Earthquake - UPSC Geography Notes

Earthquake – UPSC Geography Notes

Earthquake represents a natural occurrence triggered by the abrupt release of energy within the Earth’s crust, leading to ground shaking and trembling. These occurrences are primarily instigated by the movement of the Earth’s tectonic plates and are a familiar feature in the realm of geology. Find out more about this natural phenomenon here.

Earthquakes, much like volcanic activities, exemplify sudden shifts emerging from the Earth’s interior. While we’ve already delved into the various types of volcanoes, it’s crucial to comprehend what exactly constitutes an earthquake.

In simple terms, an earthquake refers to the seismic shaking of the Earth’s surface. It is essentially a sudden and intense trembling of the ground, stemming from movements within the Earth’s crust or volcanic disturbances.

These movements lead to the discharge of energy along a fault line, inducing seismic vibrations. Comparable to volcanoes, earthquakes are categorized as a type of endogenic process.

  • Seismograph networks worldwide detect numerous daily earthquakes, with most being imperceptible minor quakes.
  • Severe earthquakes are generally localized to specific global regions.
  • The point of origin within the Earth’s crust is known as the focus hypocenter or seismic focus, typically situated at a depth of around 6 kilometers.
  • The corresponding point directly above the focus on the Earth’s surface is termed the epicenter, where the earthquake’s intensity peaks and diminishes with distance.
  • All-natural earthquakes occur within the lithosphere, which encompasses the Earth’s crust and the rigid upper mantle.
Earthquake

Types of Earthquakes

  • Tectonic earthquakes:
    • These are the most common type of earthquakes, arising from the movement of fragmented land masses on the Earth’s crust known as tectonic plates.
  • Volcanic earthquakes:
    • Comparatively less frequent than tectonic earthquakes, these occur before or after a volcanic eruption. They result from the interaction between magma leaving the volcano and rocks being pushed towards the surface.
  • Collapse earthquakes:
    • These earthquakes take place in underground mines, primarily caused by pressure buildup within the rocks.
  • Explosion earthquakes:
    • Artificial in nature, this kind of earthquake arises from high-density explosions, such as nuclear explosions, and does not occur naturally.

Causes of Earthquakes

Earthquakes predominantly result from the shifting of tectonic plates beneath the Earth’s surface. These plates, sizable segments of the Earth’s crust, float atop the semi-fluid mantle layer.

There exist three primary types of plate boundaries where earthquakes frequently occur:

  • Divergent Boundaries: Plates separate from one another, generating tension that frequently leads to earthquakes.
  • Convergent Boundaries: Plates converge, creating compression forces that trigger earthquakes.
  • Transform Boundaries: Plates slide horizontally alongside one another, producing shear stress and causing earthquakes.

Earthquake waves or Seismic waves

  • An earthquake stemming from the lithosphere gives rise to distinct seismic waves, commonly known as earthquake waves.
  • These seismic waves can be broadly categorized into two types: body waves and surface waves.

Body waves

  • Originating from the energy release at the focus, they propagate in all directions through the Earth’s body, hence termed body waves.
  • Their travel path is restricted to the Earth’s interior, distinguishing them from surface waves.
  • Being faster, they are usually the initial waves detected on a seismograph.
  • The two main categories of body waves are primary waves (P-waves) and secondary waves (S-waves).

Primary waves (p-waves)

  • Primary waves (P-waves) are the swiftest body waves, reaching their destination first during an earthquake (twice as fast as S-waves).
  • Similar to sound waves, they are longitudinal waves where particle movement aligns with the direction of wave propagation.
  • They can travel through solid, liquid, and gaseous materials, showcasing their versatility.
  • P-waves induce density discrepancies within Earth’s materials, resulting in stretching and compression.
Primary waves (p-waves)

Secondary waves (s-waves)

