An earthquake is caused by the seismic waves generated by the transverse movement of landmasses at plate tectonic boundaries, volcanic eruption, compression in the earth’s crust, groundwater extraction, geothermal drilling, construction of tracking, and injection wells, and havoc of a powerful nuclear bomb blast. All these events cause seismic waves. The seismic waves spread spreads outwards in all directions from the focus. Basically, the seismic waves are categorized into two types;
- Body Waves: The body waves are the waves, which travel through the eath’s body. These waves are comparatively strong and can travel even across the earth, and transmit their effect over the opposite side of the focus point. Body waves can further be classified into two types: (i) Primary Waves, and (ii) Secondary Waves.
- Surface Waves: Surface waves are comparatively weak, and unlike body waves, they are unable to travel across the body of the earth. These waves only travel through the crustal surface of the earth. These waves are less dreadful and disastrous. Like the body waves, these waves are also divided into two types; (1) Long Waves (L-waves), (ii) Rayleigh Waves
All these types of waves are being discussed separately in the following paragraphs:
(1). Body Waves
- Primary Waves: These waves travel only through the surface of earth, and are unable to travel deep through the layers of earth. P-waves or primary waves are compressional waves, which cause the particles of the rock to vibrate in to and fro direction. The particles move forward and backward in high frequency. The P-waves are faster than the rest of the waves and reach at a point earlier than the other types of waves. These waves only push the buildings to and fro. Buildings are shaken with strong jerks. These waves have less time duration and die within 2 to 5 seconds. These waves do not last for long time, therefore, they are less destructive. Read more about the P-waves
- Secondary Waves: These are also included in the surface waves, and can only travel through the earth’s surface. Secondary waves also called the Shear Waves, which are longitudinal in nature. The velocity of these waves is lesser than that of the P-waves. These waves reach a point a few seconds later than the P-waves. The secondary waves are felt like the ground is floating on some liquid. As being longitudinal in nature, these waves shake the ground up and down, and to the right angle with respect to the direction of wave propagation. These waves not only move the structures up and down but also swing them. Hence these waves are longitudinal in nature, therefore, these waves make the buildings swing at large displacement.
(2). Surface Waves
- Long Waves: L-waves are also longitudinal in nature, but their velocity is lesser than that of the P and the S-waves. These waves also move the ground up and down direction, and this movement is at right angle with the direction of travel of the waves. These waves are quite similar to the secondary waves, the only difference between the two is their energy and velocity. These waves have lesser energy and lesser velocity, therefore, they are considered to be less destructive. Read more about the Long-waves
- Rayleigh Waves: Rayleigh waves are another type of surface wave, in which the motion of the particles is elliptical and characterized perpendicular to the direction of the propagation of the wave. These are the least destructive waves. Rayleigh waves are differentiated from the Love-waves on the basis of particle motion. In Rayleigh waves, the particles move up and down (perpendicular to the direction of wave propagation), while in the case of Love-waves, the particles vibrated perpendicular to the direction of propagation of waves, but move transverse to the direction of the propagation of the wave. Animated images of all types of the seismic waves