Earthquake is a transient oscillation or series of oscillations of the ground, activated by natural causes in a localized region and spreading in all directions. The source region, known as the focus, may be located at any depth to about 700 km. The surface region directly above the focus is known as the epicenter.

Causes: Many earthquakes are believed to be caused by Tectonic effects, especially the sudden release of accumulated elastic strain energy . Among other possible causes are: sudden shearing accompanying plastic flow, volcanic activity including explosive release of accumulated gas; abrupt volume changes produced by mineral crystallization and polymorphic phase transformations; and abrupt changes in circulation patterns of magma. Rock slides, meteor impacts, and collapse of subsurface caves are other minor causes. Artificial explosions, under or above the surface, also trigger ground motion which at large distances may be difficult (if not impossible) to distinguish from natural earthquakes.

Theory of Plate Tectonics: The Theory of Plate Tectonics states that the earth's crust (outermost layer) is made of plates of rock that are floating on a liquid magma layer forming the top part of the Earth's second layer, or the mantle. The places where the moving plates meet are called faults. The plates stick together and move in jumps when enough pressure has built up between them to overcome the resistance of a rock which is holding them back. The pent up potential energy of the plates, stored as elasticity, is released in this jump sometimes as heat and sometimes as waves. These waves comprise an earthquake.

Classification: There are three types of earthquakes: tectonic, volcanic and colluese. Tectonic and volcanic earthquakes are identical in their propagation. Volcanic earthquakes occur simultaneously with volcanic activity, but tectonic are caused by a ruptured fault. Colluese occurs when a mine shaft or cave implodes or explodes and they are the least common.

Effects: Earthquakes can produce striking, often highly destructive effects. The intensity of an earthquake is a composite, qualitative measure of the damage to man-made structures and of changes in the surface features of the earth, e.g. visible faults and fissures. It is a measure of the stability of man's structures and of Earth's surfaces . The Mercalli Intensity Scale ranges from degree I, for an earthquake which is felt by a few persons and with no effects on structures, to degree XII, for an earthquake which produces widespread panic, total destruction of buildings, ground accelerations exceeding that of gravity, visible ground motion, and permanent distortion of lines of sight and level. Among the visible effects on Earth's surface are: soil slumping, earth avalanches, and water and sand fountains. At sea, the elastic compressional and shear waves are transformed into sound waves when they strike the ocean bottom from beneath. These sound waves are sometimes observed aboard a ship at sea by shaking of the masts and hull (seaquakes). Under special circumstances' an earthquake with focus near or under an ocean, produces a seismic sea wave, known as TSUNAMI. The water in landlocked bays and lakes is sometimes excited by earthquakes, which produces an oscillating motion of the water surface, known as a seismic seiche. Frequently earthquakes especially those occurring near areas of alluvial cover, are accompanied and preceded by audible sounds of low pitch. These are triggered by compressional and shear waves in the alluvium, from which, under favorable circumstances, an appreciable sound wave may be transmitted into the air. Most large earthquakes are preceded and followed by a series of smaller shocks, known as foreshocks and aftershocks. A sequence of small earthquakes all occurring in the same region at small time intervals and of the same order of magnitude is known as an earthquake swarm.

Seismic Belts : Earthquakes occur principally in two elongated regions. One passes around the Pacific Ocean (circum-Pacific belt) and includes New Zealand, New Guinea, Japan, the Aleutian islands, Alaska, and the west coasts of North and South America. It has been estimated that energy released in earthquakes in this region account for 80 per cent of the total earthquake energy release. The other region underlies the Mediterranean region eastward through Asia, passing under the Atlas, Alpine, Iranian, Himalayan, and Burmese Mountains, the island chains of Indonesia and New Guinea, the Solomon Islands, and the New Hebrides. The two regions meet at Celebes. A less active region consists of the Mid-Oceanic Ridges including the submarine mountains underlying the center of the Atlantic, Indian, and South Pacific Oceans.

Depths of Disturbances: Most earthquakes occur at depths of about 25 km; the frequency of occurrence diminishes with depth. The observed depth distribution permits classification into shallow-focus (0-70 km), intermediate-focus (70-300 km), and deep-focus earthquakes (300-700 km ). The deepest-focus earthquake on record occurred at about 700 km. The deep-focus type are confined to the circum-Pacific belt.

Evidence from analysis of many quakes has suggested that the most common type of earthquakes-producing fault movement is transcurrent -i.e. involving relative horizontal displacement of the two sides of a vertical fault. Such transcurrent faults may be the most important mechanism for shallow-focus disturbances throughout the circum-Pacific belt. Some earthquakes, especially the intermediate-depth ones, may be produced by normal fault movements.

Locating an earthquake: Distance of epicenter from each of three seismic stations is determined by comparing arrival times of seismic waves. With this distance as radius, circle is drawn around location of each station on map or globe. Point of intersection of the three circles is location of epicenter.

Measurement of Earthquake: There are two main scales ( Richter & Mercalli) used to measure earthquakes. They are based on intensity and damage. The one based on intensity is the Richter scale, measuring how much energy is released by an earthquake. It operates on powers of ten. The magnitude scale was designed so that the smallest detectable disturbances, with an energy release of about 6.3X10⁵ ergs, have a magnitude 0, while the largest ones, with an energy release of about 2X10²⁵ ergs, have a magnitude of 8.5. The frequency of occurrence varies inversely with magnitude.

