A study of earthquake engineering calls for a good understanding of geophysical process that causes earthquakes and various effects of earthquakes. Seismology is the study of the generation, propagation and measurement of seismic waves through earth and the sources that generate them. The word seismology originated from Greek words, ‘seismos’ meaning earthquake and ‘logos’ meaning science. The study of seismic wave propagation through earth provides the maximum input to the understanding of internal structure of earth.

1.2 Internal Structure of Earth

The earth’s shape is an oblate spheroid with a diameter along the equator of about 12740 km with the polar diameter as 12700km. The higher diameter along equator is caused by the higher centrifugal forces generated along the equator due to rotation of earth. Though the specific gravity of materials that constitute the surface of earth is only about 2.8, the average specific gravity of earth is about 5.5 indicating presence of very heavy materials towards interior of earth. The interior of the earth can be classified into three major categories as Crust, Mantle and Core (refer Figure 1.1).