Cause of earthquakes

Earthquakes are caused due to disequilibrium in any part of the crust of the earth. A number of caused have been assigned to cause disequilibrium in the earth's crust such as volcanic eruptions, faulting and folding, up warping and down warping, hydrostatic pressure of man-made water bodies like reservoirs and lakes, and late the plate movements. The occurrence of severe devastating earthquake of San Fransisco (U.S.A) in 1960 led H.F. Reid, one of the official investigators of San Fransisco Earthquake Disaster, to advance his important ELASTIC REBOUNB THEORY to explain the mode and causes of earthquake mainly caused by fractures and faults in the earth's crust and upper mantle. Recently, PLATE TECTONIC THEORY has been accepted as the most plausible explanation of earthquakes.

As per theory of the plate tectonics the crust of the earth is composed of solid and moving plates having either continental crust or oceanic crust of even both-continental-oceanic crust. The earth's crust consists of 6 major plates (Eurasian plate, American plate, African plate, Indian plate, pacific plate, and Antarctic plate) and 20 minor plates. These plates are constantly moving in relation to each other due to thermal convective currents originating deep within the earth. Thus all the tectonic events take place along the margins of plates.

From the standpoint of movement and tectonic events and creation and destruction of geo-materials the plate margins are divided into 3 types. (i) CONSTRUCTIVE PLATE MARGINS are characterized by continuous addition of geo-materials from below along the mid-oceanic ridges. These molten hot materials are cooled and solidified and are added to the trailing margins of the divergent plates. Infect, divergent plates move in opposite direction from the mid-oceanic ridges and there is always addition of new crust to the trailing ends of these plates because of cooling and solidification of molten lavas. (ii) DESTRUCTIVE PLATE MARGINS are those where two convergent plates collide against each other and the heavier plate margin is sub-ducted below the relatively lighter plate margin, this result in constant loss of crust materials. (iii)  CONSERVATIVE PLATE MARGINS are those where two plates slip past each other without any collision. This process results neither in the creation nor in the destruction of crust. Major tectonic events associated with these plate margins are rupture and faults along the constructive plate margins; faulting and folding along the destructive plate margins and transform faults along the conservative plate margins. Thus these major tectonic events of faulting and folding and mountain building cause volcanic eruptions and earthquake of varying magnitudes along different plate margins.