Earthquake is a vibration or oscillation of the earth’s surface. Just as waves are generated on the water surface of a pond when a stone strikes it, similarly, the earth’s crust which has considerable elasticity is set into tremors by a sudden blow from internal or external sources.
At times, the shocks are highly disastrous to human life and property. General destruction takes place in a few seconds; Earthquakes may be natural or artificial.
Causes of Earthquakes:
The causes of earthquakes may be divided into three groups (i) surface causes, (ii) volcanic causes and (iii) tectonic causes.
(i) Surface causes:
Great explosions, landslides, slips on steep coasts, dashing of sea waves, avalanches, railway trains, heavy trucks, some large engineering projects cause minor tremors. Some of them are man made, others are natural.
(ii) Volcanic causes:
Vocanic eruptions produce earthquakes. Earthquakes may precede, accompany and frequently follow volcanic eruptions. They are caused by sudden violent displacments of lava within or beneath the conduit of the valcano.
(iii) Tectonic causes:
Structural disturbances resulting in the relative displacements of the parts of the lithosphere is the main cause of this type of earthquake. Most of the disastrous earthquakes belong to this category and occur in areas of great faults and fractures. Sudden yielding to strain produced on the rocks of accumulating stress causes displacements especially along old fault zones known as great transform faults.
Reid proposed the idea of elastic rebound hypothesis. Stresses accumulate on the two sides of the fault plane and produce strain. The rock deforms bends and when the stress crosses the elastic limit, sudden displacement of the two sides of the fault plane takes place. This results in a strong blow to the rocks Elastic rebound and produces tremors.
Earthquakes often occur on the ocean floor. This produces large sea waves known as tsunami that produces devastating effects on the sea coasts. Recently, the tsunami produced by the earthquake near the Sumatra coast affected far of places like Srilanka and South India and even African coast.
Tectonic earthquakes are classified according to their depth of origin into:
(i) Normal – When the depth of origin is less than 50 km.
(ii) Intermediate- When the depth is in between 50 to 400 km and
(iii) Deep seated – When the depth is more than 400 km.