8.2 Seismic Waves
Properties of Waves
Earthquakes are releases of energy that travel through seismic waves. These waves have the same properties as waves caused by moving water or sound. So let’s quickly review what the basic properties of a wave are.
The wave crest is the topmost or highest part of the wave. The wave trough is the bottommost or lowest part of the wave. The rest position is where there is no positive or negative height of the wave – in water waves, it is called sea level.
Amplitude is the total height of a wave from the rest position. Wave height is the total height of a wave from crest to trough, or highest point to the lowest point.
A wavelength is a distance between two wave crests or two wave troughs.
A wave’s period is the amount of time that it takes for one full wave to pass any given point. It is often measured in seconds.
A wave’s frequency is the number of full waves that pass a point in a specific amount of time. It is measured in Hertz, Hz.
Seismic Waves
A seismic wave is a wave of energy caused by the sudden breaking of rock within the earth or an explosive force such as an impact. Seismic waves travel through the earth as energy and they can be recorded on instruments called seismographs. A seismograph is a device that is very sensitive to vibrations. As it experiences vibrations, a needle at the tip of the device will move up and down along a piece of paper, recording the seismic wave in what is called a seismogram.
There are different types of seismic waves that an earthquake can generate, and they have had far-reaching consequences on how we view the earth and how we experience earthquake damage. Learn about the four types of seismic waves in this video below.
A P wave is a primary wave and is a type of body wave, which is a higher frequency wave that arrives before surface waves. This is the fastest of all the seismic waves and, therefore, will be the first to arrive at a seismic station. The P wave can move through solid rock as well as liquids. It moves through compressional and extensional motion, pulling and pushing on the rock.
An S wave is a secondary wave and is also a type of body wave. This is a slower wave, and it can only move through solid rock. It moves with shear motion through up and down, and sometimes side to side motion.
A Love wave is a type of surface wave, which has a lower frequency than body waves but tends to be responsible for most of the damage resulting from earthquakes. This is the fastest surface wave and moves on the ground from side to side by horizontal motion.
A Rayleigh wave is another type of surface wave that causes most of the shaking felt from earthquakes. It rolls along the ground with both up and down and side-to-side motion.
Let’s Review!
In-Depth Thinking
The reason we know some of the properties of the Earth’s interior, such as the density of the lower mantle and core, and that the outer core is liquid was due to observations with seismic waves. Think about some of the different properties of waves (S and P waves in particular), and look carefully at this image.
