What are seismic waves?

March 5, 2020 | Environment
Seismic waves: energy waves that travel through the Earth's layers | Photo: Creative Commons

A seismic wave is an energy wave that travels through the Earth's layers.

It can be created by an earthquake, a human-made explosion, a volcanic eruption, the breaking of a large rock, magma movement, large scale landslides.

Seismic waves can be detected, measured, and recorded on seismographs, seismometers, accelerometers, or hydrophones.

The frequency range of a seismic wave can be greater than 20 hertz and as low as 0.01 hertz. The human ear's audible frequency ranges from 20 hertz to 20 kilohertz.

Seismologists, the geophysicists who study the origins and propagation of seismic waves, can learn a lot about the interior of the Earth by studying this phenomenon.

There are two main types of seismic waves: body waves and surface waves.

Body Waves

A body wave is a high-frequency, energy wave that is created and travels through the interior of the Earth, and arrives before the surface waves generated by an earthquake.

There are two types of body waves: primary waves and secondary waves.

Primary Waves

Primary Waves | Illustration: seismicresilience.org.nz

A primary body wave, also known as P wave or compressional wave, is the fastest seismic wave and the first to be detected by a seismic station.

It moves fast through fluids and solid rock, and pushes and pulls (compresses and dilates) the rock it moves through - that's why they are also named pressure waves.

Although humans can only "feel" them, animals like dogs start barking just before an earthquake strikes.

Primary waves travel at between 330 meters per second in air, 1450 meters per second in water, and around 5000 meters per second in granite.

They are roughly 1.7 times faster than secondary waves.

Secondary Waves

Secondary Waves | Illustration: seismicresilience.org.nz

A secondary wave, also known as S wave or shear wave, is the second wave felt after an earthquake occurs.

It cannot move through liquids and gasses, only through solid rock.

As a result, seismologists were able to conclude that the Earth's outer core is liquid.

S waves are around 60 percent slower than P waves, and they move rock particles up and down, perpendicular to the direction that the wave is traveling.

Surface Waves

A surface wave, as the name suggests, are waves that travel along and below the Earth's surface.

They tend to be generated when the source of the earthquake is close to the Earth's surface.

They are slower than body waves and tend to lose energy as they get further away from the surface.

Nevertheless, surface waves typically are the most destructive type of seismic waves.

There are three types of surface waves: Rayleigh waves, Love waves, and Stoneley waves.

Rayleigh Waves

Rayleigh Waves | Illustration: seismicresilience.org.nz

A Rayleigh wave, also known as ground roll, spreads like rolling ocean waves, but through the ground.

Named after 3rd Baron Rayleigh, John William Strutt, these seismic waves move the ground both horizontally and vertically, and side-to-side, in the direction that the wave is traveling.

Most of the earthquake's shaking is due to the Rayleigh wave, which travels 10 percent slower than secondary waves.

Love Waves

Love Waves | Illustration: seismicresilience.org.nz

A Love wave is similar to secondary waves but does not move vertically - only from side to side across the ground, in a horizontal motion.

Named after mathematician Augustus Edward Hough Love, these energy ripples can be particularly damaging to the foundations of human-made buildings and structures.

They do not spread through water and travel slightly faster than Rayleigh waves.

Stoneley Waves

A Stoneley wave is a large-amplitude boundary wave that propagates along a solid-fluid interface.

It can be generated along the walls of a fluid-filled borehole.

Named after seismologist Robert Stoneley, Stoneley waves allow scientists to estimate the locations of fractures and permeability of the rock formations and their properties.

Seismic Waves and Tsunamis

Tsunamis can be triggered by a wide range of natural events, including underwater earthquakes, landslides, volcanic eruptions, asteroid and meteorite impacts, and submarine rockslides.

These fast and devastating, high-energy waves are formed after the collapse of large rock formations into the water or by the vertical movement of the seafloor.

Because these energy waves either pass through (primary waves) or reflect off the water interface (secondary waves), they will not be directly responsible for generating tsunamis.

However, the shaking caused by seismic waves might trigger landslides that form a tsunami.

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