Seismic Waves

SEISMIC WAVES

A number of different types of low frequency waves which are originated from the focus and travel through the earth surface are collectively called seismic waves.

These waves spread on to different points on the surface of the earth and appear as 'earthquake tremors'.

Types of seismic waves

They consist of '2' different types of waves: brodo

(i) Body waves

(ii) Surface waves.

Body waves

The waves which travel through the earth are called body waves.

There are two types of body waves,

 (i) Primary (P) or Compressional waves

(ii) Secondary (S) or Shear waves

The general pattern of these seismic waves is as shown in fig. 5.2.

(i) Primary or P Waves

• These are longitudinal waves in which the particles of the earth vibrate about their mean position along the direction of the waves themselves. (Fig. 5.3)

• If the earth is regarded to be a homogeneous sphere, these waves starting from the focus travel along the chord of huge circle of the earth.

• The velocity of wave is equal to √E/p, where E is the 'elongational elasticity' of the earth and p, its density.

• The velocity is found to be about 5 miles per second.

• These waves are also called as 'compressional' waves.

(ii) Secondary or S Waves.

• These are transverse waves. That is the particles of the earth vibrate at right angles to the direction of Lasite propagation of the waves. (Fig. 5.4)

• Starting from the focus, these waves also travel along a chord of a huge circle of the earth.

These S waves have velocity equal to √n/p where n and p represent the modulus of rigidity and the density of the earth respectively.

• The velocity of S wave is about 3 miles per second.

• They are also called as 'shear' waves.

Surface Waves

The waves which travel along the earth surface are called surface waves.

These waves can only move along the surface.

They also consist of two types of waves,

(i) Rayleigh waves

(ii) Love waves

(a) Rayleigh Waves

• These waves are found to remain confined to a comparatively thin layer in the close vicinity of the earth's surface.

• Unlike the P and S waves, they start from the epicentre 10 asli and arrive at the observing station along a huge circle of

• The displacement of the particles is being in the vertical plane containing their directions of propagation.

• These waves thus persist over long distances along the surface of the earth and they are almost unique in this respect (Fig.5.5).

• Due to heterogeneous character of earth, each single wave, starting from the epicentre, gets split up into a briose number of different sets of waves.

• Since each set has a different wavelength, velocity etc, they produce a series of oscillation instead of one single 'kick or throw' at the observing station.

(ii) Love Waves

The heterogeneity of the layers of the earth is also responsible for another type of surface waves, known as Love waves.

In these waves, the displacement of the earth is horizontal, but transverse to the direction on their propagation.

• The velocity of these waves is less in the earth's crust. Immediately after an earthquake, oscillations, corresponding vloviti to these waves can be detected almost any place on the surface of the earth (Fig.5.6).

• Unlike P and S waves, these waves get intermingled with Rayleigh waves to form a somewhat complicate system of waves

• These are called long, L waves, or the main shock. These waves are observed as a long series of oscillations.