Chapter 1

Seismology
 

The upper most part of the earth is considered to be divided into two layers with different deformation properties. The upper rigid layer, called the lithosphere, is about 100 km thick below the continents, and about 50 km under the oceans, and consists of Crust and rigid upper-mantle rocks. The lower layer, called the asthenosphere, extends down to about 700 km depth. The rigid lithospheric shell is broken into several irregularly shaped major plates and a large number of minor or secondary plates. The lithospheric plates are not stationary, on the contrary, they float in a complex pattern, with a velocity of some 2-10 cm/year on the soft rocks of the underlying asthenosphere like rafts on a lake.

Figure 1.4 The state of convection currents below the earth’s surface and their effect on plate movement (From http://www.pbs.org/gbh/aso/ tryit/ tectonics/intro.html].

This theory requires a source that can generate tremendous force is acting on the plates. The widely accepted explanation is based on the force offered by convection currents created by thermo-mechanical behavior of the earth’s subsurface. The variation of mantle density with temperature produces an unstable equilibrium. The colder and denser upper layer sinks under the action of gravity to the warmer bottom layer which is less dense. The lesser dense material rises upwards and the colder material as it sinks gets heated up and becomes less dense (refer Figure 1.4).  These convection currents create shear stresses at the bottom of the plates which drags them along the surface of earth.