Secondary School Level

An earthquake is a natural phenomenon. It is the shaking and vibration of the surface of the earth resulting from underground movement along a fault plane or from volcanic activity. It is also defined as 'the sudden, sometimes violent movement of the earth's surface due to the release of energy in the earth's crust’.

The earth is not a perfect sphere like a ball. It is slightly flattened at both poles, north and south, so that it resembles a giant orange. Inside, it is more like an onion because it consists of several layers. The outermost layer, the part we all know, is called the crust. The earth's crust is made up of a series of huge, irregularly - shaped plates, i.e. tectonic plates, on which the continents sit. In some places these plates overlap. Earthquakes are caused by the edges rubbing together of two plates and their movement in sudden jerks causing release of elastic energy which travels outward from the epicenter in the form of shock waves. Maximum destruction occurs at the epicenter.

The earth's crust is not very thick - about 8 km under the oceans and 4 times that under the continents. The great Tokyo earthquake of 1923, which is believed to have killed about 25 million people, was caused by the twisting of a section of the earth's crust in Sagami Bay.

The Richter scale, named after the US geologist Charles Richter measures the intensity of earthquakes from 1 to 10. There are over a million earthquakes annually, including those too small to be felt (Richter scale reading less than 2). These, together with three more groups’ i.e. potentially perceptible, generally- felt, and minor, account for a little under one million. Those assessed from a magnitude of 5 upwards are potentially damaging. The greatest destruction is caused by an earthquake of a magnitude of 8 or higher and it is likely to occur only once a year. The smallest vibration usually felt by human beings is 2.

In recent times, three powerful earthquakes have struck India. The first one hit the sub continent on 26 January 2001 in Bhuj district of Gujarat having a magnitude of 7.9 with the epicenter (i.e. origin) in the Rann of Kutchh (about 21 km NE of Bhuj), a desert plateau in the Western India near the border of Pakistan. Over 100,000 were estimated to have perished and at least 200,000 stranded. The tremors were felt as far as Delhi and Central India besides the whole of Gujarat. The second, with a magnitude of 6 struck on 30 September 1993 in the Latur district of Maharashtra State, killing over 10,000 people and injuring another 30,000. The third strong earthquake of 7.6 magnitude hit the northern most state of Jammu and Kashmir on 8 October 2005. This quake had its epicenter close to Muzaffarabad (138 km north of Islamabad) causing more damage to the Pakistan side. On the Indian side, over 1300 people were killed and over 6000 injured.

There is no known way to prevent earthquakes. However, the extent of devastation can be greatly diminished by building structures using earthquake resistant design and materials, making the interiors of the building safe and educating people about earthquake safety.

The ocean or sea version of an earthquake is called a Tsunami. It is defined as 'unusually large ocean wave produced by an underwater-earthquake (Seaquake) or undersea volcanic eruption and often causes extreme destruction when it strikes the land9. Such waves can have heights of up to 30m (98ft) and reach speeds of up to 950km/hr. The Indian Ocean Tsunami waves hit our shores on 26 December 2004 claiming over 11,000 lives in India's southeastern coast. Here again, there is no known way to prevent it but concerted efforts are on to devise efficient early warning systems.