Earthquake Shock Waves

Earthquakes are crazy natural disasters that we've all heard about, but maybe never experienced. Either way, we know what happens, the earth shakes rapidly and causes damage to buildings and property in the area. But did you know that the shaking that we experience during an earthquake is actually a shock wave? You may have heard of shock waves as well, examples include when a whip is whipped, or a jet travels very quickly in the sky. These are all shock waves, but shock waves that occur during an earthquake are quite different. What do shock waves have to do with earthquakes and how do earthquake shock waves work? Let's find out.

Earthquake shock waves definition

Before we get into the specifics of shock waves that occur during earthquakes, let's first make sure we know exactly what an earthquake is and what we mean by mean by shock waves in the context of earthquakes.

When an earthquake occurs, energy is released and it spreads out in all directions, moving away from the source like ripples on a pond. These perturbances that propagate through the Earth's layers are known as shock waves or seismic waves.

Specifically, this compression needs to occur at a high speed, the speed of sound. The speed of sound is 343 metres per second (\(\mathrm{m/s}\)), and if an object is to travel through a medium at this speed, a shock wave will occur.

When a shock wave occurs, the point at which it originates undergoes intense physical changes in the blink of an eye. The temperature in this region increases massively for a small time, as well as the density of the material, as the Pressure has pushed much of the medium into this single point for a short time.

So now we have an idea of what shock waves are in general,

we can go into more detail in regards to Earthquake shock waves. When it comes to earthquakes, we typically don't call them shock waves, we call them seismic waves. These seismic waves are called shock waves the same as we have described just now, however they aren't shock waves in the same sense.

Seismic waves are caused by the sudden release of pressure caused by the Friction between tectonic plates deep underground. The sudden release of this pressure results in waves of energy heading up towards the surface. There are multiple different kinds of seismic waves when an earthquake occurs, with different properties and times that they occur for each, but we'll get to that later. These shock waves are what we feel and experience when an earthquake happens where we live, the violent shaking of the ground is the result of a shock wave passing through it!

Types of shock waves earthquakes

As we've previously discussed, earthquakes generate more than one kind of shock wave. There are four different types of shock waves, all with different properties.

P-waves

P-waves are the first of the shock waves to occur after the earthquake, and one of the two main shock waves to occur overall. This wave is also known as a compressional wave, and it is also longitudinal. This means that the wave creates areas of compression and dilation that travel along the surface of the earth, disrupting anything on this ground as it travels across it. As this is the first type of shock wave to occur in an earthquake, it is the first detectable shock wave by technology.

Fig. 3: A P-wave in action. Compressions and rare factions can be seen.

S-waves

S-waves are the second type of shock wave to occur after the earthquake, and also the second of the two main shock waves that occur overall. These are different to P-waves in that they are transverse waves instead of longitudinal waves. This means these are the waves that propagate perpendicular to the direction of oscillation, therefore the oscillation is perpendicular to a P-wave. Because these waves are transverse, S-waves cannot travel through liquid, this includes magma and molten rock underneath the Earth's surface. These shock waves can also be called shear waves.

Fig. 4: An S-wave in action.

Rayleigh-waves

Rayleigh-waves might be a type of shock wave to occur in an earthquake that you haven't heard of before. Rayleigh-waves travel across the Earth's surface in an elliptical pattern. As a result of this, Rayleigh-waves have a horizontal and a vertical component to their shock waves.

Fig. 5: A Raleigh-wave in action.

Love-waves

Love-waves are also a type of shock wave to occur in an earthquake that you may not be familiar with. Love waves are also a surface wave, meaning they travel along the surface of the Earth. This makes them quite similar to Rayleigh-waves, however, the key difference is that Love Waves travel as a transverse wave as opposed to the more elliptical motion of the Rayleigh-waves.

Fig. 6: A Love-wave in action.

Earthquake shock waves speed

As we know, earthquakes generate more than one kind of shock wave. As a result, these different kinds of shock waves all move at different speeds.

P-Waves have a range of velocities, from \(629\,\,\mathrm{m}/s\) to \(2599\,\,\mathrm{m/s}\). S-waves are slower than P-waves, ranging from about \(288\,\,\mathrm{m/s}\) to \(1200\,\,\mathrm{m/s}\). We can think of Rayleigh-waves as like waves in the water, making them travel at a few kilometres a second. Similarly, Love-waves will travel between \(2-6\,\,\mathrm{km}{s}\).

The most powerful shock waves from an earthquake

Before we're able to cover the most powerful shock waves, and therefore the most powerful and destructive earthquakes to have ever occurred, we first need to learn about how the power of earthquakes is measured. The technology we use to record the intensity of earthquakes are called seismographs.

Fig. 7: A seismometer inside a seismograph. Look at how the whole thing is bolted down for it to work properly!

Fig. 8: An example of how an earthquake looks on the Richter scale.

So now that we know how earthquakes are measured, what units do we measure them with? Earthquake shock waves are measured using the Richter scale, a scale used to measure the size (also known as magnitude) of an earthquake, therefore how powerful the shock wave of an earthquake is. The measurement of an earthquakes shock waves is determined using the amplitude of the largest shock wave the seismograph registers. The amplitude of a wave is the height that it reaches, the higher the height meaning the more powerful the wave.

Now that we know how earthquake shock waves are measured, and what scale they are measured on, let's look at the most powerful recorded earthquake in history. The most powerful earthquake to ever occur happened near Valdivia in Chile, in 1960. This earthquake registered on the Richter scale as a 9.5 magnitude, also causing an enormous tsunami to occur that travelled at nearly 200 miles per hour.