(i) Earthquake triggers tremors through the interior of the Earth, these move in the form of waves radiating from the focus in all directions. Body waves are a type of earthquake wave, along with surface waves. Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth, and hence the name body waves.

(ii) The direct sources of information about the interior of the earth are:

Rock samples

The most easily available solid earth material is surface rock or the rocks from mining areas. Apart from mining and surface rocks, scientists have taken up various projects to penetrate deeper depths to explore the conditions in the crustal portions. Many deep drilling projects have provided large volume of information through the analysis of materials collected at different depths.

Volcanoes

Volcanic eruption is a major source of direct information about the interior of the earth. As and when the molten material (magma) is thrown onto the surface of the earth, during volcanic eruption, it becomes available for laboratory analysis.

(iii) Earthquake waves develop shadow zones because of the properties of each (P and S) waves as well as the core. Outer crust is liquid and hence S-waves, which can travel only through solid material, is blocked and cannot move across the core. P-waves, which can move faster and travel through gaseous, liquid and solid material, gets refracted inside the core, and in the process P-waves fails to manifests itself in certain regions.

Thus, S-waves are not found beyond 105° from focus of an Earthquake, whereas P-waves are absent between 105° and 145° from the focus. Therefore, the regions between 105° and 145° forms a shadow zone for both kinds of earthquake waves.

(iv) Altogether there are 5 different indirect sources of information of the interior of the earth. Excluding the seismic activities these are:

1. Analysis of properties of matter indirectly.

Mining activity have proved that temperature and pressure increase with depth. It is also known that the density of the material increases with depth. Knowing the total thickness of the earth, scientists have estimated the values of temperature, pressure and the density of materials at different depths.

2. Meteors

The material and the structure observed in the meteors are similar to that of the earth. They are solid bodies developed out of materials same as, or similar to, our planet. However, the material available for analysis in such a way is not from the interior of the earth.

3. Gravitation

The gravitation force (g) is not the same at different latitudes on the surface, it differs from the poles to the equator. This is because the distance from the centre of the earth varies based on the latitude (less at poles, maximum at equator). The gravity values also differ based on the mass of material. Thus, the gravity measured at different places is influenced by these factors (latitude and mass). When these gravitation readings differ from the expected values, we get gravity anomaly. These anomalies give us information about the distribution of mass of the material in the crust of the Earth.

4. Magnetic Field

Magnetic survey provides information about the distribution of magnetic materials in the crustal portion, and thus, provide information about the distribution of materials in this part.