Seismic activity is the vibration and oscillation of the Earth’s surface. Volcanic eruptions, explosions, landslides, avalanches, and even rushing rivers can also cause seismic waves. Seismic activity or earthquake waves is one of the most important sources of information about the interior of the earth. Earthquake waves are waves of energy caused by earthquakes or an explosion. An instrument called a seismograph records the waves reaching the earth's surface which are caused by earthquakes or other sources. Seismic activity is a very important concept of the geomorphology syllabus of the UPSC exam geography syllabus. We will study the same in this article.

Seismic Waves

What is Seismic Activity?

• Seismic activity refers to the frequency, type, and size of earthquakes experienced over a period of time.
• The term can also refer to the array of earthquake occurrences in a specific area or globally.
• This activity is driven by the release of energy from sudden dislocations of segments of the Earth's crust, or by the eruption of volcanoes.
• The energy released radiates in all directions from its source, the focus, in the form of seismic waves.
• When these waves reach the Earth's surface, they can cause ground shaking, a phenomenon commonly associated with earthquakes.

How is Seismic Activity Measured?

• The main equipment for measuring earthquakes is a seismograph.
• Sensitive seismographs are used in seismic monitoring to measure ground motion caused by earthquakes or other sources.
• The seismograph records the ground motion induced by seismic waves in a digital graphic format.
• The location, origin time, and magnitude (along with other properties) of earthquakes can be determined using seismograms from seismic monitoring stations.

Seismic Waves or Earthquake Waves

• Seismic waves are the vibrations or oscillations that travel through the Earth's interior and across its surface in response to various sources of energy release, such as earthquakes, volcanic activity, or human-induced events.
• These waves carry energy and information about the Earth's subsurface and are crucial for understanding the planet's interior structure, seismic activity, and geological processes.
• These seismic waves travel as vibrations through the earth. The vibrations are recorded and measured by a device known as a seismometer.
• The seismometer generates a graph called a Seismograph that depicts these vibrations. These waves travel at different speeds through various materials.
• Earthquake waves are caused by the rapid release of energy along a fault (a sharp rupture in the crustal layer).
• The force exerted on the rock layers along a fault causes them to move in opposite directions, but they are held in place by the frictional force exerted by the above rock strata.
• Over time, the pressure on the rock strata builds up and exceeds the frictional barrier, causing an abrupt movement that generates shockwaves (seismic waves) in all directions.
• Focus: The focus or hypocentre of an earthquake is the location where the energy is released.
• Epicenter: Epicenter refers to the place on the surface directly above the focus.

• There are two main types of Seismic Waves i.e. Body Waves and Surface Waves.
  • Body waves are generated due to the release of energy at the focus and move in all directions traveling through the body of the earth.
  • Surface waves are generated when body waves interact with the surface rocks. These waves move along the surface.

The Focus of an Earthquake; and Epicentre

Seismic Waves Classification

Body Waves

• Body waves are seismic waves that travel through the Earth's interior.
• They are generated by the sudden release of energy from events such as earthquakes, volcanic eruptions, or even man-made explosions.
• Since they move through the Earth, they are instrumental in providing information about the Earth's internal structure, including layer densities, thicknesses, and states of matter.
• By analyzing the travel times and paths of these waves, seismologists can infer a lot about the Earth's interior structure and the properties of different layers.
• The variations in the speed and direction of body waves as they interact with different materials inside the Earth help scientists construct models of the Earth's interior, including the identification of the core, mantle, and crust layers.
• There are two primary types of body waves: Primary Waves (P-waves) and Secondary Waves (S-waves)

Primary Waves (P-waves)

• They are the fastest among seismic waves and thus reach the recording instruments first.
• P-waves are compressional or longitudinal waves, which means the ground particles move in the direction of the wave propagation (similar to sound waves).
• They can travel through solid, liquid, and gaseous materials, which means they can pass through the Earth's crust, mantle, outer core, and inner core.
• When they pass through the Earth's surface, they can shake the ground in the direction of their movement, often feeling like a sudden jolt during an earthquake.

Secondary Waves (S-waves)

• They are slower than P-waves and arrive at the recording instruments after P-waves.
• S-waves are shear or transverse waves, which means the ground particles move perpendicular to the direction of wave propagation.
• They can only travel through solid materials. This characteristic of S-waves has provided critical evidence that the Earth's outer core is liquid because S-waves do not pass through it.
• They cause more intense shaking than P-waves, moving the ground up and down and side-to-side, and are generally responsible for the damage and destruction during earthquakes.

