How do the waves of different types tell us about the changes in the nature of different layers of the earth's interior ?

The most important indirect source to know the interior of the earth is the data collected through earthquake waves.

There are three types of earthquake waves :

(i) ‘P’ waves or primary waves or longitudinal waves. These waves can travel through solid as well as liquid media.

(ii) The ‘S’ or secondary or transverse waves can travel in solid media only. These waves are slow in motion.

(iii) ‘L’ waves or surface waves move along the boundaries between solid, liquid and gaseous media. They do not go deep into the earth. They have a very low velocity.

The velocity of the waves increase if they travel through denser media. The velocity of first two types of waves increases with depth but only upto 2900 kilometres. Afterwards ‘S’ waves do not pass and ‘P’ waves pass at a reduced velocity. The velocity of ‘P’ waves passing through inner core again increases as compared to their passage through the outer core. Based on this finding the scientists have estimated that the iron and nickel of the core of the earth may be in liquid state.

Fig. 3.8. Earthquake Shadow Zones