Seismic Waves and Their Types

At a Glance

• Body waves P (primary) and S (secondary)
• Surface waves Love and Rayleigh; slowest, most destructive
• P waves Pass through solids, liquids and gases
• S waves Pass through solids only
• P shadow zone Between about 105 and 145 degrees from the epicentre
• Exam link UPSC Prelims, Geography: Interior of the Earth and Magnetic Field

Where Does It Fit?

This is the second topic of the category on the interior of the Earth and its magnetic field. The previous topic named the sources of information, and this one explains seismic waves, the best source. The next topic covers the crust, mantle and core. Previous topic: Sources of Information about the Interior of the Earth. Next topic: Crust, Mantle and Core – Internal Structure of the Earth.

What Are Seismic Waves?

In short: seismic waves are the energy of an earthquake travelling through the Earth.

An earthquake begins at a point inside the Earth called the focus or hypocentre, where the rocks break and release energy. The point on the surface directly above the focus is the epicentre, and it is the first place to experience the shaking. The energy spreads outward as seismic waves in all directions. Instruments called seismographs record these waves as a seismogram, which shows the arrival time and size of each wave. Seismic waves are of two broad kinds: body waves, which travel through the interior, and surface waves, which travel along the outer layer of the Earth.

What Are P Waves and S Waves?

Body waves are generated by the release of energy at the focus and move in all directions through the body of the Earth. They are of two types.

The velocity of both waves increases with the density and rigidity of the material. They also bend, or refract, when they enter a layer of different density, and they reflect when they strike a boundary. These changes make it possible to detect layers inside the Earth.

What Are Surface Waves?

When body waves reach the surface they produce a third type, the surface waves, also called long waves or L waves. They move only along the surface of the Earth and are the slowest, but they have the largest amplitude and are the most destructive, which is why most building damage is caused by them. Their effect fades with depth. Two main kinds are Love waves, which shake the ground horizontally from side to side, and Rayleigh waves, which move the ground in a rolling motion like ocean waves. Seismographs record the order of arrival as P waves, then S waves, then surface waves.

What Are Shadow Zones?

Seismographs at different distances from the epicentre record the waves, but there are regions where some waves are not recorded. These are shadow zones. The shadow zone of P waves lies roughly between 105 and 145 degrees from the epicentre, because P waves are refracted as they enter the liquid outer core. S waves are not received at all beyond about 105 degrees from the epicentre, because they cannot pass through the liquid outer core. The shadow zone of S waves is therefore much larger than that of P waves, covering more than 40 per cent of the surface of the Earth.

The absence of S waves beyond 105 degrees is the key evidence that the outer core is liquid. The layers they reveal are described in Crust, Mantle and Core – Internal Structure of the Earth.

How Is an Earthquake Located?

The distance of a seismograph from the epicentre can be found from the time gap between the arrival of P and S waves: the larger the gap, the greater the distance. Readings from at least three stations are then used to fix the epicentre by triangulation, where three circles drawn with the distances as radii meet at one point. As an analytical point, the same method that locates earthquakes is used to study the interior, so each large earthquake is a natural experiment that sends waves through the whole planet. The causes and scales of earthquakes are discussed in Earthquakes – Causes, Focus and Richter Scale.

Why Is This Topic Important?

• Interior Waves reveal layers
• Earthquakes Basic vocabulary
• Prelims P versus S comparisons

What Came Next?

The next topic describes the crust, mantle and core. Next topic: Crust, Mantle and Core – Internal Structure of the Earth.

Key People and Terms

• Focus Point of origin of an earthquake
• Epicentre Surface point above the focus
• Seismograph Records seismic waves
• Shadow zone Area where waves are not recorded
• Love wave Horizontal surface shaking
• Rayleigh wave Rolling surface motion

Exam Corner

Mains Angle

Questions ask how waves prove the structure of the Earth. These points add depth.

• Speed Changes with density and rigidity
• Shadow Absence of S waves shows liquid
• Damage Surface waves cause it