What is an earthquake? Where do they occur?

An earthquake is the sudden release of strain energy in the Earth's crust resulting in waves of shaking that radiate outwards from the earthquake source.

india Updated: Mar 13, 2003 14:53 IST
PTI

Any sudden disturbance within the Earth manifested at the surface by a shaking of the ground”, is how the Encyclopaedia Britannica defines an earthquake.

An earthquake is the sudden release of strain energy in the Earth's crust resulting in waves of shaking that radiate outwards from the earthquake source. When stresses in the crust exceed the strength of the rock, it breaks along lines of weakness, either in a pre-existing or new fault plane. The point where an earthquake starts is termed the focus or hypocentre and may be many kilometres deep within the earth. The point at the surface directly above the focus is called the earthquake epicentre.

Though we have evidence of humans being aware of earthquakes, proper understanding and study of the subject, seismology, developed only recently. Derived from the Greek word, seismos, Seismology is the scientific study of the sudden violent movements of the earth connected with earthquakes.

Earthquakes take place as a series of vibrations that originate below the surface of the earth. The vibrations are a manifestation of the elastic or seismic waves due sudden movements within the earth during the release of accumulated strain, which causes the generation of energy too.

The most likely place for earthquakes to occur is at the meeting point of two crustal plates. Earth’s crust, instead of being one continuous body, is actually divided into a number of parts, called plates. These plates may be large or small, continental or oceanic. And they move.

And it this movement that causes them to either move towards or away from each other. So even as you read this some plates are moving. The Indo-Australian plate is buckling under the mega Eurasian Plate. This, an example of converging plate boundary, causes enormous pressure to be built up in the area where the two plates meet, i.e. the Himalayan region. Prospects for divergent plate boundary regions are better as the pressure is much lesser.

However, as has been amply brought out over the years, plate boundaries are not the only areas getting affected. The recent large earthquakes affecting India, eg those of Latur (1993), Jabalpur (1997) or the mega Gujarat quake (2001) have all occurred in areas fairly distant from the Himalayas.

First Published: Feb 14, 2003 13:32 IST