2 Types of Seismic waves in Geophysics

When the shaking of the Earth's ground (known as earthquake) occurs at several times, then it is due to release and movement of energetic waves called seismic waves. Without ruining time,. I don't want to dig deeper on this topic, but just give you the basics regarding these waves. Stay alerted!

Seismology is the study of seismic waves and Earthquakes. This may include the study of vibration, propagation, types and application of these seismic waves through the Earth.

The term seismology originated from two Greek words "Seismos" means Earthquake and "Logos" means study of.

The study of these waves is important as it gives clues regarding,

➔ It used to study the structure and nature of Earth's interior.

➔ It is used in seismic exploration geophysics.

Note: The main source of seismic waves in nature is produced from Earthquakes. However there are man made sources of seismic waves such as dynamites, Vibrators forexample vibroseis.

Although due to heterogeneous nature of the Earth, in order to study seismic waves propagation within the Earth two important assumptions must be considered,

(i) The Earth must be considered as homogeneous medium

(ii) Seismic waves travel by elastic displacement in the medium.

Categories of seismic waves

There are mainly two (2) categories of seismic waves based on where they propagates within or on the surface of the Earth,

(i) Body waves 

(ii) Surface waves.

1: BODY WAVES

These waves propagate from the source within the Earth's interior through the medium.

As adopted by Huygens principle, that as the distance increases from the source the wavefront of body waves acquire spherical shape then body waves are considered as spherical waves.

At great distances as the curvature of spherical wavefronts decreases to flat shape then body waves are considered as planes waves. See figure 1 here below.

Types of body seismic waves

There are main two (2) types of body seismic waves based on how they propagates

(i) Compressional waves

(ii) Transverse waves

1: Compressional Waves

These are transmitted by expansion (rarefaction) and compression through the medium.

They are also known as primary waves (P-waves), because they travel faster than all seismic waves hence they are the first to reach a seismic recording station. See animation figure 2.

These waves produce only changes in volume but not rotation during propagation. See figure 3.

They can travel in all medium such as in liquid,solids and gases.

Figure 2: P - waves travel faster and reach seismic station first than S - waves

P-waves are the most important for controlled source seismology because they are,

➔ Easier to observe them at seismic recording stations since they travel faster than S - waves.

➔ Most explosion sources are compressional. Hence it is hard to produce a shear source.

2: Transverse Waves

These waves have particles that vibrate perpendicular to the direction of propagation.

They are also known as secondary waves (S-waves),or shear waves since they travel slower by shearing and reach seismic recording stations later after P-waves.

These waves produces rotation and shearing but no volume change during propagation. See figure 3.

VELOCITIES OF  P and S - WAVES.

For P - waves, let say Vp  represent P - wave velocity and Vs  represent S wave velocity

Then, Vp = √(K + 1.33U)/P

Where K - Bulk modulus, U - Shear modulus, and P - Pressure.

This means P - waves has change in shape and volume

Also, Vs = √(U/P)

This means S - waves have changed in shape only.

Since for liquids and gases, U = 0

Hence Vs = 0 , means no propagation and Vp = √(K/P), means Vp has been reduced. For example in highly fractured or porous rocks.

Note: K is always positive, then Vs < Vp .

2: SURFACE WAVES

These waves travel on the surface of the Earth.

These waves are slower than body waves hence they are more destructive due to effective ground shaking. Surface seismic waves can be grouped into two (2) types based on type particle motion in their wavefront.

(i) Rayleigh waves

(ii) Love waves.

1: RAYLEIGH WAVES

They have wave particles in the wavefront that are polarized to vibrate in the vertical plane.

The resulting particles motion as the combination of the P and SV-vibration. See figure 3

2: LOVE WAVES

They have particle motion that is horizontal and perpendicular to the direction of propagation. See figure 3

Figure 3: Seismic waves P - wave (a), S - wave (b), Love wave (c), Rayleigh wave (d).

Thanks for reading!

Follow this blog on whatapp group to gain more benefits.