Seismic Waves – P waves, S waves, L waves

An earthquake in simple words is the shaking of the Earth’s surface. It happens when energy stored inside the Earth is suddenly released, producing waves that travel in all directions.

Why does the earth shake?

The shaking of the Earth is mainly related to the release of energy along a fault.

• A fault is a crack or break in the Earth’s crust.
• Rocks on either side of a fault tend to move in opposite directions.
• However, friction between rock layers prevents immediate movement.

Over time:

• Pressure gradually builds up due to continuous forces inside the Earth.
• When this pressure becomes too strong, it overcomes the friction.
• The rocks suddenly slip or break, releasing energy in the form of shockwaves.

This sudden release of energy is what causes an earthquake.

Important terms explained:

• The point inside the Earth where energy is released is called the focus (hypocentre).
• The point on the Earth’s surface directly above the focus is called the epicenter.
• The epicenter experiences the strongest shaking first, as it is closest to the origin.

Earthquake Waves

All natural earthquakes occur within the lithosphere, which extends up to about 200 km below the Earth’s surface.

• The waves generated during an earthquake are recorded using an instrument called a seismograph.
• These waves are known as earthquake waves or seismic waves.

There are two main types of earthquake waves:

• • Body waves – travel through the interior of the Earth
  • Surface waves – travel along the Earth’s surface

Body waves:

• Generated at the focus
• Travel in all directions through the Earth
• When they reach the surface, they create surface waves, which are usually more destructive

Important concept:

• • The speed (velocity) of waves changes depending on the material they pass through.
  • Materials that are more rigid (elastic) allow waves to travel faster.
  • Waves may also change direction due to:
    • Reflection (bouncing back)
    • Refraction (bending while passing through different materials)

There are two types of body waves:

• P-waves (Primary waves)
• S-waves (Secondary waves)

The behavior of Earthquake Waves

Earthquake waves are recorded using a seismograph and are mainly of three types:

• • P-waves (Primary waves) – longitudinal in nature
  • S-waves (Secondary waves) – transverse in nature
  • Surface waves (L-waves) – long waves traveling on the surface

Key concept:

• The velocity and direction of these waves change when they pass through materials of different density and temperature.
• Cooler and more rigid regions allow waves to travel faster, while hotter regions slow them down.

Primary Waves (P waves)

• Also called longitudinal or compressional waves
• Particles vibrate in the same direction as the wave travels

Key features:

• Fastest waves – first to reach the surface
• High frequency
• Can travel through:
  • Solids
  • Liquids
  • Gases

Velocity pattern:

• Solids > Liquids > Gases

Additional concept:

• The shadow zone of P-waves lies between 103° and 142° from the epicenter
• This helps scientists understand the structure of the Earth’s inner core

Secondary Waves (S waves)

• Also called transverse or distortion waves
• Similar to ripples in water

Key features:

• Slower than P-waves
• Arrive at the surface after a time lag
• Particles move perpendicular to the direction of the wave

Important limitation:

• Cannot travel through:
  • Liquids
  • Gases

This property is very important in understanding the Earth’s interior.

Additional concept:

• The shadow zone of S-waves covers a large portion of the Earth
• Their absence beyond certain regions led to the discovery of the liquid outer core

Surface Waves (L waves)

• Also called long-period waves
• Travel only along the Earth’s surface

Key features:

• Low frequency but long wavelength
• Cause movement:
  • Up and down
  • Side to side
• Affect only the upper layers of the Earth

Impact:

• Responsible for maximum destruction
• Cause:
  • Collapse of buildings
  • Ground displacement
• Recorded last on a seismograph

Propagation of Earthquake Waves

Different types of earthquake waves move in different ways, causing vibrations in the materials they pass through.

• P-waves:
  • Vibrate parallel to the direction of travel
  • Cause compression and expansion (squeezing and stretching)
• S-waves and surface waves:
  • Vibrate perpendicular to the direction of travel
  • Create crests and troughs, similar to waves in water

These movements are responsible for the shaking effect experienced during earthquakes.

The emergence of Shadow Zone

When earthquake waves travel through the Earth, they are recorded by seismographs placed at different locations. However, there are certain regions where these waves are not detected.

Such regions are called shadow zones.

Key observations:

• • Areas within 105° from the epicenter record both P and S-waves
  • Areas beyond 145° record only P-waves
  • The region between 105° and 145° is the shadow zone for both waves
  • Beyond 105°, S-waves are completely absent

Important conclusions:

• The S-wave shadow zone is larger than that of P-waves
• It covers about 40% of the Earth’s surface
• The P-wave shadow zone appears as a band around the Earth

How these properties of ‘P’ and ‘S’ waves help in determining the earth’s interior?

The study of earthquake waves helps scientists understand the internal structure of the Earth.

Key principles:

• • Reflection – waves bounce back when they hit a boundary
  • Refraction – waves bend when passing through different materials

By analyzing:

• Changes in wave speed
• Changes in direction
• Presence of shadow zones

Scientists can:

• Estimate the density of Earth’s layers
• Identify different internal layers like crust, mantle, and core

Why S-waves cannot travel through liquids?

S-waves are shear waves, which means they move particles sideways (perpendicular to direction of travel).

For this movement, the material must have shear strength (ability to resist shape change).

• • Solid rocks have enough shear strength → S-waves can pass through them
  • Liquids do not have shear strength → S-waves cannot travel through them

Simple example:

• If you remove a glass holding water, the water does not retain its shape and flows away
• This shows that liquids lack rigidity