Earthquake waves Upsc – p-waves, s-waves and Surface Waves

Earthquake Waves

Earthquake Waves are or seismic waves are the vibrations generated by an earthquake or similar energetic sources such as explosions. These are propagated within the Earth or along its surface.

Earthquake waves - P-waves and S-waves Upsc
Earth quake in Haiti By UN Photo/Logan Abassi United Nations Development Programme – originally posted to Flickr as Haiti Earthquake, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=8985699

All the earthquakes take place in the Lithosphere. A seismograph device records the waves that reach the surface.

Earthquake waves are of two types they are body waves and surface waves. The body waves are the one which is generated by the release of energy at the focus and move in all direction by traveling through the body of the earth.

These body waves interact with the surface rocks and generate a new set of waves called surface waves.

The speed of the waves varies as it passes via different densities of materials. The speed of waves is faster in the case of denser material.

The wave’s direction changes when the waves get reflected or refracted when tries to pass through the material with varying densities.

Waves of earthquake
body and surface waves Earthquake waves – P-waves and S-waves Upsc

There are two types of body waves: P and S-Waves.

P-Waves

P- Waves move faster as it the one which first arrives at the surface. This wave is similar to sound waves as they travel through gas, liquid, and solid materials.

S-Waves

S-Waves arrive at the surface after P-waves. This wave travels only through solid materials. This characteristic of S-waves that it travels only through solid materials is an important thing as it helped the scientist to understand the interior of the earth.

The reflection causes the waves to rebound and refraction makes the waves scatter in all directions.

This variation in the direction of waves is studied with help of a seismograph. The surface waves are last to get recorded in the Seismograph. The Surface waves are the most dangerous as displace rocks, collapse the building structure, etc.

Magnitude and intensity of earthquake

The earthquake is scaled according to its magnitude and intensity. The Richter scale is the magnitude scale. It relates to the energy released during the quake, it is expressed in absolute numbers, 0-10.

Another Scale is the Intensity scale, where the intensity scale takes the visible damage into account. Its scale ranges from 1-12. It is named after an Italian seismologist, Mercalli.

Propagation of Earthquake Wave’s

Different types of earthquake waves travel in different manners. As they move or propagate, they cause vibration in the body of the rocks through which they pass. P-waves vibrate parallel to the direction of the wave and it exerts pressure on the material in the direction of the propagation.

As a result, it creates density differences in the material leading to stretching and squeezing of the material. The other three waves vibrate perpendicular to the direction of propagation. The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane.

Hence, they create troughs and crests in the material through which they pass. Surface waves are considered to be the most damaging waves. The emergence of Shadow Zone Earthquake waves gets recorded in seismographs located at far-off locations.

However, there exist some specific areas where the waves are not reported. Such a zone is called the ‘shadow zone’. The study of different events reveals that for each earthquake, there exists an altogether different shadow zone.

It was observed that seismographs located at any distance within 105° from the epicenter, recorded the arrival of both P and s-waves.  However, the seismographs located beyond 145° from the epicenter, record the arrival of P-waves, but not that of S-waves.

Thus, a zone between 105° and 145° from the epicenter was identified as the shadow zone for both types of waves. The entire zone beyond 105° does not receive S-waves.

The shadow zone of the S-wave is much larger than that of the P-waves. The shadow zone of P-waves appears as a band around the earth between 105° and 145° away from the epicenter.

The shadow zone of S-waves is not only larger in extent but is also a little over 40 percent of the earth’s surface.

Earthquake

It is simply the shaking of the earth. The cause of the earthquake is the release of energy, which generates waves that travel in all directions.

Causes of earthquake

The release of energy occurs along a fault. The fault is a sharp break in the crustal rocks. The rocks along the fault tend to move in opposite directions.

The overlying rock presses, the friction locks the rock together but the tendency to move overcomes the friction. Then the blocks get deformed and eventually they slide on one another abruptly and release energy, these energy waves travel in all directions.

The point where the energy is released is called the focus of the earthquake and is also called a hypocentre.

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* * All the Notes in this blog, are referred from Tamil Nadu State Board Books and Samacheer Kalvi Books. Kindly check with the original Tamil Nadu state board books and Ncert Books.
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