1. Most people do not notice, animals may be uneasy, can be detected by a seismograph
2. Hanging objects sway back and forth
3. Many people feel the movement, parked cars may rock
4. Doors, windows and shelves may rattle, people indoors can feel movement
5. Light furniture moves, pictures fall off walls, objects fall from shelves
6. Nearly everyone feels movement, light furniture falls over, windows may crack
7. Some people fall over, walls may crack
8. Heavy furniture falls over, some walls crumble
9. Many people panic, some buildings collapse, dams crack
10. Railroad lines are bent, most buildings are damaged, roads crack
11. Bridges collapse, buried pipes break, most buildings collapse
12. All manmade structures are destroyed

• October 10, 1980; Algeria: 2,590 killed. 3,30,000 rendered homeless. Intensity of quake 7.3.
• November 23, 1980; Italy: 2,735 people killed and over 7,500 injured. Intensity of quake 7.2
• June 11,1981: Iran: 1.027 people killed and over 800 injured.

Intensity of quake 6.8.

• December 13,1982: Yemen:3,000 killed and over 2,000 injured.

Intensity of quake 6.0.

• October 30, 1983: Turkey: About 1,300 people killed and 500 injured. Rocked the city of Erzuram. Intensity of quake 6.9.
• September 19, 1985: Mexico: over 9,500 killed and 40,000 injured. Intensity of quake 8.1
• October 10, 1986: EI Salvador: 1,500 killed. Intensity of quake 5.5
• March 5, 1987: Ecuador: over 1,000 people killed. Intensity of quake 7.0
• December 7,1988: USSR : More than 25,000 people killed.

Intensity of quake 6.9

• June 21, 1990: Iran: Over 35,000 killed in one of the worst recorded disaster in Iran. Intensity of quake 7.7
• July 16, 1990: Philippines: 1,620 people killed. City of Cabanatuan is the epicentre. Intensity of quake 7.8
• February 1, 1991: Pakistan/Afghanistan: Around 1,200 people (Pakistan 200, Afghanistan 1,000) killed. Intensity of quake 6.8
• October 20, 1991: India: 1,600 killed near Uttarkashi,.

Intensity of quake 6.1

• December 12, 1992: Indonesia: Around 2,200 people killed on a string of islands in the province of East Nusa Tenggara. Intensity of quake 6.8.
• September 30, 1993: India; A series of quakes killed almost 10,000 people in western and southern India. The first tremor measured 6.4.
• June 6, 1994: Colombia: Over 1,00 killed in Paez River Valley in South/West Colombia
• January 17, 1995: Japan; 6,430 people killed in a major quake that rocked Kobe. The biggest to hit Japan in Half a century.

Intensity of quake 7.2

• May 28, 1995: Russia: Over 1,989 killed in Oil-produing Far-East Russia. Intensity of quake 7.5
• February 28, 1997: Iran: At least 1,000 killed in north-western Iran. Intensity of quake 5.5
• May 10, 1997: Iran: over 1,560 people killed in rural areas of eastern Iran near the Afghan border. Intensity of quake 7.1
• February 4, 1998: Afghanistan: At least 4,500 killed in the Takhar province. Intensity of quake 6.1
• May 30, 1998: Afghanistan: over 4,000 people killed in northern Afghanistan's Takhar province. Intensity of quake 6.9
• July 17, 1998: Papua New Guinea: An under sea quake created three tidal waves and killed over 2,100 people. Intensity of quake 7.1
• January 25, 1999: Colombia: At least 1,170 people killed in Colombia's central coffee-growing region. Intensity of quake 6.3
• August 17,1999: Turkey: Around 17,800 killed. Intensity of quake 7.4.
• September 21. 1999: Taiwan: Around 2,000 killed and thousands rendered homeless in central Taiwan. Intensity of quake 7.6.

Quakes occur more frequently in India because it lies next to the region where the greatest continental collusion on Earth is taking place. A hundered million years ago, when dinosaurs roamed the continental masses, the so-called Tethys Sea lay partly between the Indo-Australian and the Eurasian plate. Forty million years later, the Indo-Australian plate collided with the Eurasian plate, neither giving way. The collision is in slow-motion, the closing speed only 2 cm (1 inch) a year, but the energies generated are colossal - the collision area between the two plates has crumpled and pushed upwards to form the Himalayas, the greatest mountain range on the planet. The impact is still being felt and the Himalayas are still rising. Satellite measurements put the ascent of the Himalayas at about 5-millimetres a year.

• April 4, 1905: Thousands killed in Kangra quakes in Himachal Pradesh. Intensity of quake 8.0
• January 15,1934: Strong quake near Bihar-Nepal border. Extensive devastation in Kathmandu. Intensity of quake 8.3
• June 26,1941: Heavy quake in Andaman Islands. Intensity of quake 8.1
• August 15, 1950: Major quake rocks Assam. Intensity of quake 8.5
• January 19,1975: Kinnaur and Lahul Spiti rocked in the Himalayas. Intensity of quake 6.2
• August 21, 1988: Quake kills 900 people near Bihar-Nepal border. . Intensity of quake 6.5
• October 20, 1991: Over 1,600 killed in the Uttarkashi quake.

Intensity of quake 6.6

• September 30, 1993: Quake in Latur, Maharashtra, kills more than 10,000 people. Intensity of quake 6.3
• May 22, 1997: About 40 killed in Jabalpur region, Madhya Pradesh. Intensity of quake 6.0
• March 29, 1999,:Quake causes extensive damage in Chamoli area in Uttar Pradesh. Intensity of quake 6.8
• January 26, 2001: Severe quake sends tremors across Indian subcontinent,; extensive damage in Gujarat. Thousands die, lakhs rendered homeless and hundreds of buildings destroyed.