P Wave

To understand P waves, we have to first look into the basics of seismology and seismic waves. The waves of energy which travel through the earth and cause earthquakes and related phenomena are seismic waves. There are two types of seismic waves :

  1. Body waves
  2. Surface waves.

Body waves are the waves that can travel through the layers of the earth. They are the fastest waves and as a result, the first waves that seismographs can record. Body waves can move through all states of matter including rocks and molten lava. Surface waves can only travel on the surface of the earth. Their frequency is lower than body waves.

Body Waves

Body waves can be further sub-categorized into

  1. P waves (Primary waves)
  2. S waves (Secondary waves)

P waves and S waves

P waves and S waves

P waves

P waves, or Primary waves, are the first waves to arrive at a seismograph. P waves are the fastest seismic waves and can move through solid, liquid or gas. They leave behind a trail of compressions and rarefactions on the medium they move through. P waves are also called Pressure waves for this reason. Certain animals, such as dogs, can feel the P waves much before an earthquake hits the crust (surface waves arrive). Humans can only feel the ramifications it has on the crust.

S waves

S waves, or Secondary waves, are the second waves to arrive during an earthquake. They are much slower than P waves and can travel only through solids. It is after studying the trajectory of S waves through the layers of earth, scientists were able to conclude that the earth’s outer core is liquid.

Following is the table explaining concepts related to waves:

Difference between s waves and p waves

P waves S waves
First wave to hit seismographs Second waves to hit seismographs
They are compression waves They are shear waves
Can move through solids and liquids Can only move through solids
Shake the medium in the direction in which they are propagating Shake the medium in the direction perpendicular to which they are moving

P Wave And S Wave Formula

Following is the table for P and S waves formula based on the velocity of P and S waves:

P wave \(\alpha ^{2}=\frac{\lambda +2\mu }{\rho }\)
S wave \(\beta ^{2}=\frac{\mu }{\rho }\)

Following is the table for P and S waves formula based on the distance between the earthquake location and the location of the seismic station:

Local earthquakes with epicenter distance up to 0-500km d=t(S-P).8
Regional earthquakes with epicenter distance up to 1000km d=t(S-P).10

Notations Used In The P And S Waves Formula

  • ⍴ is the density through which the wave propagates
  • 𝜇 is the shear modulus
  • Λ is the first Lame parameter

Stay tuned with BYJU’S to learn more on Physics related concepts.

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