P-Waves - Geography (Jigyasa)

Primary waves (P-Waves) are the first waves to arrive at a seismograph. P-waves move faster and are first to arrive at the surface hence they are the first one to recorded. P-waves travel through gaseous, liquid and solid materials. Their velocity is determined by the medium's shear strength (elasticity). P-wave is a very important concept for the UPSC exam geography syllabus. In this article, you will read about the p-wave in brief for the IAS exam. We must first study the fundamentals of seismology and seismic waves in understanding P waves.

Seismic Wave or Earthquake Wave:

• Seismic waves are energy waves that occur as a result of earthquakes or explosions. Body waves and surface waves are the two types of earthquake waves.
• Body wave are generated due to release of energy at the focus and move in all directions travelling through the body of the earth.
• These wave travel into the body of the earth.
• These waves are somewhat like sound wave.
• These are faster than surface wave.
• Primary wave (P-Wave) and secondary wave (S Wave) are the parts of body wave.
• Surface wave are generated when body waves interact with the surface rocks. These waves moves along the surface.

Formation of Seismic Wave:

• Earthquake waves are caused by the rapid release of energy along a fault (a sharp rupture in the crustal layer). 
• The force excreted 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.
• The focus or hypocentre of an earthquake is the location where the energy is released.
• Epicentre refers to the place on the surface directly above the focus.

Primary wave (P-Wave):

• Primary wave (P-Wave) and secondary wave (S Wave) are the parts of body wave.
• Body Wave:
• These wave travel into the body of the earth.
• These waves are somewhat like sound wave.
• These are faster than surface wave.
• P-waves move faster and are first to arrive at the surface hence they are the 1st one to recorded.
• P-waves travel through gaseous, liquid and solid materials. Their velocity is determined by the medium's shear strength (elasticity). Hence, the velocity of the P-waves in Solids > Liquids > Gases.
• P-wave velocity in earthquakes is in the range 5 to 8 km/s.
• They are compressional and push-pull wave.
• They move the material forward and backward in the same direction that the wave themselves are moving.
• The material through which P-wave travel is expanded and compressed as the wave move through it and return to its original shape and size after the wave passes by.
• In fact, some p-waves emerging from the earth are transmitted as a sound wave that at certain frequencies can be heard by human and animal.
• P-waves are also known as longitudinal waves because the displacement of the medium occurs in the same or opposite direction to (parallel to) the wave's propagation direction.
• P-waves cause density differences in the material, causing it to expand (rarefaction) and compress (compression).
• These waves have a low frequency and are the least destructive of the earthquake waves.
• The earth's surface trembles in an up-down motion as a result of these waves (vertical).
• P-waves are also known as pressure waves because they create pressure changes in the medium.

Why do P-waves travel faster than S-waves?

• P-waves travel at 1.7 times the speed of S-waves.
• P-waves are compression waves that apply a force in the propagation direction, allowing them to easily convey their energy through the medium and move swiftly.
• S-waves, on the other hand, are transverse waves or shear waves (the motion of the medium is perpendicular to the wave's propagation direction) and are thus more difficult to transmit through the medium.

Act as an earthquake warning:

• The detection of non-destructive primary waves, which move faster through the Earth's crust than destructive secondary and surface waves, allows for early earthquake warning.
• The time between the arrival of the P-wave and the arrival of subsequent damaging waves might range from 60 to 90 seconds, depending on the depth of focus of the earthquake (depends of the depth of the focus).

Emergence of Shadow Zone of P-Wave and S-Wave:

• 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 epicentre recorded both P and S-wave arrivals.
• Seismographs placed beyond 145 degrees from the epicentre, 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 epicentre.

Conclusion:

• To summarise, a p-wave is a disturbance that travels through a medium, transporting energy from one location to another while avoiding the transport of matter. Each medium particle is moved for a short time before returning to its original equilibrium position.

MCQs 

Question. Consider the following statement: 

1. P-waves move faster and are first to arrive at the surface hence they are the 1st one to recorded.
2. P-waves travel through gaseous, liquid and solid materials.
3. The velocity of the P-waves in Liquids> Solids > Gases.

Select the correct code:

1. 1 and 2 only
2. 2 and 3 only
3. 1 and 3 only
4. All of these

Answer: (a)

Explanation:

• P-waves move faster and are first to arrive at the surface hence they are the 1st one to recorded. Hence statement 1 is correct.
• P-waves travel through gaseous, liquid and solid materials. Hence statement 2 is correct.
• The velocity of the P-waves in Solids > Liquids > Gases. Hence statement 3 is incorrect. Therefore (a) is the correct option.

Question. With reference to the body wave consider the following statements? 

1. These wave travel into the body of the earth.
2. These waves are somewhat like sound wave.
3. These are faster than surface wave.

Select the correct code:

1. 1 and 2 only
2. 2 and 3 only
3. 1 and 3 only
4. All of these

Answer: (d)

Explanation:

• These wave travel into the body of the earth. Hence statement 1 is correct.
• These waves are somewhat like sound wave. Hence statement 2 is correct.
• These are faster than surface wave. Hence statement 3 is correct. Therefore (d) is the correct option.