What is an Earthquake or seismology and how many types of waves and their properties

What is an Earthquake?

Earthquake literally to earth-shaking or ground movement. It is a phenomenon associated with ground failure or movement of the ground resulting from sudden or abrupt release of an immense amount of energy. Earthquake is considered to be one of the most destructive forces occurring on the earth’s surface that cause not only loss of life and property to a great extend but also shakes the morale of the people. The science of studying earthquake engineering is in gaining importance day by day the knowledge of civil engineer is considered as insufficient and incomplete and incomplete without the basis of knowledge of civil engineer is considered codes of practice that have been introduced by the Bureau of India standard.

Definition of earthquake

The definition of the earthquake is generally the ground vibration, both feeble and strong, produced on the surface of the earth due to the other any reason what so ever is described as earthquakes. Whenever this vibration traverse, an earthquake is said to have taken place. It may also be defined as ‘Violent shaking of the earth’s surface due to the abrupt release of a large amount of energy that has accumulated over a long time inside the earth. It is important to mention here that about 90% (percent) of all the earthquakes are caused naturally whereas 10% (percent) earthquakes are related to men made such as nuclear explosions reservoir induced tremors, etc. 

Earthquake engineering or (seismology)

The branch of science that deals with the study of earthquakes and the structure of the earth are termed as earthquake engineering or seismology. Its research about all about the earthquake.

Seismic waves 

The seismic waves transmitted in all directions passing through the earth’s surface during an earthquake due to the release of large strain energy are termed as seismic waves. These waves obey the laws of reflation at the interface as they pass through earth materials of densities.

There are two types of seismic waves are following below.

1. Body waves
2. Surface waves

1. Body waves

The body waves which travel through the earth layers in all directions and are not restricted to any depth are termed as body waves. These waves are very important for the scientists and geologists for carrying out a detailed study of the earthquake. Body waves are mainly two types.

• Primary waves or P-waves
• Secondary waves or S-waves

Primary waves (P-waves)

The primary waves in materials particles vibrate in the direction of propagation of the wave with a push and pull effect are termed as primary waves or P-waves. These are also termed as longitudinal waves or compressional waves. Under the effect of P-waves, the particles undergo extensional strain (Pull effect) and compression strain (Push effect).

Characteristics of (P-waves) - These waves having the following important characteristics.

• These waves are fastest to travel and hence are first to be recorded at the recording station.
• In granite rocks, P-waves have a velocity of 5 Km/sec.
• These waves are capable of passing through the solids as well as liquids. 
• These waves are responsible for the preliminary tremors or shocks upon the earth’s surface.
• These are responsible or compressional in nature like sound waves.
• Thes3e waves cause a volumetric change in the materials through which they pass.

Secondary waves (S-waves)

The body waves in which materials particles vibrate at the right angle to the direction of propagation are termed as secondary waves or S-waves. These are also termed as shear waves or shake waves. 

Characteristics of (S-waves) - Following are the important characteristics of S-waves.

• These waves are slower than the P-waves in the same materials and are seemed to be recorded at the recording station.
• The granite rocks, the velocity of S-waves are approximately 3 Km/sec.
• These waves are capable of passing through the solids but are unable to propagate through fluids as they do not have any shear strength.
• These waves are distortions or transverse in nature like light waves.
• These waves, unlike P-waves, do not change the volume of the materials through which they pass.
• vertically polarized S-waves are termed as SV waves and horizontally polarized S-waves are termed as SH waves.

2. Surface waves (L-waves)

The seismic waves which travel along the surface of the earth in a circumferential path are termed as surface waves or long waves or long waves. Their propagation inside the surface of the earth is limited to a few seismic wavelengths. The surface waves or L-waves are important to study the layered earth crust materials.

Characteristics of (L-waves) - Following are the important characteristics of L-waves are given below.

• These waves are the slower to travel and therefore last to be recorded at the recording station.
• These waves are also transverse in nature and their behavior is similar to that of sea waves.
• These waves are more destructive in nature and are responsible for all the damage on the surface of the earth during the earthquake.

Types of surface waves

These waves are distinguished from each other by the types of motion of their wave particles. There are mainly two types that are given below.

• Rayleigh waves (Lr-waves)
• Love waves (LL-waves)

Rayleigh waves (Lr-waves)

The surface waves in which materials particle vibrates in an elliptical path in the vertical plane are termed as Rayleigh waves.

The waves first explained by Lord Rayleigh in the year 1885 and are therefore termed as ‘Rayleigh waves’. in the described ‘ellipse’, the major axis is along the vertical direction and the minor axis is in the direction of wave propagation. Thus resulting elliptical particle motion can be described as a combination of P and SV motion. The important feature of Rayleigh waves is that the particle displacement is not confined entirely to the earth but these also penetrate below the surface up to the equal to the wavelength.

Love waves (LL-waves)

The surface waves in which particle motion is in the horizontal plane and vibrates at right angles to the direction of propagation are termed as love waves. The surface motion of love waves is similar to the S-waves but with no vertical component.

These waves were first described as A.E.H. love in the year 1911 and are therefore termed as ‘Love waves’ after this name. The particle motion of love waves can be described as SH vibration. 

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