Seismic Waves, Definition, Types, Formation, Shadow Zone

Seismic waves, also called Earthquake Waves, are energy waves generated by the sudden release of energy during an earthquake, volcanic activity, or explosion. They travel through the Earth’s layers, providing vital information about its internal structure. The study of seismic waves forms the foundation of seismology, which helps in understanding earthquakes, predicting hazards, and investigating the Earth’s crust, mantle, and core composition.

Seismic Waves

Seismic waves can be broadly classified into body waves and surface waves, depending on how they travel through the Earth. They are recorded using instruments called seismographs, which measure ground motion. These records, known as seismograms, help geologists locate the epicenter, determine the magnitude, and study the internal structure of the Earth through the analysis of wave speed and path variation.

Read About: Tsunami

Seismic Waves Classification

Seismic waves are categorized into two main types: Body Waves and Surface Waves, based on their mode of propagation through the Earth.

1. Body Waves

Body waves move through the interior of the Earth and are further divided into Primary (P) waves and Secondary (S) waves.

(a) Primary or P Waves

• • Also called compressional or longitudinal waves.
  • They are the fastest seismic waves, travelling at speeds between 5 to 14 km/s, depending on the medium.
  • Can travel through solids, liquids, and gases.
  • Cause particles to move parallel to the direction of wave propagation.
  • Recorded first on a seismogram, hence used to estimate the distance to the epicenter.
  • Types-
    • Pn Waves: Travel along the upper mantle and are refracted back to the surface.
    • Pg Waves: Move through the Earth’s crust, often recorded in near-surface seismic studies.
    • Pp Waves: Travel deeper through the mantle, providing insights into mantle structure.

(b) Secondary or S Waves

• • Also known as shear waves or transverse waves.
  • Travel slower than P waves, with speeds of 3-8 km/s.
  • Can move only through solid materials, not liquids or gases, because they require rigidity to propagate.
  • Cause particles to move perpendicular to the direction of wave propagation.
  • Their absence in the Earth’s outer core proved the liquid nature of the outer core.
  • Types-
    • Sn Waves: Travel through the upper mantle, refracted similarly to Pn waves.
    • Sg Waves: Move within the crust, detected in shallow seismic surveys.
    • ScS Waves: Reflect from the outer core, confirming its liquid nature since S-waves cannot pass through liquids.

2. Surface Waves

Surface waves travel along the Earth’s surface and are responsible for most of the destruction during earthquakes. They are slower than body waves but have higher amplitudes. Surface waves are typically last to arrive but cause maximum damage, especially in densely built urban areas.

(a) Love Waves

• Move in a horizontal, side-to-side motion.
• Cause ground shaking and structural damage to buildings.
• Named after British mathematician A.E.H. Love (1911).

(b) Rayleigh Waves

• Move in a rolling motion, similar to ocean waves.
• Cause both vertical and horizontal ground movement.
• Named after Lord Rayleigh, who first mathematically described them.

‹ ›

Seismic Waves Formation Process

Seismic waves are generated when energy stored in the Earth’s crust is suddenly released due to a rupture or displacement along a fault line. This energy release causes vibrations that propagate through the Earth’s layers in all directions. Stages in the Formation Process:

1. Stress Accumulation: The tectonic plates of the Earth are in constant motion. Over time, immense stress builds up along faults due to plate interactions such as collision, subduction, or lateral movement.
2. Elastic Deformation: Rocks near the fault zone deform elastically as stress increases, storing potential energy within the rock layers.
3. Rupture and Energy Release: When the stress exceeds the elastic limit of rocks, they break suddenly, releasing the accumulated energy in the form of seismic or earthquake waves.
4. Propagation of Seismic Waves: The released energy travels outward from the focus (hypocenter), where the rupture starts. The epicenter is the point directly above the focus on the Earth’s surface.
   • Body Waves (P and S) travel through the Earth’s interior.
   • Surface Waves (Love and Rayleigh) travel along the crust and cause maximum damage.

According to the Elastic Rebound Theory proposed by H.F. Reid (1911), earthquakes result from the elastic strain energy that is suddenly released when rocks fracture and return to their original shape.

Emergence of Shadow Zone

A shadow zone is a region on the Earth’s surface where no direct seismic waves from a particular earthquake are detected. It occurs due to refraction, reflection, and absorption of seismic waves as they travel through layers of different densities within the Earth. The study of shadow zones was instrumental in discovering the liquid nature of the outer core and the solid inner core.

