt.
    • Mid-Oceanic Ridges.
  • Magnitude measures the energy released at the source; measured on the Richter Scale or Moment Magnitude Scale.
  • The Richter scale is logarithmic; a 1-unit increase means ~32 times more energy release.
  • Intensity measures the visible damage at a location; measured on the Modified Mercalli Scale (I-XII).
  • Reservoir-Induced Seismicity (RIS) is a type of anthropogenic earthquake. The Koyna earthquake (1967) is a key example in India.
  • Tsunami is a series of long-wavelength sea waves, also known as seismic sea waves.
  • The most common cause of a tsunami is an underwater earthquake involving vertical displacement of the seafloor.
  • Shoaling Effect: As a tsunami enters shallow water, its speed decreases and its height (amplitude) increases.
  • Notable Earthquakes (Magnitude):
    • Chile (1960): 9.5
    • Indian Ocean (2004): 9.1
    • Japan (2011): 9.1
    • Bhuj (2001): 7.7
    • Kashmir (2005): 7.6
    • Latur (1993): 6.2

Mains Insights

Geo-physical phenomena and understanding the Earth’s Interior (GS-I)

  1. From Phenomenon to Theory: The study of seismic waves is a prime example of how indirect observation can lead to profound scientific discoveries. The analysis of P and S wave travel times and their shadow zones was instrumental in moving from a theoretical, homogenous model of the Earth to a scientifically validated, layered structure.
    • Discovery Progression:
      • Richard Oldham (1906): Used the absence of S-waves beyond a certain distance to propose the existence of a liquid core.
      • Beno Gutenberg (1914): Used the P-wave shadow zone to accurately calculate the depth of the core-mantle boundary.
      • Inge Lehmann (1936): Used faint P-wave arrivals in the shadow zone to deduce the existence of a solid inner core.
    • Significance: This progression demonstrates the power of the scientific method and how seismology provides a ‘CAT scan’ of our planet, revealing its internal composition and dynamics.

Disaster Management and Preparedness (GS-III)

  1. Prediction vs. Forecasting: While the long-term probability of an earthquake in a seismic zone can be forecasted (e.g., “seismic gap” theory), the precise short-term prediction (date, time, location) of earthquakes remains elusive. This has critical implications for disaster management.
    • Policy Shift: The focus has shifted from prediction to mitigation, preparedness, and rapid response. This includes:
      • Strict enforcement of building codes (e.g., Bureau of Indian Standards codes for earthquake-resistant structures).
      • Seismic zonation and micro-zonation for land-use planning.
      • Public awareness campaigns and drills.
      • Strengthening institutions like the National Disaster Management Authority (NDMA) and National Disaster Response Force (NDRF).
  2. Tsunami Early Warning Systems: The catastrophic 2004 Indian Ocean tsunami was a wake-up call, highlighting the vulnerability of coastlines.
    • Cause & Effect: The lack of an early warning system in the Indian Ocean led to massive loss of life. In response, India established the Indian Tsunami Early Warning System (ITEWS) at INCOIS, Hyderabad.
    • Analysis: This system, part of a global network, showcases the importance of scientific infrastructure and international cooperation in disaster risk reduction. Its effectiveness depends on the “last-mile connectivity”—the ability to disseminate warnings to coastal communities in real-time.

Anthropogenic Factors and the Development-Environment Debate (GS-I & GS-III)

  1. Reservoir-Induced Seismicity (RIS): The link between large dams and earthquakes presents a complex dilemma.
    • The Dilemma: Large dams are crucial for water security, irrigation, and hydropower (development), but they can pose seismic risks (environment and safety). The Koyna Dam case is a classic example that continues to be studied.
    • Policy Implication: This necessitates extremely rigorous Environmental Impact Assessments (EIA) and Social Impact Assessments (SIA) for large infrastructure projects in tectonically sensitive areas. It fuels the debate on the viability of large dams versus smaller, decentralized water management solutions.
  2. Other Human Triggers: Emerging anthropogenic causes, such as wastewater injection from oil/gas extraction (fracking), are increasing the frequency of minor to moderate earthquakes in regions previously considered seismically stable. This raises questions of corporate responsibility, regulatory oversight, and the long-term geological consequences of resource extraction.

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