The Earth’s Inner Structure and Seismic Waves


By Robert Hazen, Ph.D.George Mason University

The Earth has a radial structure, quite like the layers of an onion. We know about the Earth’s inner structure from the study of seismology. Seismology is the study of the Earth’s inner structure using sound waves, and it provides us with our clearest picture by far of the Earth’s deep interior.

Image of a male hand showing the seismological activity on a measuring paper.
Seismology is the study of the Earth’s inner structure using sound waves. (Image: Belish/Shutterstock)

Seismic Waves

Sound waves pass through the Earth at speeds, typically, of a few kilometers per second. Different materials have different speeds. For example, the speed is much faster if you have a dense, cold, solid material like rocks near the surface. Speed decreases if the material is less dense; it also decreases if it’s hotter or even if it’s partially molten.

These waves also bend. As the waves pass through the Earth, they tend to bend as they go through materials of different densities, partially molten materials, and so forth. In addition, these seismic waves, these waves of sound, bounce off different layers of the Earth. For example, at the core-mantle boundary, much of the seismic energy, much of these waves bounce off and reflect back up towards the surface.

This is a transcript from the video series The Joy of ScienceWatch it now, on Wondrium.

P-waves and S-waves

We have two different kinds of waves in seismology. First, we have primary or p-waves. These are compressional waves. The motion of the wave is the same as the motion of the atoms.

Illustration showing the difference between p-waves and s-waves.
There are two different kinds of waves in seismology: p-waves and s-waves. (Image: Fouad A. Saad/Shutterstock)

Suppose you have a material, and you push on it suddenly. Each atom tends to push on the atom next to it. So a wave of energy passes through the material as an atom pushes against an atom pushing another atom. That’s the compressional wave, a primary wave, or a p-wave, as it’s called in seismology. 

We also have secondary waves, s-waves. These are somewhat slower, and they’re also known as shear waves. These are the waves that, when you move an atom up and down, jostle the atom up and down next to it. This is like “the wave” in a stadium; when you do a wave in the stadium, one person stands up, and then the person next to him stands up; and you see a wave moving around the stadium, even though the motion of an individual is up and down. That’s a shear wave, also a secondary wave, or an s-wave, in seismology. 

P-waves always travel faster than s-waves. Another important distinction is that s-waves quickly die out in a liquid. If you have a liquid region anywhere inside the Earth, the p-waves travel right through it, but the s-waves stop.

The seismic waves are measured by instruments called seismometers. These are sensitive to very slight variations of ground movement. They usually involve a heavy weight sitting on a delicate spring, so the slightest movement of the ground causes the arm to move, causing the pen to jiggle on a paper.

Learn more about the organization of atoms.

Seismology and Nuclear Test-ban Treaties 

Image of a seismometers
Seismometers are used to measure earthquakes. (Image: Belish/Shutterstock)

Most of these seismic waves, most of these sound waves in the Earth, are generated by earthquakes. That’s a situation where rocks suddenly crack and break, and suddenly move, and they generate a big pulse of sound and energy that goes swinging through the planet in various paths. 

One can also set off local explosions. At least for local seismic studies, where one is trying to study the detailed structure of a very small volume of rock, the best way to do it is to set off small explosive charges at the surface, and that also provides sound waves for seismology. For this reason, because you can set off explosions and detect them, seismology has been greatly advanced because it’s the key to nuclear test-ban treaties. 

You can’t have a nuclear test-ban treaty unless you have a seismic network to look for those explosions. So the U.S. Government and the Department of Defense have put a considerable amount of money into seismology.

Learn more about earthquakes, volcanoes, and plate motions.

Earth’s Interior Design

Seismology reveals the details of the Earth’s deep interior, and it shows that the core is divided into two separate layers. We have a solid-iron inner core with a radius of 1,200 kilometers. That’s the centermost part of the Earth, and the maximum pressures at that core approach 3.5 million atmospheres with 5,000 degrees centigrade, or greater, temperatures. 

Then we have an outer core, which is liquid. Remember, s-waves don’t pass through liquid, so that’s how we know it’s a liquid outer core. It’s formed primarily of a nickel-iron metal, but it has to have a small amount of some lighter element because the density of that outer core is not quite great enough to just be nickel-iron. 

Indeed, the composition of this outer core is one of those as yet unanswered questions that allow scientists to suppose highly speculative models.

Common Questions about the Earth’s Inner Structure and Seismic Waves

Q: What are the two different kinds of seismic waves?

The two different kinds of seismic waves are p-waves and s-waves. P stands for primary, and s stands for secondary.

Q: What are two main distinctions between p-waves and s-waves?

The two main distinctions between p-waves and s-waves are: firstly, p-waves always travel faster than s-waves, and secondly, s-waves quickly die out in a liquid.

Q: What is the main structure of the Earth’s core?

Seismology reveals the details of the Earth’s deep interior, and it shows that the core is divided into two separate layers. There is a solid-iron inner core and an outer core, which is liquid. The composition of the outer core not confirmed yet.

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