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What is Earthquake? Why an Earthquake occur? Where most of the Earthquake occur over the world?

                      



 In simple language, the shaking of the surface of the Earth due to the sudden release of energy in the form of seismic waves in the earth’s lithosphere (including the crust and upper mantle) is known as Earthquake.

 

The brunch, which is associated with the study of earthquakes such brunch is called seismology.

   


The instrument, which is used to detect and record an earthquake, such instrument is called a seismograph. Seismograph has two types of scale-

1)    Richter scale: Richter scale is used to determine the intensity of an earthquake.

2)    Mercalli scale: Mercalli scale is used to determine the magnitude of an earthquake.

Seismic Wave: Seismic waves are the waves of energy, which can travel through the earth’s lithosphere, as a result forming earthquakes, volcanic eruption, large landslide, etc.

      There are two types of seismic waves-

1)    Body waves: It is a type of seismic wave, which can travel below the earth’s surface. They have a higher frequency than the surface wave. These are the fastest seismic waves. Since they fast-mover so, they arrive at the earthquake station before the surface wave.

    The body waves are further divided into two types-



a)     Primary or P wave: It is a type of body wave, which can travel through all mediums like solid, liquid, gaseous, etc. they are the fastest body wave. They travel through Earth’s layer with pull-push motion. The primary waves are moving in the same direction that they are propagating. These waves are also known as compressional waves due to their pull-push motion like a sound wave traveling in the medium of air.

b)    Secondary or S wave:  It is the other type of body wave, which can only travel through the solid medium. Due to this unique property of the secondary waves, the geologists are able to conclude that the outer core of the earth is in a liquid state. They travel through the rocks layer with side to side and up-down motion. The secondary waves are traveling in the perpendicular that they are propagating. It is also known as shear waves because they do not change the volume of materials through which they propagate, they shear it.

2)    Surface waves: It is another type of seismic wave, which can travel along the earth’s surface. They have a lower frequency as compare to the surface wave but they are most destructive seismic waves due to their motion.

    The surface waves are further divided into two types-



a)     Love or L wave: It is a type of surface wave, named after the A.E.H Love, a British mathematician, who’s worked out the mathematical model of L wave in 1911. They travel along the earth’s surface with a side-to-side motion. 

b)    Rayleigh wave:  It is the other types of surface, which are slower as compare to the Love wave. It is named after John William Strutt, Lord Rayleigh, who’s predicted the existence of the Rayleigh wave in 1885. They travel along the earth’s surface with the side to side and up-down motion. According to geologists, they are the most destructive seismic waves found on the earth.


Effects of Earthquake:

1)    Due to the occurrence of an earthquake under the ocean, this produces Tsunami in oceanic area.

2)    Large landslides, volcanic eruption, ground rupture, etc. are the major result of an earthquake.  

3)    Buildings are damaged, people getting injured, etc. due to the occurrence of earthquakes.

Some of the important terminology:



1)    Focus: Focus is the point of origin of an earthquake below the earth’s surface.

2)    Epicenter: the point on the surface of the earth, which lies above the focus point, such point is called the epicenter.

3)    Earthquake intensity: It is may be defined as the number, which gives information about the degree of intensity of an earthquake.

4)    Isoseismal line: It is a line joining all points at which the intensity of the earthquake is the same.

 

Classification of Earthquake:

1)    On the basis of reasons of occurrence, earthquakes are classified into three types –

a)     Earthquakes due to surface cause:

      Sometimes, earthquakes maybe produce due to landslip but, these are very minor. 

b)    Earthquakes due to volcanic causes:

      A volcanic eruption may produce earthquakes, which are not destructive, but leaves an observe the able effect on the surface.                                 c) Earthquakes due to plate tectonic:

      Most of the earthquakes are formed due to movement of plates beneath the Earth’s surface. Earthquake occurs when plates are colliding, subducting, slipping, etc. with each other.

2)    On the basis of depth of focus, the earthquakes are classified into three types:

a)     Shallow focus earthquakes:

     This category includes the earthquakes, which have a depth of focus is up to 55 km.

b)    Intermediate focus earthquakes:

      This category includes earthquakes, which have a depth of focus in between 55 km to 300 km.

c)     Deep focus earthquakes:

     This category includes earthquakes, which have a depth of focus in between 300 km to 650 km.

Earthquake Belt:

All over the world the distribution of earthquake is well defined and well-marked because they are found in a specific zone or belt.  There are three major zones or belt in the world, where most of the earthquakes occur –



1)    The circum Pacific: This belt covers the eastern and western areas of the pacific ocean or the eastern coastal margin of Asia and the western coastal margin of North and South America

                                     


2)    The mid-continental: This belt represents the epicenter located along the Alpine-Himalaya chain of Eurasia and North Africa as well as the epicenter of East African fault zones.

 


3)    Mid-Oceanic-ridge: this belt includes the mid-Atlantic-ridge and it’s offshore.

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