P-Wave Shadow Zone

1. Nature of P-Waves: P-waves (Primary waves) can travel through both solids and liquids, but their speed and direction change when they pass through different mediums.
2. Reason for Shadow Zone Formation:
   • When P-waves pass from the mantle (solid) to the outer core (liquid), they slow down and refract sharply due to a sudden drop in density and rigidity.
   • This refraction bends the waves away from a certain region on the opposite side of the Earth, creating a shadow zone.
3. Shadow Zone Range:
   • Found between 103° and 142° angular distance from the earthquake’s epicenter.
   • Within this zone, no direct P-waves are recorded on seismographs.
4. Scientific Significance: The existence of the P-wave shadow zone helped scientists confirm the liquid composition of the outer core and estimate its depth (~2,900 km).

S-Wave Shadow Zone

1. Nature of S-Waves: S-waves (Secondary or shear waves) can only move through solid materials; they cannot propagate through liquids or gases.
2. Reason for Shadow Zone Formation:
   • As S-waves reach the liquid outer core, they are completely absorbed or stopped because liquids do not support shear stress.
   • This creates a large zone where no S-waves are recorded.
3. Shadow Zone Range:
   • Found beyond 103° from the epicenter on both sides of the globe.
   • Covers almost 40% of the Earth’s surface, making it larger than the P-wave shadow zone.
4. Scientific Significance: This absence of S-waves provided conclusive proof that the outer core is in a liquid state.

Surface Wave Shadow Zone

1. Nature of Surface Waves: Surface waves (Love and Rayleigh) travel only along the Earth’s crust and lose energy rapidly with depth.
2. Reason for Shadow Zone Formation:
   • Since surface waves are confined to the outermost layer, they do not penetrate the Earth’s interior.
   • The curvature of the Earth and attenuation of energy with distance cause limited propagation around the globe.
3. Shadow Zone Range:
   • Surface waves typically dissipate after traveling a few thousand kilometers.
   • Beyond this range, their amplitude drops below detectable levels, forming a practical shadow zone rather than a fixed angular one.
4. Scientific Significance: Surface wave attenuation patterns are used to assess local crustal structure and earthquake intensity distribution, crucial for seismic hazard mapping.

Read About: Continental Drift Theory

Seismic Waves Properties

The major properties and features of the Seismic Wave has been discussed here:

1. Velocity Variation: The speed of seismic waves depends on the density and elasticity of the medium.
   • Higher density and elasticity = higher velocity.
   • P-waves generally travel faster than S-waves.
2. Refraction and Reflection: Seismic waves bend (refract) and bounce (reflect) at boundaries between layers of different densities, helping scientists study internal Earth layers.
3. Attenuation: Wave energy decreases with distance due to absorption and scattering in the medium.
4. Shadow Zones:
   • P-wave shadow zone: Between 103°-142° from the epicenter.
   • S-wave shadow zone: Beyond 103°, as S-waves cannot pass through the liquid outer core.
     These zones were crucial in discovering the layered structure of the Earth.

Seismic Waves in Earth’s Interior

The study of seismic waves has helped scientists understand the three major layers of the Earth- crust, mantle, and core.

• Crust: The outermost layer where seismic waves first originate. P and S waves travel fastest through the continental crust.
• Mantle: The layer below the crust, extending to about 2,900 km deep. Seismic wave velocity increases here due to higher density and rigidity.
• Core: Divided into an outer liquid core (where S-waves disappear) and a solid inner core (where P-waves speed up).

This forms the basis of the seismic discontinuities, these are boundary layers inside the Earth where seismic wave velocity suddenly changes due to variations in composition and density. These discontinuities together reveal Earth’s layered internal structure and are essential for understanding plate tectonics and seismic behavior.

• Mohorovičić Discontinuity (Moho): Discovered by Andrija Mohorovičić in 1909, it marks the boundary between the crust and mantle, found at depths of 30-50 km beneath continents and 5-10 km under oceans.
• Gutenberg Discontinuity: Located at about 2,900 km depth, it separates the mantle and the liquid outer core. P-waves slow sharply, while S-waves disappear completely here, confirming the outer core’s liquid nature.
• Lehmann Discontinuity: Identified by Inge Lehmann in 1936, it lies around 5,100 km deep, marking the boundary between the liquid outer core and the solid inner core, where P-wave velocity increases again.