  • They reach the surface with a delay after the primary waves.
  • S-waves are comparatively slower than primary waves and can traverse solely through solid materials.
  • Seismologists deduced the liquid state of the Earth’s outer core due to the absence of S-waves beyond 105 degrees from the epicenter.
  • S-waves are transverse waves where the direction of particle movement is perpendicular to the direction of wave propagation.
Secondary waves (s-waves)

Surface Waves

  • When body waves interact with surface rocks, they give rise to a distinct set of waves known as surface waves.
  • These waves travel along the Earth’s surface, creating crests and troughs in the materials they traverse.
  • Surface waves, like their counterparts, are transverse waves where particle movement is perpendicular to wave propagation.
  • Among the various seismic waves, surface waves are recognized as the most destructive.
  • Love waves and Rayleigh waves are two well-known examples of surface waves.

Love waves

  • Responsible for the lateral shifting of the earth’s surface during an earthquake, Love waves are a type of surface wave.
  • While they are slower than body waves, they are faster than Rayleigh waves.
  • Love waves specifically require the presence of a semi-infinite medium covered by a finite upper layer.
  • Confined to the crust’s surface, Love waves induce exclusively horizontal motion.
Love waves

Rayleigh waves

  • Following an elliptical path, Rayleigh waves exhibit a rolling motion along the ground, akin to waves on a lake or an ocean.
  • As it rolls, the Rayleigh wave induces vertical and horizontal movement in the same direction as the wave’s propagation.
  • The significant shaking experienced during an earthquake is primarily caused by the Rayleigh wave, often exceeding the magnitude of other waves.
Rayleigh waves

Shadow regions of waves

  • As previously mentioned, P-waves traverse through all materials, whereas S-waves solely propagate through solid substances.
  • Leveraging these characteristics of primary waves, seismologists have gained valuable insights into the Earth’s interior.
  • Despite their ability to travel through all materials, P-waves experience reflection when transitioning from one medium to another.
  • Seismograph records are used to deduce the alterations in wave direction.
  • Regions where seismographs fail to detect any waves are termed the ‘shadow zone’ of that wave.
  • Specifically, it has been observed that beyond 105 degrees, S-waves do not reach, while between 105 and 140 degrees, P-waves do not propagate.
Shadow regions of waves

Measuring Earthquake

  • Seismometers are devices utilized to measure ground motion, including seismic waves generated by earthquakes, volcanic eruptions, and other seismic events.
  • While the term seismograph is also used interchangeably with seismometer, it typically refers to older instruments.
  • The graphical output recorded by a seismometer or seismograph is known as a seismogram. (Note: Do not confuse seismograph with seismogram. A seismograph is an instrument while a seismogram is the recorded output).
  • Two primary scales employed in seismometers are the Mercalli Scale and the Richter Scale.

Mercalli Scale:

This scale assesses the earthquake’s intensity by examining its aftermath, including factors such as the number of people affected and the extent of destruction. Intensity ranges from 1 to 12.

Richter Scale:

The Richter Scale measures the earthquake’s magnitude, expressed in absolute numbers from 1 to 10. Each whole number increase on the Richter scale corresponds to a tenfold increase in the earthquake’s power.

Distribution of Earthquakes

  • The global distribution of earthquakes closely mirrors that of volcanoes, indicating a strong correlation between the two phenomena.
  • The Circum-Pacific regions exhibit the highest seismic activity, with epicenters and frequent occurrences clustered along the ‘Pacific Ring of Fire.’
  • Approximately 70% of earthquakes are recorded within the Circum-Pacific belt.
  • An additional 20% of earthquakes transpire in the Mediterranean-Himalayan belt, encompassing areas such as Asia Minor, the Himalayas, and sections of northwest China.
  • The remaining earthquakes occur within plate interiors and at spreading ridge centers.
Distribution of Earthquakes

