# Seismic Wave: Definition, Types & Frequency

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• 0:02 Seismic Waves
• 2:26 Body Waves
• 3:25 Surface Waves
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

In this lesson, you will learn what seismic waves are, the various types of seismic waves, and their frequencies. You will learn about P-waves, S-waves, Rayleigh waves and Love waves. A short quiz will follow.

## Seismic Waves

Earthquakes are scary, right? Well, they're scary because they create seismic waves, and those seismic waves carry energy. Technically all waves carry energy, but seismic waves carry a lot of it. And that is enough to send people diving for the nearest sturdy table or door-frame.

Seismic waves are low-frequency waves that travel through the Earth. They're generally caused by an earthquake, explosion or volcano. Seismic waves are studied by seismologists and detected using seismometers. These waves are described using a number on the Richter scale. The largest earthquake ever detected was in 1960, in Chile, and was given a rating of 9.5 on the Richter scale.

Because of their high energy, earthquake waves are definitely the most damaging and dramatic. But why is it that they have so much energy?

Earthquakes happen when sections of the Earth's crust hit each other. The Earth's surface is split into many sections called tectonic plates that float on the sea of magma underneath. For example, most of the Pacific Ocean makes up the Pacific Plate. When these tectonic plates hit each other, or scrape alongside each other, the result is an earthquake. Because of the huge size of the plates, and the significance of the collisions, there is a lot of energy released when this happens. And that energy can cause huge amounts of damage.

Most people don't notice the changes that happen during an earthquake from start to finish. To be fair, that's probably not surprising considering they spend most of that time running around in a panic or hiding underneath the kitchen table. But earthquakes contain at least four distinct types of seismic waves, which are grouped into two categories of surface and body waves. An earthquake is like a four-course meal, with every course different than the last... and every course increasingly bad for you!

One difference between types of waves is the way some move through the interior of the Earth and others ride along the surface. As you can see in the image above, there are two general categories of waves: surface waves and body waves. Surface waves are, unsurprisingly, the ones that move along the surface of the Earth, while body waves go through the interior.

## Two Types of Body Waves: P-Waves vs. S-Waves

The first thing you feel when an earthquake happens is the P-wave, or primary wave. This is like the salad course - it gets to the table quickly, and isn't particularly bad for you. The primary wave moves faster and arrives first. It's a longitudinal wave, meaning it vibrates the ground parallel to the direction of motion - it basically shakes the ground up and down or side to side. Despite having the highest frequency (the number of vibrations per second), P-waves cause relatively minor damage.

Next comes the S-wave, or secondary wave, which is like the main course - it can be sizable and have a big impact on your health. The S-wave is pretty much the exact opposite of the P-wave: It's transverse (the vibration is 90 degrees to the direction of motion), it's slow moving, but it causes far more damage. Imagine it as a ripple through the ground. S-waves have lower frequencies than P-waves but still much higher frequency than surface waves.

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