Magnitude (M) of an earthquake gives size of an earthquake. It is measured on the basis of ground motion (amplitude) recorded by an instrument and applying standard correction for the epicentral distance from the recording station. It is related to logarithm (to the base of 10) of amount of energy (E) released by an earthquake and normally measured on Richter Scale, named after the scientist C F Richter, who introduced magnitude in seismology.

The smallest earthquake perceptible by human beings corresponds to 2 whereas largest and most destructive earthquake so far known to have occurred has been assigned a magnitude of 8.7 on the Richter Scale. The damage from an earthquake generally starts from magnitude 5 and above.

Intensity of an earthquake at a particular place is determined by the extent of shaking that has been felt or the damage at that site. Thus, intensity of an earthquake at a given place depends on (a) distance of the site from the epicentre, (b) focal depth,(c) magnitude of the earthquake and (d) soil condition at the site. The intensity is maximum at the site close to epicentre and decreases with increase in distance from the epicentre. It is normally measured on Modified Mercalli Scale and expressed in Roman numerals, I to XII, forming a qualitative scale with most disastrous being intensity XII.

A classification of earthquake on the basis of magnitude:

Microearthquake or                              M<3

tremor                                                3<M<5

Slight earthquake                                5<M<7

Moderate earthquake                           7<M<8

Great earthquake                                8<M

The distribution of earthquake is not uniform over the globe. 85 per cent of earthquakes occur in Circum-Pacific belt while 10 per cent of earthquakes occur in Alpide-Himalayan zone, which begins in the Azores in Europe, passes through the Mediterranean, Himalayas and then passes through Andaman Sea, Sumatra and Indonesia to join the Circum-Pacific belt in New Guinea. The remaining 5 per cent occur in mid-Atlantic Ocean ridge and mid-Indian Ocean ridge.

In India, the main seismic zone passes through the Himalayan mountain range, Northeast India, Andaman & Nicobar Islands and Rann of Kutch which fall in Zone-V of seismic zoning map of India.

The number (N) of earthquake decreases with increase of magnitude. Following is the statistics of world-wide earthquakes per year.

Magnitude (M)              Average number above M

8                                                  2

7                                                  20

6                                                  100

5                                                  3,000

4                                                  15,000

3                                                  more than 100,000

Due to release of earthquake energy, seismic waves are generated which propagate through the earth in all directions. The vibrations caused by the seismic waves are recorded by an instrument called seismograph. A seismograph consists of a sensor called seismometer which picks up the ground vibration and a recoder to make a permanent record of the vibrations. The records thus produced are known as seismograms.

The most common cause of earthquakes is the tectonic movement of the lithosphere which consists of upper part of the earth down to a depth of about 100-150 km. The lithosphere forms the cover of the earth and consists of different plates which move relative to each other. As a result, stresses are developed within the lithosphere. When the stresses exceed the breaking strength of the rock, faulting takes places at weak zones called a fault. This causes release of energy in the form of elastic waves which vibrates the earth. Most of the earthquakes occur along the boundaries of plates. Such earthquakes are known as interplate earthquakes. However, a few earthquakes also occur within the plate and these are known as intraplate earthquakes. The plate on which India is lying is known as Indian plate and consists of eastern Indian Ocean and whole of Australia. This plate is moving in north-north-east direction at an average rate of 4-5 cm per year and colliding with Eurasian plate along the Himalayan region. Forces originating in the magma chamber and conduits of volcanoes also cause earthquakes, which occur near the surface or within the volcano cone. This class of earthquakes are called volcanic earthquakes. Earthquakes are sometimes triggered by man-made activities like reservoir impounding and mining activities.

- Amanpriet