The Inside of the Earth

How Seismic Waves Travel Through Different Materials

The amount of time that it takes for a seismic wave to pass through the earth is dependent on the material that it encounters along its path.

By monitoring arrival times of seismic waves throughout the earth we can make determinations about what types of materials are found in the earth.

For example, S waves do not reach from an earthquake to the opposite side of the earth--indicates presence of a liquid core.

precise recordings of seismic waves after atomic explosions revealed the presence of the asthenosphere.
 

Reflection and Refraction

When traveling through the earth, seismic waves frequently pass through materials that transmit them more or less quickly. Remember that the speed of seismic waves depends on variations in strength and density of the rocks that they pass through.

Reflection: When seismic waves hit a surface (boundary) between very different materials they may bounce off this surface.   If you have heard an echo, you have experienced a reflection of sound waves. In the earth, seismic waves reflect at the boundaries between the major earth layers.

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Diagram of Reflection showing how light from the fisher is reflected by the air-water interface into the "roving eye". Note that the angle that the light hits the surface is the same as the angle that it leaves the surface.  The roving eye sees a reflection of the fisher

Refraction: When seismic waves pass through different types of materials their speed is altered and consequently their path through the materials may be bent. You experience refraction when you see that a straw inserted in water appears bent. Refraction may occur at a distinct boundary like reflection, but it also occurs when the material changes slowly.

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Diagram of Refraction showing how light from the fish is refracted through the air-water interface to the fisherman. Note that the light hits the air-water interface at a high angle, and emerges from the interface at a much lower angle.  This is because speed of light in water is slower than in air. The result is that the fisher, no matter how low he/she gets to the water, can only see the top side of the fish.

Refraction works both ways

 

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Diagram of Refraction showing how light from the fisher is refracted through the air-water interface to the fish. Note that the angles are the same as the picture above, but the light is moving the other way. The result is that the fish has to look up in order to see the fisher who stands to the side.

Refraction and reflection happen simultaneously at the same spot.

 

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Diagram of Simultaneous Reflection and Refraction showing how part of the light from the fisher is refracted through the air-water interface to the fish and how, simultaneously, the remainder of the light from the fisher is reflected by the air-water interface into the "roving eye". Note that whenever light or any other wave form hits any interface, there can be both reflection and refraction. A lower angle of incidence promotes more reflection at the expense of refraction (if the fisher looks across the water he/she is more likely to see a reflection, but if looking down directly into the water, he/she is more likely to see into the water (a refraction).

Refraction and reflection on the inside of the earth

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Cross section of Earth showing some of the possible paths of P waves and how they reflect and refract inside the earth before emerging at the surface somewhere else. Note that whenever any of these paths intersect with an interface, both reflection and refraction can occur, and in some cases, both paths are shown.  This is a simple illustration: just because a path is not shown does not mean that is does not exist.

Note that the all paths through the earth tend to bend upward.  This is a result of refraction because materials deeper in the earth conduct seismic waves faster than materials above.

When an Earthquake occurs, seismic waves are emitted from the focus (=hypocenter) there are several paths that it can take through the earth before emerging again at the surface.  These paths are symbolized by letters (refer to the above figure):

  • p = P wave arrival from a path that traveled upward from the focus (hypocenter)
  • pP = P wave arrival from a path that traveled upward from the focus, reflected off the surface of the earth, then arrived back at the surface.
  • P = P wave arrival from a path that traveled downward from the focus (hypocenter)
  • PP = P wave reflected off the surface once
  • PPP = P wave reflected off the surface twice
  • c = a reflection off the outside of the outer surface of the outer core. Note that this is the principle cause of multiple arrivals of p and s waves right at the epicenter
  • K = a travel path (refraction) through the outer core
  • KK = one reflection on the inside outer surface of the outer core
  • KKK = two reflections off the inside outer surface of the outer core
  • i = a reflection off the outside of the outer surface of the inner core
  • I = a travel path (refraction) through the inner core

These letters can be used to indicate the path of a seismic wave through the earth (refer to the above figure).  For example PKiKP indicates that the wave traveled downward from the focus, refracted through the outer core, reflected off the surface of the inner core, traveled through the outer core, then traveled through the mantle to arrive at the surface. SKiKS is the same path, but an S wave.

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Chart of Elapsed time vs. Angular Distance from Epicenter showing the arrival times for different seismic wave paths through the earth. The angular distance is in degrees with 180 degrees being the point on earth directly opposite the earthquake epicenter. This chart is constructed by lining up seismograms according to their distance from the epicenter, and it helps seismologists determine which arrival times represent different paths in the earth.  This is how we know what the inside of the earth looks like.

The arrival times of different waves are constructed by lining up many seismograms according to their position with respect to the original earthquake epicenter. By carefully studying graphs like this one, seismologists are able to determine how the velocity of seismic waves varies with depth.  Along with other constraints, this information is used to determine what the inside of the earth is made of.

One of the more telling phenomena is the S-wave Shadow zone.  S-waves cannot travel through the outer core because the outer core is made of liquid iron.

Free Oscillations

Free oscillations are also called "standing waves". If you are a bathroom singer (and who isn't?) you might have noticed that, in the bathroom, certain notes are accentuated and others are dampened out. A similar phenomenon is found in the ringing of a bell or gong or the vibration of a piano string. Each of these rings (oscillates) only at certain frequencies.

When the earth "rings" it is called a free oscillation. Because of the shape and construction of the earth, there are several frequencies and modes of free oscillation.