Earthquake in India

  • Mild-intensity earthquakes occur daily, whereas severe tremors causing widespread destruction are less frequent.
  • Earthquakes are more common along plate boundaries, especially at convergent boundaries.
  • The Himalayan region in India, where the Indian Plate and the Eurasian Plate converge, is particularly vulnerable to earthquakes.
  • The peninsular part of India is generally considered stable, but occasional earthquakes occur along the margins of minor plates, such as the Koyna earthquake of 1967 and the Latur earthquake of 1993.
  • Indian Seismology experts have divided India into four seismic zones: Zone-II, Zone-III, Zone-IV, and Zone-V.
  • The highest and high-risk seismic zones (Zone V and IV) include the entire Himalayan region, the states of North-East India, Western and Northern Punjab, Haryana, Uttar Pradesh, Delhi, and parts of Gujarat.
  • Moderate-risk zones encompass the remaining parts of the northern plains and western coastal areas.
  • A significant part of the peninsular region falls into the low-risk seismic zone.
Earthquake in India

Earthquake Fault Types

  • Normal, reverse (thrust), and strike-slip faults represent the three main fault types that can lead to interplate earthquakes.
  • Dip-slip faulting involves normal and reverse faulting, with vertical component movement and displacement occurring within the fault plane.
  • Normal faults typically emerge along divergent boundaries or regions experiencing crustal extension.
  • Reverse faults form in areas where the crust is undergoing compression, such as near convergent boundaries.
  • Strike-slip faults, which are often seen at transform boundaries, involve horizontal movement where the opposing sides of the fault slide past each other. Earthquakes frequently arise from faults exhibiting both dip-slip and strike-slip movement components.

Shallow-focus and Deep-focus Earthquakes

  • The majority of tectonic earthquakes originate within the Ring of Fire, with focal depths typically reaching up to ten kilometers.
  • Shallow-focus earthquakes occur with focal depths below 70 km, while mid-focus earthquakes range from 70 to 300 km.
  • Deep-focus earthquakes can occur at significantly greater depths, ranging from 300 to 700 km, within subduction zones. In these regions, older and colder oceanic plates submerge beneath another tectonic plate.
  • The extreme temperature and pressure within these regions cause deep-focus earthquakes to transpire, often beyond the point where the subducted lithosphere remains brittle.
  • One potential mechanism for the occurrence of deep-focus earthquakes involves faulting induced by olivine undergoing a phase transformation into a spinel structure.

Wadati–Benioff zone

Wadati–Benioff zone

Earthquake Clusters

  • The majority of earthquakes are interrelated spatially and temporally, often appearing in succession.
  • There is a notion that earthquakes might recur in a predictable sequence, yet many earthquake clusters primarily consist of minor tremors that cause negligible to no damage.

Aftershocks

  • Aftershocks, subsequent earthquakes following the mainshock, are a common occurrence.
  • Sources of aftershocks include the crust readjusting around the ruptured fault line, rapid shifts in stress among rocks, and stress from the initial earthquake.
  • Despite being smaller in size, aftershocks can still cause further damage to structures already affected by the mainshock.
  • If an aftershock surpasses the mainshock in magnitude, the original mainshock is classified as a foreshock, and the subsequent aftershock is considered the mainshock.
  • Aftershocks result from the adaptation of the crust surrounding the shifted fault plane to the effects of the mainshock.

Earthquake Swarms

  • Earthquake swarms denote a series of earthquakes occurring in a specific location within a short time frame.
  • Unlike earthquakes followed by a series of aftershocks, no individual earthquake within the swarm is distinctly the main shock, as none exhibits a notably higher magnitude than the others.
  • When multiple earthquakes simultaneously impact a fault, it is referred to as an earthquake storm.
  • Each cluster of earthquakes in the storm results from the shifting or stress reorganization triggered by the preceding earthquakes.
  • Similar to aftershocks but occurring on neighboring fault segments, these earthquake storms can persist over several years, with some of the later earthquakes being equally as destructive as the earlier ones.

