Types of Seismic Waves
1. Body Waves - travel in all directions through the body of the Earth.
There are two types of body waves:
o P - waves - are Primary waves. They travel with a velocity that
depends on the elastic properties of the rock through which they
travel. (rigidity, compressibility, density)
Vp = [(K + 4/3 )/ ]
Where, Vp is the velocity of the P-wave, K is the incompressibility of the
material, is the rigidity of the material, and is the density of the material.
P-waves are the same thing as sound waves. They move through the
material by compressing it, but after it has been compressed it expands, so
that the wave moves by compressing and expanding the material as it
travels. Thus the velocity of the P-wave depends on how easily the material
can be compressed (the incompressibility), how rigid the material is (the
rigidity), and the density of the material. P-waves have the highest velocity
of all seismic waves and thus will reach all seismographs first.
S-Waves - Secondary waves, also called shear waves, travel with a
velocity that depends only on the rigidity and density of the material through
which they travel: Vs = [( )/ ]
S-waves travel through material by shearing it or changing its shape in the
direction perpendicular to the direction of travel. The resistance to shearing
of a material is the property called the rigidity. Liquids have no rigidity, so
that the velocity of an S-wave is zero in a liquid. S-waves travel slower than
P-waves, so they will reach a seismograph after the P-wave.
Surface Waves - Surface waves differ from body waves in that they do
not travel through the Earth, but instead travel along paths nearly parallel to
the surface of the Earth. Surface waves behave like S-waves in that they
cause up and down and side to side movement as they pass, but they travel
slower than S-waves and do not travel through the body of the Earth. Thus
they can give us information about the properties of rocks near the surface,
but not about the properties of the Earth deep in the interior.
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Because seismic waves reflect from and refract through boundaries where
there is sudden change in the physical properties of the rock, by tracing the
waves we can see different layers in the Earth. This allows us to look at the
structure of the Earth based on layers of differing physical properties. Note
that we know that density must increase with depth in the Earth because the
density of crustal rocks are about 2,700 kg/m3 and the average density of the
Earth is about 5,200 kg/m3. Also note from the velocity equations that if
density increases, wave velocity decreases. Thus, the other properties,
incompressibility and rigidity must increase with depth in the Earth at a
greater rate than density increases.
Once we know the seismic wave velocities throughout the Earth, then we can perform experiments on different possible materials and make
estimates of what the chemical composition. Thus, we can also divide the Earth into layers of differing chemical composition.
Layers of Differing Chemical Composition
Crust - variable thickness and composition
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o Continental 10 - 70 km thick, underlies all continental areas, has an average composition that is andesitic.
o Oceanic 8 - 10 km thick, underlies all ocean basins, has an average composition that is basaltic.
Mantle - 3488 km thick, made up of a rock called peridotite. Evidence comes from Seismic wave velocities,
experiments, and peridotite xenoliths (foreign rocks) brought to the surface by magmas.
Core - 2883 km radius, made up of Iron (Fe) and small amount of Nickel (Ni). Evidence comes from seismic wave velocities, experiments, a
iron meteorites, thought to be remnants of other differentiated planets that were broken apart due to collisions.
Layers of Differing Physical Properties
Lithosphere - about 100 km thick (up to 200 km thick beneath continents, thinner beneath oceanic ridges and rift valleys), very brittle,
easily fractures at low temperature. Note that the lithosphere is comprised of both crust and part of the upper mantle. The plates are made
up of the lithosphere, and appear to float on the underlying asthenosphere.
Asthenosphere - about 250 km thick - solid rock, but soft and flows easily (ductile). The top of the asthenosphere is called the Low
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Velocity Zone (LVZ) because the velocities of both P- and S-waves are lower than the in the lithosphere above. Neither P- nor S-wave
velocities go to zero so the LVZ is not completely liquid.
Mesosphere - about 2500 km thick, solid rock, but still capable of flowing.
Outer Core - 2250 km thick - liquid. We know this because S-wave velocities are zero in the outer core (implies liquid state)
Inner core - 1230 km radius, solid
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