How earthquake waves help determine interior of the Earth?

Earthquake generally occurs due to the occurrence of a fault in the earth’s interior. The movement of the tectonic plates results in the release of energy. This energy comes out as waves. On the earth’s surface, they are recorded using instruments and then analyzed based on their velocities, and in this way, we determine the structure in the interior i.e. the layers and the state of the material present. Also, the interior of the earth could be determined using below sources :

1. Direct sources: Volcanic Eruption, Mining.
2. Indirect sources: Meteorites, Seismic waves, gravity anomaly, magnetic fields.

Since the interior of the earth is very hot we cannot dig deeper. Hence indirect sources are used to find the material, density, pressure, and temperature within the earth.

Seismic Waves

Interior of the Earth

The interior of the earth is divided into the following three different layers:

1. The Crust: The topmost layer of the earth is known as the crust. It extends up to a depth of 40km. It is thin and brittle mostly made of rocks. The crust is further classified as :

• Oceanic crust: It is found on the oceanic basin and made up of basaltic magma. It is younger than continental crust and is only 6-7 km thick.
• Continental crust: It is found on the continent and made up of andesitic magma. It extends up to a depth of 35-40 km.

2. The Mantle: It occupies 84 percent of the Earth’s volume. The mantle is around 2900 km thick. It is hot and made up of solid rocks. Major elements of mantle are Silicon and Magnesium. Also known as SiMa Layer.

3. The Core: It is the innermost layer forming the center of the earth. It occupies 15 percent of the Earth’s volume and is in molten state. It is mainly made up of Nickel and Iron. Also known as NiFe Layer.

Now, let us look how seismic waves help scientists determining the interior of the earth.

Seismic Waves

Seismic waves is the energy which comes out during an earthquake. Earthquake occurs due to the release of energy from the faults within the earth. The point from where this energy is released is called the focus of an earthquake. and epicentre is the point on the surface nearest to the focus. These seismic waves can be classified as further:

Seismic Waves

1. Body Waves: These are generated due to the release of energy at the focus and it move in all the directions travelling through the body of the earth.

Body waves

2.Surface Waves: When body waves interact with the rocks on the surface, they generate a new set of waves called surface waves. These waves move along the surface.

As the waves travel through different densities, the velocity changes. Velocity is directly proportional to the density of the material i.e. the denser the material, the higher will be the velocity. Also reflection or refraction occurs while passing through various densities.

Body waves are further classified into P-waves and S-waves.

P-waves: They are the first to arrive at the surface and can pass through all the three types of material that is solid, liquid and gaseous material. Their vibration is parallel to the direction of the wave.

S-waves: They arrive at the surface after a time gap and can travel only through solid materials. They vibrate perpendicular to the direction of the propagation.

Seismic Waves and Structure of the Earth

The seismic waves are measured using an instrument known as Seismograph. There are specific areas where these waves are not reported. Such a zone is known as Shadow Zone. P-waves shadow zone lies between 105° and 145° whereas zone beyond 105° is the shadow zone for S-waves because the outer core is liquid and S-wave cannot pass through liquid. These characteristics of the waves tells us that earth consist of concentric layers namely crust, mantle and core.

Frequently Asked Questions

What are the indirect sources to determine Earth’s interior?

The indirect sources includes seismic waves, meteorites, gravity anomaly and magnetic fields.

What are the different layers within the earth?

The Crust, Mantle and Core.

How P-waves differ from S-waves?

P-waves are the first to arrive at surface & passes through solid, liquid and gaseous material whereas S-waves arrive after some time lag & can pass only through solid material.

Which type of seismic wave is more disastrous?

S-Waves or Secondary waves causes more destruction.

What is the Shadow zone of P-wave and S-wave?

Shadow zone of P-wave lies between 105° & 145° and shadow zone of S-waves lies beyond 105°.