# What are seismic waves? How have they helped in understanding the structure of the earth? (250 W/ 15 M)

Mentor’s Comment:

• Introduction should explain about Seismic waves as it has been the basis of most hypotheses developed on earth’s structure. How it gets generated.
• Further, mention the types and their respective features concerning the earth’s structure. How seismic waves are measured.
• Next, mention how these waves helped in understanding the features of earth. As we know today, that Inner core is solid, outer core is liquid. Also the way waves travel gives ideas regarding properties of material through which it is travelling. It also helps in understanding the earth’s velocity, regions of earth quakes, shadow zones etc.
• Mention conclusion based on the points of main part(s).

• Most hypotheses about the internal structure of the earth are developed by studying seismic waves that travel through the earth and are measured at seismometer stations. Seismic waves are generated in earthquakes and they travel differently through different types of material.

Seismic Waves:

• The slipping of land generates seismic waves and these waves travel in all directions.
• Earthquake is caused by vibrations in rocks. And the vibrations in rocks are produced by seismic waves.
• Seismic waves are produced when some form of energy stored in Earth’s crust is suddenly released, usually when masses of rock straining against one another suddenly fracture and slip.
• Seismic waves are basically of two types – body waves and surface waves.
1. Body 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. Hence, the name body waves.
• There are two types of body waves. They are called P and S-waves.
• P waves are also called as the longitudinal or compressional waves. They are analogous to sound waves. P-waves move faster and are the first to arrive at the surface.
• Secondary Waves (S waves) are also called as transverse or distortional waves. They are analogous to water ripples or light waves. S-waves arrive at the surface with some time lag. These waves are of high frequency waves and travel at varying velocities through the solid part of the Earth’s crust, mantle.
1. Surface Waves
• The body waves interact with the surface rocks and generate new set of waves called Surface Waves. These waves move along the surface.
• The velocity of waves changes as they travel through materials with different elasticity or stiffness. The more elastic the material is, the higher is the velocity. Their direction also changes as they reflect or refract when coming across materials with different densities.
• Surface waves are of two types viz. Rayleigh Waves and Love waves.
• L waves are also called as long period waves. They are low frequency, long wavelength, and transverse vibration. Generally affect the surface of the Earth only and die out at smaller depth. They cause displacement of rocks, and hence, the collapse of structures occurs. These waves are the most destructive and are recorded last on the seismograph.

Seismic Waves and Structure of Earth:

• Seismic waves can tell us a lot about the internal structure of the Earth because these waves travel at different speeds in different materials.
• Reflection causes P and S waves to rebound whereas refraction makes waves move in different directions.
• The variations in the direction of these waves are inferred with the help of their record on seismograph.
• Change in densities greatly varies the wave velocity. By observing the changes in velocity, the density of the earth as a whole can be estimated. By the observing the changes in direction of the waves (emergence of shadow zones), different layers can be identified.
• For both kinds of waves, the speed at which the wave travels also depends on the properties of the material through which it is traveling.
• Scientists are able to learn about Earth’s internal structure by measuring the arrival of seismic waves at stations around the world.
• For example, we know that Earth’s outer core is liquid because s-waves are not able to pass through it; when an earthquake occurs there is a “shadow zone” on the opposite side of the earth where no s-waves arrive.
• Similarly, we know that the earth has a solid inner core because some p-waves are reflected off the boundary between the inner core and the outer core.
• By measuring the time it takes for seismic waves to travel along many different paths through the earth, we can figure out the velocity structure of the earth.
• Abrupt changes in velocity with depth correspond to boundaries between different layers of the Earth composed of different materials.

The structure of Earth’s deep interior cannot be studied directly. But geologists use seismic (earthquake) waves to determine the depths of layers of molten and semi-molten material within Earth. Geologists are now using these records to establish the structure of Earth’s interior.