An earthquake is defined as a tremor below the surface of the earth which causes shaking of the crust. These devastating tremors cannot be regarded as movements as they are caused by the stresses that break the earth's crust.
Causes of earthquake
The main factors that cause earthquakes are the following: (i) Plate tectonics : Most earthquakes occur on account of plate movements. When two plates slip past each other or collide against each other, their edges produce faults along the lines of weakness. The tectonic earthquakes are frequent and immensely powerful. (ii) Volcanoes : Almost all volcanic eruptions are accompanied by earthquakes.
Anatomy of an earthquake
The sudden movement of rocks along a fault causes vibrations that transmit energy through the Earth in the form of waves. Waves that travel in the rocks below the surface of the Earth are called body waves, and there are two types of body waves: primary, or P, waves, and secondary, or S, waves. The S waves, also known as shearing waves, move the ground back and forth. Earthquakes also contain surface waves that travel out from the epicenter along the surface of the Earth. Two types of these surface waves occur: Rayleigh waves, named after British physicist Lord Rayleigh, and Love waves, named after British geophysicist A. E. H. Love.
Distribution of earthquakes
Most earthquakes originate from plate boundaries. i) The Circum-Pacific Mountain Belt: This belt has 70% of all earthquakes. A part of San Andreas Fault in the USA lies in this belt. The area of Japan, the Philippines and Indonesia are a part of this belt and prone to severe earthquakes. ii) The Midworld Mountain Belt: It stretches from Eastern Europe covering Alpine-Himalayan ranges in Europe and Asia. 20% earthquakes occur in this zone. The balance 10% are associated with smaller submarine ridges, ocean floors, rift valleys and other fault zones.
An epicentre is a point on the earth's surface directly above where the earth begins to fracture in an earthquake. The focus lies beneath the epicentre. The epicentre is located from records of earthquake waves created by a seismograph.
Cause of earthquake
An earthquake is the shaking and trembling of the earth's surface due to the sudden release of earth's energy in the form of waves. Earthquakes are caused to due various reasons. Some of them are:
Due to the movement of the tectonic plates.
Colliding of the tectonic plates.
When the tectonic plates slides one below the other earthquake occurs.
Forming of fractures in the rock layers.
Surface or L-waves
The waves which are responsible for earthquake are called seismic waves. The seismic waves can be divided into primary, secondary and surface waves. Surface or L-waves are the highly destructive waves among all the three waves. These waves are generated after the primary and secondary waves reach the epicenter.
Secondary or S-waves
The waves which are responsible for earthquake are called seismic waves. The seismic waves can be divided into primary, secondary and surface waves. Secondary or S-waves: The waves which reach the earth's surface after the primary waves are called secondary waves. The velocity of these waves are lesser than the primary waves and they scatter in all direction from the focus. These waves can travel only through the solid medium.
Primary or P-waves
The waves which are responsible for earthquake are called seismic waves. The seismic waves can be divided into primary, secondary and surface waves. Primary or P-waves: These waves are the first waves to reach the earth after the energy is transmitted in the interior. They travel at a very fast speed and can travel through all the medium, solid, liquid and gaseous but while travelling through the liquid medium, its direction gets changed.
Ring of Fire
About two third of the earthquakes in the world occur in the vast region of Pacific Ocean, "Ring of Fire" which is closely linked with the region of volcanic activity and crustal dislocation. Second belt is the Mid World Belt which accounts for about 20 per cent of earthquakes run through Mid Atlantic Islands, Alps mountains, Himalayas and Indonesia.
Major earthquakes in India
The earthquakes in India are mainly confined to the Himalayan Region and its foothills. There have been a number of violent earthquakes in historic time causing large-scale damages. The Kutch earthquake (1819), Assam earthquake (1897 & 1950), Kashmir valley earthquakes (1823 & 1885), Kumaon Hills (1803), Bihar (1934), Kinnaur-Himachal (1975), Uttar Pradesh (1990 & 1999), Gujarat (2001) are some of the severe earthquakes occurred in India.