Propagation of P-Waves and S-Waves

Surface Waves (L Waves)

• When body waves interact with surface rocks, a new set of waves known as surface waves is created. These waves propagate across the earth's surface.
• Surface waves are transverse waves in which particle motion is perpendicular to the propagation of the wave. As a result, they create crests and troughs in the material they pass through.
• Surface waves travel more slowly through Earth's material and are generally lower in frequency than body waves.
• Deeper earthquakes produce weaker surface waves; shallow earthquakes produce stronger surface waves.
• Wave velocity changes as they travel through different densities of materials. The higher the velocity, the denser the material.
• When they come into contact with materials of varying densities, their direction changes as they reflect or refract.
• Because they cover the greatest distances of any seismic wave, they are also known as Long Period Waves or "L" waves.
• They are the last to reach the surface of the Earth and are the most powerful and destructive seismic waves.
• Surface waves are classified into two types: Rayleigh waves and Love waves.

Rayleigh Waves

• Rayleigh waves are another type of earthquake surface wave that causes both vertical and horizontal motion of particles.
• Rayleigh waves create an elliptical, rolling motion of particles. This motion includes both vertical and horizontal components.
• Rayleigh waves travel along the Earth's surface and are typically slower than Love waves.
• Rayleigh waves can have larger amplitudes than body waves like P and S waves, contributing to ground displacement and shaking.
• The vertical motion of Rayleigh waves can cause uplift and subsidence of the ground, leading to ground cracks and surface deformations.

Love Waves

• Love waves are a type of earthquake surface wave that causes horizontal, side-to-side motion of particles parallel to the Earth's surface.
• Love waves cause particles to move in a horizontal, snake-like motion similar to the way a snake slithers across the ground.
• These waves propagate along the Earth's surface, primarily within the uppermost layers of the crust.
• Love waves can have large amplitudes, contributing to significant ground shaking during earthquakes.
• The side-to-side motion of Love waves can cause structural damage to buildings, bridges, and other infrastructure, particularly if the structures are not well-designed to withstand horizontal shaking.

Propagation of Love Wave and Rayleigh Wave

Emergence of Shadow Zone

Shadow Zone

• Seismographs installed in remote regions record earthquake waves. There are, however, some regions where the waves are not reported. The 'shadow zone' is a term used to describe such a region.
• The examination of several disasters demonstrates that each earthquake has a distinct shadow zone.
• It was discovered that seismographs placed within 105 degrees of the epicenter recorded both P and S-wave arrivals.
• Seismographs placed beyond 145 degrees from the epicenter, on the other hand, record the arrival of P-waves but not S-waves.
• As a result, the shadow zone for both types of waves was found as a zone between 105° and 145° from the epicenter.

Seismic Zones in India

• India is one of the most earthquake-prone countries because of the existence of technically active young fold mountains in the Himalayas.
• The Bureau of Indian Standards has grouped the country into four seismic zones viz. Zones-II, III, IV, and V.
• Zone V is the most seismically active region while Zone II is the least.
• The Modified Mercalli (MM) intensity, which measures the impact of the earthquakes on the surface of the earth, broadly associated with various zones is as follows:

Seismic Zones in India

• Zone-V comprises entire north-eastern India, parts of Jammu and Kashmir, Himachal Pradesh, Uttarakhand, Rann of Kutch in Gujarat, part of North Bihar and Andaman and Nicobar Islands.
• Zone-IV covers the remaining parts of Jammu and Kashmir and Himachal Pradesh, Delhi, Sikkim, the northern part of Uttar Pradesh, Bihar, West Bengal, parts of Gujarat, a small portion of Maharashtra near the west coast and Rajasthan.
• Zone-III comprises Kerala, Goa, Lakshadweep, and the remaining parts of Uttar Pradesh, Gujarat, and West Bengal, parts of Punjab, Rajasthan, Madhya Pradesh, Bihar, Jharkhand, Chhattisgarh, Maharashtra, Odisha, Andhra Pradesh, Tamil Nadu, and Karnataka.
• Zone-II covers the remaining parts of the country.

Conclusion

Seismic activity, though destructive, has been instrumental in understanding our planet's inner composition and structure. It is a natural process resulting from the dynamic movement of the Earth's tectonic plates. Understanding this activity is essential for both predicting potential earthquakes and mitigating their impacts.