Consequences of Earthquake

Damage to human life and property

  • The displacement of the earth’s crust vertically and horizontally leads to significant damage and devastation to human infrastructure and buildings.
  • Example: The Nepal earthquake of 2015 serves as an urban disaster case study. This 7.8 magnitude earthquake occurred at a depth of 8.2 kilometers.
  • Due to factors such as poorly planned urban construction, inadequately designed buildings, and non-scientific structural designs, the Nepal earthquake resulted in a significant loss of life and property.
  • Urban regions, particularly Kathmandu, experienced severe destruction, resulting in a death toll of approximately 8,000 people and an estimated economic loss of 10 billion USD.

Landslides and Avalanches

  • Earthquakes, particularly in mountainous regions, can induce slope instability and failure, resulting in debris descending the slopes and triggering landslides.
  • The impact of tremors can cause large masses of ice to cascade down snow-covered peaks, resulting in avalanches.
  • Example: The Nepal earthquake of 2015 led to multiple avalanches on and around the Mount Everest peak.
  • The Sikkim earthquake of 2011 resulted in landslides and significant damage to life and property, notably affecting the Singik and Upper Teesta hydel projects.

Floods

  • Earthquakes can result in catastrophic disruptions to dams and reservoirs, potentially leading to flash floods.
  • Landslides and avalanches triggered by earthquakes have the potential to obstruct river courses, consequently causing floods.
  • Example: The Assam earthquake of 1950 created a barrier in the Dihang River due to the accumulation of substantial debris, resulting in flash floods in the upstream section.

Tsunami

  • Tsunamis are massive waves generated by the disruption of the ocean basin and the displacement of a significant volume of water, often caused by seismic waves from an earthquake.
  • Example: The Indian Ocean Tsunami of December 26, 2004, resulted from an earthquake off the coast of Sumatra, caused by the subduction of the Indian plate beneath the Burmese plate. It caused the loss of approximately 240,000 lives in countries around the Indian Ocean.
  • Fukushima Nuclear Accident: The colossal Tohoku earthquake in Japan in 2011 triggered tsunami waves measuring 10 meters, which stemmed from an undersea earthquake with a magnitude of 9. This event led to the destruction of emergency generators cooling the reactors and resulted in a nuclear meltdown, with the radioactive fallout from the Fukushima Daiichi plant causing global concerns.

Earthquake Management

  • Earthquake management involves the coordination and utilization of resources to address all humanitarian aspects of emergencies caused by earthquakes.
  • Its primary objective is to mitigate the detrimental impacts of such disasters.
  • The process of earthquake management encompasses activities ranging from pre-earthquake risk reduction measures to post-earthquake recovery efforts.
    • Risk Recognition: Identifying regions more susceptible to earthquakes is the initial step in earthquake management.
    • Earthquake Monitoring System/Early Warning System: Accurately forecasting earthquake occurrences remains a challenge, but seismologists are increasingly focusing on enhancing earthquake forecasting capabilities to reduce disaster impacts.
      • Example: Japan has implemented an earthquake early warning system utilizing electronic signals that propagate faster than earthquake waves.
    • Structural Solutions: Past earthquake data emphasizes that over 95% of casualties were attributed to the collapse of non-earthquake-resistant structures. Constructing earthquake-resistant buildings tends to be more expensive, posing a challenge for countries like India. Implementing seismic strengthening requires prioritizing structures, and this necessitates the establishment of earthquake hazard maps for different vulnerable zones.
Earthquake Management

Induced Seismicity

  • Tectonic plate movement serves as the primary cause of earthquakes, although human activities can also induce seismic events.
  • Activities such as constructing reservoirs, mining resources like coal or oil, and subsurface fluid pumping for waste disposal or hydraulic fracturing can modify the stress and strain on the Earth’s crust, resulting in minor-magnitude earthquakes.
  • Several earthquakes in Oklahoma over the last century have been linked to the state’s oil sector, with the 5.7 magnitude earthquake in 2011 believed to have been triggered by the disposal of wastewater from oil extraction into injection wells.
  • While the connection between the 2008 Sichuan earthquake and the loading from the Zipingpu Dam has not been definitively established, a report from Columbia University has made this assertion.

