Waves Wave- A self-propagating disturbance. Medium- The substance through which the wave propagates. Source- The object that disturbed the medium to create the wave. Waves carry or transport energy from the source through the medium. As the wave passes through one point of the medium, the medium is disturbed. After the wave has passed, the medium returns to its original location.

There are three types of waves: Transverse, Longitudinal, and Surface There are three types of waves: Transverse, Longitudinal, and Surface. For now, we will focus on only longitudinal and transverse waves. More on surface waves later.

Notice in both this slide and the previous, the waves are moving from right to left. In a transverse wave, the medium is displaced up and down, perpendicular to the direction the wave is moving. In the longitudinal wave, the medium is displaced right and left, parallel to the direction the wave the wave is moving.

All of the red lines shown above are wavelengths of the wave. Wavelength- The distance from a point on a wave to the corresponding point on the next wave. This is a distance, so it is measured in meters. The symbol for wavelength is λ (lamda) . All of the red lines shown above are wavelengths of the wave. The black lines are NOT correct wavelengths.

Amplitude- A measure of the magnitude of the disturbance. Midline of Vibration (AKA Equilibrium Position)- The “zero value” of the wave. The value or line about which a wave oscillates. The point where a wave has zero amplitude. Crest- The high point of a wave. Point where the wave has maximum amplitude. Trough- The low point of a wave. Point where the wave has minimum amplitude. Notice in the next picture that the amplitude is measured from the midline to a crest or from the midline to a trough. Amplitude is NOT measured from the crest to the trough.

Midline of Vibration AKA Equilibrium Position

Period- The amount of time it takes for a wave to oscillate (vibrate) once. Measured in seconds. Symbol for period is T. Frequency- The number of times a wave oscillates (vibrates) in one second. Measured in Hertz (Hz). 1 Hertz = 1 oscillation per second. The symbol for frequency is f. There are two equations that relate Period and Frequency: T = 1/f and f = 1/T In other words, Frequency and Period are reciprocals of each other.

As wavelength increases (as the wave gets longer), the frequency decreases (oscillates fewer times per second) Notice both of those waves have the same amplitude.

The blue wave has the greatest amplitude, the red wave has the lowest amplitude. Notice all the waves have the same wavelength, period, and frequency.

Surface wave:

Notice the size of the circle the medium particles make decreases the deeper you go. When you are at the top of the circle, the crest of the wave is directly over you. When you are at the bottom of the circle, the trough of the wave is directly over you.

When the water depth is less than half a wavelength, there is friction between the deeper water and the bottom. This slows the movement of the bottom of the wave. The top of the wave then moving faster than the bottom, and the wave begins to “break”.

Earthquakes produce “seismic waves” which are a combination of longitudinal, transverse, and surface waves. Black arrow shows the seismic waves are travelling to the right P wave: Longitudinal Wave, ground moves front to back S wave: Transverse Wave, ground moves side to side Surface wave: Ground moves in a circular motion

P and S waves travel from the location of the earthquake inside the earth outward in all directions. P stands for Primary Wave, so named because it travels the fastest and arrives first. P waves are similar to sound waves due to the medium being compressed and expanded. S stands for Secondary Wave, so named because it travels slower than the P wave, and so arrives second. Surface waves are generated when the P and S waves reach the surface. They travel along the surface of the Earth and arrive after the P and S waves. Surface waves are the most destructive of the three types of seismic waves.

Epicenter: Point on the Earth’s surface directly above the location of an earthquake. Can be located by a process of Triangulation: 1. Need three locations that detected both the P and S waves 2. Since both waves travel at constant, known speeds, and P waves are faster, the time between the arrival of the two waves at a location can be used to determine the distance to the epicenter of the earthquake. All three locations calculate the distance to the epicenter. 3. All three locations draw circles on a map. The center of each circle is the location and the radius of each circle is the distance to the epicenter. 4. The location where all three circles overlap is the epicenter of the earthquake

The center of each circle is a location that has detected P and S waves from an earthquake using a seismograph. Each location measures the time between the arrival of the P and S waves, then calculates the distance to the epicenter. The radius of each circle is that distance. Where the three circles overlap is the location of the epicenter of the earthquake.

Damage caused by S- waves

Damage caused by Surface Waves

We know the Earth has a liquid outer core based on the properties of S and P waves. S waves can only travel through solids, not liquids. P waves can travel through both solids and liquids. When there is an earthquake, both waves travel through the solid crust and mantle, but only P waves reach the opposite side of the Earth. Something blocks the S waves from reaching the other side of the Earth. That something is the liquid outer core.

S- wave shadow zone No S waves are detected in the “S wave shadow zone”. However, P waves are detected everywhere around the globe. The only thing that would block the S waves is a liquid core at the center of the Earth.