Question

What are the four types of seismic waves? Which are body waves, and which are surface waves?

Short answer

The four types of seismic waves are Primary waves (P waves), Secondary waves (S waves), Love waves (L waves), and Rayleigh waves (R waves). P waves and S waves are classified as body waves since they travel through the Earth's interior. On the other hand, L waves and R waves are surface waves that travel along the Earth's surface. P waves are the fastest and can travel through solid, liquid, and gaseous materials, while S waves only travel through solid materials. L waves cause horizontal shaking, and R waves cause a rolling motion similar to ocean waves.

Step-by-step solution

1. Understanding Seismic Waves

Seismic waves are waves of energy that travel through the Earth and are result of an earthquake, explosion, or volcanic eruption. There are four main types of seismic waves: Primary waves (P waves), Secondary waves (S waves), Love waves (L waves), and Rayleigh waves (R waves).

2. Body Waves

Body waves are seismic waves that travel through the Earth's interior. This category includes two types of waves: Primary waves (P waves) and Secondary waves (S waves).

2.1 Primary Waves (P Waves)

Primary waves, also known as P waves, are the fastest type of seismic waves. They travel through solid, liquid, and gaseous materials by alternately compressing and stretching the material. In other words, they travel through the Earth by pushing and pulling particles in the same direction as the wave is moving.

2.2 Secondary Waves (S Waves)

Secondary waves, or S waves, are slower than P waves and only travel through solid materials. They move particles in a perpendicular motion to their direction of travel. In simpler terms, they travel through the Earth by moving particles up and down or side to side as they propagate.

3. Surface Waves

Surface waves are seismic waves that travel along the Earth's surface, rather than through its interior. They are generally slower than body waves and cause most of the ground motion during an earthquake. This category includes two types of waves: Love waves (L waves) and Rayleigh waves (R waves).

3.1 Love Waves (L Waves)

Love waves, also known as L waves, are surface waves that cause horizontal shaking. They are the fastest of the two surface waves and are responsible for the side-to-side motion experienced during an earthquake. Love waves only travel through solid materials as they require the material to be rigid in order to exist.

3.2 Rayleigh Waves (R Waves)

Rayleigh waves, or R waves, are surface waves that cause a rolling motion, similar to ocean waves. They are slower than Love waves, and their movement tends to be more intense at the surface and decrease in intensity with depth. Rayleigh waves can travel through solid, liquid, and gaseous materials. To summarize, the four types of seismic waves are P waves, S waves, L waves, and R waves. P waves and S waves are body waves, while L waves and R waves are surface waves.

Key concepts

Primary waves (P waves)

Imagine a slinky being stretched and compressed; this is similar to how Primary waves, or P waves, travel. They are the fastest type of seismic waves and one of the first indicators that an earthquake has occurred.

P waves can move through all types of materials—solids, liquids, and gases—because their motion involves the compression and extension of matter. The particles of the medium through which P waves travel move in a parallel direction to the wave energy itself. For instance, if a P wave is moving to the right, the particles are also pushing and pulling in the same rightward direction.

This feature allows P waves to be extremely fast, racing through the Earth's crust and mantle, and even penetrating its liquid outer core.

Secondary waves (S waves)

Imagine cracking a whip or the ripples from throwing a stone into water; this is akin to how Secondary waves, or S waves, move through the Earth. They are slower than P waves and surface waves because their movement is more complex.

S waves are characterized by their movement that shakes the ground perpendicularly to the direction they are traveling. If an S wave is traveling toward the east, it will move particles up and down or side-to-side in a north-south direction. This shearing motion can only occur within solid materials, making S waves unable to travel through liquids or gases, such as Earth’s outer core or the atmosphere.

During an earthquake, these waves trail behind P waves and can cause significant damage due to their shaking motion.

Love waves (L waves)

Imagine sliding a rug across a floor; this back-and-forth motion resembles the horizontal movement produced by Love waves, or L waves. Among surface waves, L waves travel quickly, but they are slower than the body waves P and S.

Love waves are trapped close to the Earth’s surface, gliding horizontally through the uppermost layers of the crust. They cause the ground to sway side-to-side, perpendicular to the direction of the wave. Efficient energy transmission makes Love waves particularly damaging to the foundations of structures. Because they require a rigid medium, you will not find Love waves moving through water or air but only through the solid crust of the Earth.

Rayleigh waves (R waves)

For a visualization of Rayleigh waves, or R waves, picture ocean waves rolling onto a beach. Rayleigh waves move in a rolling motion that involves both an up-and-down and a side-to-side movement as the wave energy travels forward.

These waves can travel through any type of material—solid, liquid, or gas—but they tend to be more intense at the surface and weaken with depth. This characteristic motion of Rayleigh waves can be especially destructive to buildings during an earthquake, as it results in the ground moving in elliptical patterns similar to the way particles move in water waves. R waves commonly follow L waves and are usually responsible for the rolling sensation felt during an earthquake.