Earthquake Prediction

  • The field of earthquake prediction within seismology involves identifying the potential date, location, and magnitude of future earthquakes within predetermined ranges.
  • Various methodologies exist for determining the timing and location of potential earthquakes.
  • Seismologists have dedicated extensive research to this area, yet it remains scientifically unfeasible to make precise predictions for a specific day or month.

Earthquake Forecasting

  • Although forecasting is often distinct from prediction, prediction is commonly considered a form of forecasting.
  • Earthquake forecasting primarily involves probabilistic assessment of the overall earthquake risk, including the frequency and magnitude of potentially destructive earthquakes in a specific region over extended periods, spanning years or decades.
  • Well-characterized faults allow for the calculation of the likelihood of a segment rupturing within the next several decades.
  • The development of earthquake warning systems enables individuals within the system’s area to seek shelter before feeling the effects of the earthquake, as these systems can notify a region of an ongoing earthquake before the ground starts moving.

Earthquake Preparedness

  • The aim of earthquake engineering is to predict the impact of earthquakes on buildings and infrastructure and to design them in a way that minimizes the risk of damage.
  • Seismic retrofitting is a technique used to modify existing buildings to enhance their earthquake resistance.
  • Obtaining earthquake insurance can help building owners safeguard their finances from earthquake-related losses.
  • Governments or organizations can implement emergency management strategies to mitigate risks and prepare for potential consequences.
  • The Igor expert system, integrated into a portable laboratory, streamlines the processes involved in the seismic analysis of masonry structures and the design of retrofitting measures.
  • Individuals can take precautionary measures such as securing water heaters and large objects, identifying utility shutoffs, and learning how to respond when an earthquake occurs.
  • Earthquake preparedness includes considering the risk of a tsunami triggered by a significant earthquake in regions situated near large bodies of water.

FAQs of Earthquake

Q: What are the different earthquake zones in India as per the UPSC syllabus?

A: According to the UPSC syllabus, India is divided into four seismic zones, namely Zone II, Zone III, Zone IV, and Zone V, based on the intensity of earthquakes in different regions.

Q: What are the different types of earthquakes as per the UPSC syllabus?

A: The UPSC syllabus covers three main types of earthquakes: tectonic earthquakes, volcanic earthquakes, and collapse earthquakes, which occur due to different geological processes.

Q: What are the essential components of an earthquake diagram?

A: An earthquake diagram typically includes the focus, epicenter, seismic waves, and various geological layers, representing the occurrence and propagation of seismic activity in a visual format.

Q: What is the concept of seismic gap as per the UPSC syllabus?

A: The concept of seismic gap refers to the segment of an active fault line that has not experienced a significant earthquake over a considerable period, indicating a potential for future seismic activity.

Q: What is the earthquake shadow zone in the context of the UPSC syllabus?

A: The earthquake shadow zone refers to the specific area on the Earth’s surface where seismic waves from a particular earthquake are not detected, indicating the presence of an outer core that refracts seismic waves.

Q: What are the significant earthquake events in the history of India as per the UPSC syllabus?

A: The UPSC syllabus may cover major historical earthquakes in India, such as the 2001 Gujarat earthquake, the 2004 Indian Ocean earthquake, and the 1934 Bihar-Nepal earthquake, among others.

Q: What is the concept of seismic discontinuity in the context of the UPSC syllabus?

A: Seismic discontinuity refers to the abrupt changes in the velocity of seismic waves as they pass through different layers of the Earth’s interior, providing crucial insights into the Earth’s composition and structure.

Q: What type of multiple-choice questions (MCQs) can be expected in the UPSC examination regarding earthquakes?

A: MCQs in the UPSC examination may cover topics such as the Richter scale, earthquake-resistant building techniques, historical seismic events, types of seismic waves, and the geological causes of earthquakes.

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