Seismic Waves

Earthquakes are one of the planet's most dangerous natural forces. They originate in the crust of the Earth which is a thin layer of a depth of around$50\mathrm{km}$below the surface. Below the crust is a thick layer of molten rock, which is solid but can move due to high heat and high pressures. The Earth's outer core lies beneath the mantle, is mostly fluid and surrounds a solid inner core. Earthquakes are caused when volcanoes erupt or when tectonic plates in the Earth's crust are in relative motion with each other. This causes a seismic wave on the surface of the Earth which can be violent enough to destroy the land and buildings.

Definition of seismic waves

It is clear that earthquakes cause disturbances in the Earth and these disturbances carry energy through the Earth to areas away from the point where the earthquake occurs.

In this article, we will learn about the different types of seismic waves and the energy that is carried by each.

Types of seismic waves

The crust of the Earth is solid rock and lies directly beneath the continents and the oceans. The crust is separated into plates that can sometimes move against each other with large amounts of friction occurring between them. This friction can lead to vibrations (disturbances) in the crust which is the source of seismic waves. Seismometers are instruments that can be used to measure the magnitudes of earthquakes by producing a seismogram which can be read to determine the amplitude and intensity of the related seismic waves.

We will discuss four seismic wave types that differ in how they propagate energy and the media through which they move. These seismic waves can be separated into two broader groups; body waves and surface waves. Body waves are seismic waves that travel through the inner layers of the Earth, whereas surface waves travel only along the Earth's surface. From the definition of the seismic wave, both surface and body waves carry energy through the Earth.

Types of seismic waves: P-waves

The first type of seismic wave that we will study is the P-wave. P-waves or primary waves are usually the first waves to be felt when an earthquake occurs. This is because P-waves travel the fastest of all waves generated by an earthquake. P-waves are body waves since they move along the inner layers of the Earth with a speed of about$1500\mathrm{m}/\mathrm{s}$in water and$5000\mathrm{m}/\mathrm{s}$in granite. P-waves can travel through both solid ground and water and travel through the inner core, outer core and mantle.

P-waves are also longitudinal waves. That is, the material of the medium, through which a P-wave passes, will vibrate parallel to the direction in which the wave is travelling. P-waves do not have large amplitudes and so their effects on the ground are not severe and generally do not cause much damage. Due to their longitudinal nature, P-waves cause material to compress and expand as they move through the medium.

Types of seismic waves: S-waves

Secondary waves or S-waves are seismic waves that travel at speeds of about 50% to 60% of the speed of the primary waves. Their effects are hence only felt after the effects of the P-waves during an earthquake. S-waves are also classified as body waves as they move along the inner layers of the Earth. S-waves waves cannot travel through water and can only travel through solid material and hence travel through the mantle only.

S-waves are transverse waves. This means that the material of the medium moves from side to side while the wave moves perpendicularly to the motion of the material. S-waves have an amplitude that is greater than P-waves and their effects are felt more severely and can cause greater damage. S-waves cause shear in the material of the medium of propagation.

Types of seismic waves: Rayleigh waves

Rayleigh waves are seismic waves that travel at speeds of 80% to 90% of the speed of S-waves. The effects of these waves are usually felt shortly after the S-wave has passed. Rayleigh waves are created by a combination of transverse and longitudinal motion. The material of the Earth moves along circular paths whilst the wave itself propagates in a direction that is perpendicular to the plane of these circles.

The effects are much more dangerous than primary and secondary waves since Rayleigh waves are classified as surface waves. The circular nature of the motion of surface material means that the ground moves up and down as the Rayleigh wave moves. This means that they carry all their energy along the Earth's surface and can cause a large degree of shaking and damage.

Types of seismic waves: Love waves

Love waves are the last of those that are seismic. They travel slower than S-waves or P-waves but slightly faster than Rayleigh waves. Love waves are transverse and the material of the Earth's surface moves perpendicular to the direction of wave motion, with an amplitude that is parallel to the surface. Love waves are classified as surface waves and their energy can be immediately transferred to buildings and objects on the ground. Love waves can cause the greatest destruction and damage outside of the epicentre of an earthquake.

Seismic Wave Diagrams

The diagram below shows the compressions and rarefactions of P-waves which are consistent with longitudinal waves. We can see that the materials vibrate in the same direction as the wave's motion.

A depiction of a primary seismic wave (P-wave) in which the particles oscillate in a direction that is parallel to the direction of wave motion (left to right). P-waves can travel through solids and liquids, adapted from image by Davezelenka CC BY-SA 4.0

The diagram below shows the transverse nature of S-waves that has materials vibrating in a direction that is perpendicular to the direction of motion of the wave. S-waves and P-waves travel beneath the surface of the Earth.

A depiction of a secondary seismic wave (S-wave) where the particles oscillate perpendicular (up and down) to the direction of wave motion (left to right). S-waves can travel through solids only, adapted from image by Davezelenka CC BY-SA 4.0

The following image depicts a Rayleigh wave that contains properties of both longitudinal and transverse waves, with materials moving in circular paths in planes perpendicular to the direction of wave motion.

The Love wave is shown in the diagram below and indicates that the wave is transverse, with the material moving perpendicular to the direction of propagation of the wave. The amplitude of the material is parallel to the surface. Love waves and Rayleigh waves travel on the surface of the Earth.

A diagram of a Love wave that shows the direction of the vibrating material to be perpendicular to the direction of wave motion. The amplitude of the wave is parallel to the Earth's surface, Wikimedia Commons CC 4.0