Plate Tectonics

How Do We Know What’s Down There?

P and S Waves

Despite what Hollywood may tell you, we cannot drill down to the center of the Earth. So, how do we know what’s down there? We can use a variety of tools to indirectly "see" inside the Earth, just like a doctor uses X-rays to "see" a broken bone. The best set of tools scientists have are seismic waves. Seismic waves are generated by earthquakes, volcanic eruptions, or other things that go bump on the Earth's surface.

There are two basic types: body waves, which travel through the body of the Earth, and surface waves, that travel only along the surface of the Earth.

Body waves are the ones scientists use to see inside the Earth.  There are two types - P waves and S waves.

Seismic Body Wave Movement

P waves	S waves
Primary or Push-Pull waves  Fastest of all waves  Liquids and solids	Secondary or Shear waves  Second fastest of all waves  Solids only - the wave's energy gets absorbed by liquids.
Image source:  "Onde compression impulsion 1d 30 petit" by Christophe Dang Ngoc Chan is licensed under CC BY-SA 3.0	Image source:  "Onde cisaillement impulsion 1d 30 petit" by Christophe Dang Ngoc Chan is licensed under CC BY-SA 3.0

P and S Wave Shadow Zone

When an earthquake releases energy, all of the seismic waves are released at the same time. The P-waves and S-waves are the only ones that travel through the body of our planet. All waves (seismic, sound, light, etc). will travel in a more-or-less straight path and at the same velocity so long as the conditions (density, composition, etc) remain unchanged. Thus the P and the S waves will travel in a straight path throughout the mantle.

At the mantle-core boundary, there is a change not only in density but also in composition and phase; the core is now a denser material, composed of metal, and is a liquid. When waves transition from a medium with one set of conditions into another the waves will bend (or refract) and will start moving in a different direction. Thus, we know where the mantle-core boundary is based upon the following:

• P-waves refract and begin to travel in a new direction
• P-waves speed up
• S-waves stop

We also know that the inner core is solid using these same facts. While the composition and density haven't changed, we know that the inner core is solid because the P-waves refract again slightly and travel in a new direction. The image below shows the paths each of the waves take as they travel through the Earth.

Image source:  "shadow zone" by USGS is in the Public Domain

The refraction of the P waves creates what is called the "P-Wave Shadow Zone" where no seismic waves from the originating earthquake will be present. The same is true for the S-waves. Because their energy gets absorbed by liquids, a large "S-Wave Shadow Zone" where no S-waves from the originating earthquake will be present. Do this with thousands of earthquakes all over the planet over several years, a pattern starts to emerge.

Sometimes it's helpful to visually see what's going on.  Watch this video, "Seismic Shadow Zones—Introduction to P & S wave shadow zones" by IRIS Earthquake Science. As you do, note the following:

• Why and where the P waves refract.
• Why the S waves don't move through the core.
• How the P and S wave shadow zones form.

Interesting new data suggests that the inner core has an inner core. By studying how the seismic waves travel through the inner core scientists have discovered that the iron crystals are aligned in an east-west direction in the inner-inner core and north-south in the outer-inner core. This inner-inner core appears to be about half the diameter of the whole inner core. Read more here: Earth's Inner Core Found To Have A Core Of Its Own