The most important tool that has been used by scientists to gain indirect knowledge about the interior of the earth is the behavior of earthquake waves as they pass through the earth. Such vibratory waves are recorded by an instrument called a Seismograph.

There are three types of earthquake waves which travel at different speeds in materials of different densities and states (solid or liquid).

Scientists compare the speeds and paths of different types of waves produced by earthquakes. By doing so, scientists are able to determine that very important changes are taking place in the nature of the material below the earth’s surface.

Of course, this type of knowledge gained is supplemented by studies of earth’s magnetism and gravitational pull.


Seismic waves and interior of the earth:

An earthquake generates the following three types of waves during its occurrence: (i) Primary waves also called P waves, (ii) Secondary waves, also called S waves, and (iii) Surface waves- L waves.

(i) P waves are high frequency longitudinal waves:

These waves travel through the solid part of the earth as well as through the liquid part of the earth’s core. Their fastest speeds are recorded while passing through the solid part of the earth, but in the liquid part their speed slows down. They are the first to arrive at the surface.


(ii) S waves are also called transverse or distortional waves:

They cannot pas through the liquid geomaterials. These are high frequency, short wave-length waves. They travel in all directions from the focus of an earthquake. Their velocities while passing through the crust, mantle and core vary proportional to the density of the medium.

This is a very important characteristic of secondary waves. These waves have helped scientists to understand the structure of the interior of the earth.

(iii) Surface waves or L waves are the last to report on seismograph:


These waves are very destructive and affect the earth’s surface only. These waves travel through the surface regions of the earth’s crust and do not go into the interior of the earth. L waves are the slowest and are characterized by largest oscillation.

It has been observed that the velocities of the P and S waves increase with depth up to the boundary of the core. In the upper crust the velocity of P waves is 5.6 km per second, but their velocity is raised to 8 km/sec at the Moho.

From there their velocity increases again till it reaches 13.6 km/sec at the Oldham or Gutenberg discontinuity that marks the upper boundary of the outer core.

After entering the core the velocity suddenly drops across the Oldham discontinuity to about 8.10 km/sec. However, in the Inner core it rises again and reaches 11.2 km/sec.


It is the changes in the velocity and courses of the P and S waves and their variants which have provided information on the earth’s interior. If the earth were homogeneously constituted the P and S waves would have followed straight paths.

However, they are subject to reflection and refraction like light rays at the boundary of the various zones of different constitution and density. Following the principle of optics the P and S waves are concave up­wards. It is due to the increase of density with depth.

These figures show that Pg and Sg waves travel at lower velocities than P* and S* waves. It was shown by laboratory experiment that these three velocities are the same with which the earthquake waves travel in layers of granite (Pg and Sg), basalt (P* and S*) and peridotite.

So it clear that below the sediments the crust is made of three parallel layers, viz, the granitic layer, the basaltic layer and the peridotite layer.


The thickness of the granitic layer is about 10 km and that of the basaltic layer about 20 to 25 km. below the above two layers is the peridotite layer whose thickness is greater than that of the upper or intermediate layers. The lower layer may be in a glassy state and not in a crystalline condition.

The intermediate layer is thought to be amphibolites. Some scientists consider it to be glassy basalt. The upper layer might include denser igneous rocks and some metamorphic or sedimen­tary rocks in the upper sections.

The velocity of earthquake waves provides information about the mantle. The sharp boundary between the crust and the high velocity portion of the upper mantle was noted by the Yugoslavian seismologist A.

Mohorovicic in 1909, and has been called the Mohorovicic discontinuity or Moho. He determined that seismic waves change at this depth, owing to sharp contrast of materials and densities.


As the P waves cross the boundary and enter deeper region the velocity suddenly rises to about 8 km/sec. This sudden increase in the velocity suggests denser matter of the mantle.

The shadow zone extending from 105° to 143° does not receive the P and S waves. Up to about 105° these waves are transmitted across the interior to the surface showing that up to the upper surface of the core of the earth, the mantle is composed of solid matter.

Further, inwards is the core which behaves as liquid in not trans­mitting S waves towards the antipodal region. It was discovered by Oldham as fluid.

When P wave enters the core at the boun­dary called Oldham or Gutenberg Discontinuity its velocity suddenly decreases and the wave is refracted. It has been found that the velocity of P waves suddenly increases as they pass from the outer to the inner core.

From the foregoing discussion it is clear that there are several physical properties of the earth materials that enable us to know, though only approximately, the nature of the earth’s interior.

For example, when an earthquake or underground nuclear test sends shock waves through the earth, the cooler areas which gene­rally are more rigid, transmit these seismic waves at a higher velocity than do the hotter areas.