Types Of Seismic Waves: Introduction, Primary & Secondary Waves

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In this article, we will discuss the types of seismic waves.

1. Seismic Waves

The Seismic wave in the fields is calculated by a seismometer, hydrophone (in water), or accelerometer. The propagation velocity of seismic waves is based on the density and elasticity of the materials as well as the type of wave. Velocity tries to rise with the bottom through Earth’s crust and mantle but falls steadily moving from the mantle to the outer core.

In geophysics, the refraction or reflection of seismic waves is utilized for investigating the structure of Earth’s interior, and man-made vibrations are usually produced to study shallow, subsurface structures.

When any disturbance causes an earthquake, the shaking‘ travels in form of waves.
These elastic waves, called the seismic waves are generated at focus and are radiated like ripples.
These elastic waves are considered to be the outcome of the elastic deformation of rocks. As all elastic bodies are subjected to 2 types of deformation viz. compression and shear, the seismic waves are also related to this.

2. Types Of Seismic Waves

There are broadly two types of seismic waves:

a. Body waves

These waves travel within the earth from the focus. Surface waves: when the body waves reached the epicenter, they radiate in all directions by traveling on the surface of the earth. So these waves are surface waves.

Types of Body waves

a. Primary or P- waves

The seismic P-waves are mechanically equivalent to sound waves i.e. they pass through solid, liquid, and gases, by setting up back and forth movement of
compression and dilation in the direction of travel.

The effect resembles the motion of an accordion. Therefore P-waves are also called Push-Pull waves or Primary waves.

These are the first group of waves to be recorded in seismogram hence called Primary waves.

b. Secondary or S-Waves
Seismic S-waves cause material through which they pass to be sheared back and forth along lines at right angles to the direction of travel. Thus they are named Shear waves.
Shear waves can pass through solid only. Fluids lack the shearing resistance necessary to transmit S-waves.
These are longitudinal waves and are 1.7 times slower than P-waves.

b. Surface waves

When body waves travel are generated from the focus, they travel in all directions.
When these waves reach the surface of the earth, they travel along the surface of the earth in all
directions. These seismic waves now are called Surface waves.

There are two kinds of surface waves:
1. Love waves
2. Rayleigh waves

1. Love Wave
Named after a geophysicist AEH Love this wave though slower than S-wave has a different type of vibration.
They cause horizontal ground movement perpendicular to the path of propagation.
During an earthquake, Love waves are felt like a gentle side-to-side swaying. It is a surface motion where only horizontal motion is present.

2. Rayleigh wave
The second kind of seismic surface wave causes the land surface to roll up
and down, like water waves on the sea. In this case, both vertical and
horizontal motion is present.

• Named after Lord Rayleigh, British physicist.
• The motion is in the vertical plane. The motion is an ellipse.
• The short, choppy Rayleigh waves cause extensive damage to buildings
during an earthquake.

 Read Also: Types of Earthquake

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