An earthquake produces four types of waves:
(1). P-Waves (Primary Waves),
(2). S-Waves (Secondary Waves),
(3). L-Waves (Surface Waves), and
(4). Rayleigh Waves:
The tectonic movement releases a huge amount of energy, which is known as Seismic energy. Seismic energy express itself in the form of seismic waves. These seismic waves are known as earthquakes. Out of the above mentioned four types of waves, the P, S, and L waves are the main types of seismic waves. All these four types of waves are classified into two main groups. One is body waves (Primary and Secondary), and the second is Surface Waves (L-Waves and Raleigh Waves).
The waves which travel through the body of the earth, or in other words, the waves which can travel deeply into the earth’s layers, are known as the body waves. The body waves can be divided into two types; the P-waves and the S-waves. Explanation of these two waves is as under.
(a). Primary waves (P-waves): The foremost waves produced after the movement of two massive rocks are known as P-waves, which are also called primary waves. As these waves hit the ground first and foremost, that is why they are called primary waves. These waves are compressional type waves. A compressional wave moves the medium to-and-fro (push and pull), or in other words, the movement of the P-waves is in the direction of the wave’s propagation. They are also known as the pushing waves. These waves have the ability to travel through solid and liquid. These are the fastest moving waves among the seismic waves. They can move with a velocity of 1500 km/second, but the average speed of these waves is 8 km/sec. The path of the p-waves is concave. As the P-waves just move the objects to-and-fro (moving forward and backward), therefore, they bring the least destruction. When P-waves travel from a solid to liquid their velocity decreases down, but when they travel from a denser medium, their velocity increases in the denser medium. The shock of an earthquake is first felt by means of P-waves. P-waves, suddenly, start pushing and pulling the objects in their ways. These waves push and pull the buildings with very high frequency. They just vibrate the building, therefore, cause the least destruction.
(b). Secondary waves (S-waves):
These waves are also called the Shear waves, or transverse waves, but commonly known as the secondary waves. As the name implies, secondary waves are the waves transmitted after the primary waves, as a result of seismic activity. Unlike the primary waves (P-waves), the secondary waves are longitudinal in nature. Because of being longitudinal in nature, and having a higher wavelength and lesser frequency the velocity of these waves is lesser than the primary waves. After the first compressional jerk felt as a result of an earthquake, 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. The traveling waves under the building move up its one side at a time, and then the other side at the next moment, thus making the building swing.
The secondary waves are the most destructive waves among their four types. The average speed of these waves is 5.5 km/second, but during earthquakes of higher magnitude, their speed may exceed to 13 km/second. These waves do not travel through the fluids (gas, liquid), as fluids (gases and liquids) do not respond elastically to changes in shape, they will not transmit S-waves. This sharp contrast in the properties of P and S waves can be used to detect the presence of liquid zones deep in the earth. The weak transmission of these waves through the lower mantle proves that the lower layer of the mantle is molten (fluid).
As the name implies, these waves do not travel through the deeper layers of the earth. Their propagation is limited to the upper surface. Their amplitude decreases with the depth. These waves are also divided into two types. One is Surface/Love waves and the other is Raliegh Waves. Details and the difference between the two are in the following headings.
(a). Long Waves (L-waves): The third type of wave is called the L-wave, or Long waves (having longer wavelength). These waves are also known as surface or love waves. As the name implies, these waves only travel through the surface of the earth, and their amplitude decreases with depth. These waves are the most less destructive among the seismic waves. These waves are named after ”Augustus Edward Hough Love” which is why they are known as the Love Waves. Love waves also travel with a lower velocity is compared to body waves. Like the secondary waves, the love waves are also longitudinal in nature and act as the secondary waves while traveling through different states of the medium. The only difference between the secondary waves and the Love waves is, the Love waves travel through the surface, while the secondary waves travel through the body of the earth. Love Waves are created when particles vibrate perpendicular to the propagation direction. While the particle motion is similar to that of shear waves, Love wave amplitude is much higher and decreases rapidly with depth. Love waves are the most destructive waves in earthquakes because of their high amplitude and transverse particle motion.
(b). Rayleigh Waves:
Rayleigh waves are another type of surface waves, 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.