What are Seismic Waves?
Seismic Waves are caused by a sudden movement of materials within the Earth, such as a fault slip during an earthquake. An Earthquake is caused by the sudden release of energy built between crustal plates This energy is released in the form of waves that travel throughout the earth’s interior. These waves generated by an earthquake are called seismic waves.
These seismic waves travel through the earth in the form of vibrations. The vibrations are recorded and measured by an instrument called the seismometer. The seismometer produces a graph called a seismogram which shows these vibrations. These waves travel through different materials at different speeds. Scientists have applied the knowledge of how they interact with different materials to understand the earth’s internal structure.
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Seismic Waves Types
Seismic Waves are broadly classified into two types, viz. Body waves and Surface waves. This classification is based on the place of travel of the seismic waves. The body waves originate from inside the earth and travel through different layers of the earth. When these waves react with rocks on the earth’s surface, they produce surface waves. The surface waves travel on the earth’s surface.
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The body waves are further divided into two types, Primary Waves, and Secondary Waves. They are called primary or secondary based on when they are recorded at the seismometer.
These waves are the fastest-travelling seismic waves; they attain a speed of 5 to 14 km per second. They are the first to reach the earth’s surface. Hence, they are called Primary waves or “P” waves. The waves travel through a medium by compression and expansion. As a result, they push and pull the material as they travel through it, leading to a change in volume and density. Due to this type of movement, the P waves are also called compressional waves. When a particle is subjected to a P wave, they move in the same direction as the wave is travelling in.
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These waves have a speed lower than P waves, around 3.5-7.2 km per second. They move in up and down motion, perpendicular to the wave direction. Thus, they are also called transverse waves. They deform and change the material through which they pass. Hence, they are also called shear waves.
They are also called Long Period Waves or “L” waves because they cover the longest distances of all seismic waves. They are the last to reach the earth’s surface. They only affect the surface of the earth and act obliquely. They are the most violent and destructive of all seismic waves.
There are two types of surface waves: Rayleigh waves and Love waves. Love waves have been named after A.E.H. Love, a British mathematician who first predicted their existence in 1911. It is the fastest surface wave. It moves the ground from side to side. Rayleigh waves have been named after Lord Rayleigh, who discovered them in 1885. These waves roll over the ground like waves in oceans or seas, causing the ground to move in an elliptical motion.
Seismic Waves Diagram
For a better understanding of the different types of Seismic Waves, here we are providing with you a Seismic Waves Diagram:
Seismic Waves Shadow Zones
By studying how different waves and seismic waves travel through the earth, a lot of information is collected about the earth’s interior. If the earth was composed of homogenous materials, the seismic waves would have travelled in straight lines. However, this is not the case. As the seismic waves move through the earth’s interior, they are reflected and refracted. Seismic waves help us understand the earth’s interior:
- The P waves were the first to be recorded in the seismometer.
- The P waves travelled through the entire earth’s interior.
- No P waves were recorded in the seismometer between 105 degrees to 145 degrees from the epicentre of an earthquake.
- After 140 degrees, the P waves were again recorded in the seismometer.
- The S waves were recorded after the P waves in the seismometer
- No S waves were recorded in the seismometer between 105 degrees to 145 degrees from the epicentre of an earthquake.
- The S waves never resurfaced again.
Based on the above observations, i.e., the absence of S waves beyond 104 degrees and the slowing of the P waves, the scientists concluded that the outer core is liquid.
Beyond 145 degrees, the P waves again emerged. The speed of the P wave also increased. This helped scientists conclude that the inner core is solid.
Note: The zone where seismic waves from an earthquake are not recorded in the seismometer is known as the shadow zone.
Seismic Waves Shadow Zone Diagram
Below is the Seismic Waves Shadow Zone Diagram for a better understanding:
Comparison Among Primary, Secondary & Surface Seismic Waves
Below is the comparison of Primary, Secondary & Surface Seismic Waves:
|Criteria||Primary Waves||Secondary Waves||Surface Waves|
|Time to reach the Earth’s Surface||They are the first to strike the crust of the earth.||After the primary wave, the secondary wave reaches the earth’s crust.||Reach the earth’s crust last.|
|Wavelength||The wavelength is very short in nature.||The wavelength is of medium size.||The wavelength is the longest in size.|
|Direction||It strikes a parallel direction on the structure of rock where tremor occurs.||It strikes at a right angle. Hence it acts obliquely.||It also acts obliquely.|
|Movement through Different Materials||They can move through solid, liquid and gaseous||It can move only through solid substances.||It can move only through solid materials.|
|Speed||It has the highest speed- about 5 to 14 km per second.||The speed is lower than the ‘P’ wave- 3.5-7.2 km per second.||t moves slower than ‘P’ and ‘S’ waves- about 3 to 5 km per second.|
Seismic Waves Disaster
Seismic Waves cause disasters named Tsunamis. Tsunamis are oceanic gravity waves caused by geological processes such as earthquakes, landslides, or Volcanic Eruptions. Large shallow earthquakes along subduction zones cause the majority of tsunamis. Tsunami is a Japanese word that means “harbour wave,” and it is also known as a seismic sea wave or, incorrectly, a tidal wave.
Tsunamis are usually small and unnoticed in the deep sea, but they grow large and cause damage when they approach coasts or harbours. The 2004 Indian Ocean tsunami demonstrated that a tsunami can travel trans-oceanic distances and cause damage far from its source.
Seismic Waves UPSC
In simple terms, an earthquake is the shaking of the earth. It is a natural occurrence. It is caused by the release of energy, which causes waves to propagate in all directions.
Seismic waves are vibrations generated by earthquakes as they travel through the Earth and are recorded on seismographs.
The hypocenter is the location beneath the earth’s surface where the earthquake begins, and the epicentre is the location directly above it on the earth’s surface.
A UPSC aspirant should be well versed in the topic of Seismic Waves and Types from both Prelims and Mains points of View. The details in the article would help candidates prepare for UPSC 2023.