Have you ever felt the shaking of the Earth's ground at times? Today I want to help you to know through this post that it is the release and movement of energetic waves called seismic waves that contribute to such shaking! Without ruining time,. I don't want to dig deeper on this topic, but just give you the basics on these waves. Stay alerted!
What is Seismology? It is the study of Seismic waves and Earthquakes.
It originated from two Greek words "Seismos" means Earthquake and "Logos" means study of.
Why is the study of Seismic waves so important ?
- 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 (Vibroseis).
Due to heterogeneous nature of the Earth, in order to study seismic waves propagation within the Earth two important assumptions must be considered,
(i) 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 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.
Types of Body seismic waves
There are main two 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.
These waves produce only changes in volume but not rotation during propagation.
They can travel in all medium e.g Liquid,solids and gases.
Fig. P - Waves travel with expansion (Rarefaction) and compression.
Why are P-waves the most important for controlled source seismology ?
- Easier to observe them at seismic recording stations since they travel faster.
- 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
Fig: Shear waves
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.
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.
Fig: Rayleigh wave (Lowrie 2007)
2: Love waves
They have particle motion that is horizontal and perpendicular to the direction of propagation.
Fig: Love wave (Lowrie 2007)
Thanks for reading! Next post I will be happy to let you know about how these waves are detected/measured. Stay Tuned!.
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