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.
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.
Refraction works both ways
Refraction and reflection happen simultaneously at the same spot.
Refraction and reflection on the inside of the earth
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):
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.
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 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.