Seismic Waves Measurement

The analysis and measurement of the Seismic Waves can be done by using the below listed instruments and methodologies:

1. Seismograph: A seismograph is an instrument that records seismic waves. It consists of a mass-spring system that detects ground motion and produces a seismogram, showing the amplitude and frequency of waves.
2. Richter Scale: Developed by Charles F. Richter (1935), it measures the magnitude of an earthquake based on the amplitude of seismic waves. It is logarithmic in nature, each whole number increase represents a tenfold increase in amplitude.
3. Moment Magnitude Scale (Mw): Adopted by seismologists as a more accurate measure of total energy released, replacing the Richter scale for large quakes.
4. Modified Mercalli Intensity (MMI) Scale: Assesses the intensity of shaking and damage caused by an earthquake, ranging from I (not felt) to XII (total destruction).

Seismic Waves Monitoring in India

India has set up various organizations and bodies for the monitoring of the Seismic Waves. According to NCS Annual Report 2023, India recorded 109 seismic events above magnitude 4.0, with the Himalayan region being the most active zone due to plate tectonics.

1. National Center for Seismology (NCS)

• Operates under the Ministry of Earth Sciences (MoES).
• Maintains a network of 155 seismic stations across India.
• Provides real-time earthquake monitoring and epicenter mapping.
• Data from NCS is used for earthquake hazard zoning and early warning systems.

2. Indian Meteorological Department (IMD)

• Started earthquake observation in 1898.
• Currently operates 115 digital broadband seismograph stations nationwide.
• Issues alerts for seismic activity through its Earthquake Monitoring Network.

3. Indian National Centre for Ocean Information Services (INCOIS)

• Monitors undersea earthquakes and provides tsunami early warnings, especially for the Indian Ocean region.

Seismic Zones in India

India is divided into four seismic zones based on the frequency and intensity of earthquakes (as per Bureau of Indian Standards, IS 1893:2016). More than 58% of India’s land area is prone to moderate to severe seismic hazard, according to the National Disaster Management Authority (NDMA).

Seismic Waves Applications

The various applications for analysis and measurement of the Seismic Waves has been given below

1. Earthquake Prediction and Hazard Assessment: Seismic data help assess fault lines and potential earthquake-prone zones.
2. Oil and Mineral Exploration: Reflection seismology is used by ONGC and GSI to locate oil, gas, and mineral deposits.
3. Nuclear Test Monitoring: Seismic waves detect underground nuclear tests, monitored by Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO).
4. Structural Engineering: Seismic designs are based on expected ground motion patterns from wave studies.
5. Geophysical Research: Helps map subsurface structures for understanding plate tectonics and crustal deformation.

Seismic Waves Study Challenges

Despite numerous advancements, the study for Seismic Waves face several challenges and criticisms:

• Lack of Dense Monitoring Networks: Sparse coverage in rural and hilly areas affects real-time data accuracy.
• Unpredictable Nature: Despite advancements, precise earthquake prediction remains scientifically uncertain.
• Data Sharing Barriers: Limited international collaboration restricts cross-border data exchange in seismically active regions.
• Infrastructure Vulnerability: Non-compliance with building codes increases disaster risks.

Way Forward:

• Strengthen Seismic Networks: Expand digital seismograph coverage in the Himalayas and northeast India.
• Promote Research and Collaboration: Encourage partnerships with USGS, JMA, and European seismic agencies.
• Integrate AI and Big Data: Use machine learning for real-time pattern recognition in seismic signals.
• Public Awareness and Preparedness: Conduct earthquake drills, school education, and building code enforcement.
• International Coordination: Engage in global seismic observation initiatives for data sharing and rapid response.

Seismic Waves UPSC

The Seismic Waves Study has elaborated through several recent advancements. These developments aim to make India a global leader in earthquake risk management and seismic resilience.

• NCS and IIT Roorkee Collaboration (2023): Established an advanced Seismic Microzonation Framework for Himalayan urban centers.
• NDMA’s 2024 Earthquake Risk Mitigation Project: Targets 50 high-risk cities with improved structural safety audits.
• ISRO’s Remote Sensing Support: Provides satellite-based crustal deformation mapping.
• India’s inclusion in the Global Seismographic Network (GSN): Strengthens international data cooperation.