M A S T E R P A G E
EarthquakeWave Background
3.1a
The major types of seismic waves are classified as
body waves and surface waves. The two have
different shapes and properties. All waves in matter
depend upon the interaction of forces among the
particles of some material. These forces transmit
movement of one particle to movement in adjacent
particles.
Body Waves
Body waves, so called because they travel through
the body of the Earth, consist of two types: primary
(P) and secondary (S). S waves are also called shear
waves and transverse waves.
Primary (P) waves consist of alternating
compressions and expansions (dilations), so they are
also referred to as compressional waves. P waves are
longitudinal; they cause particle motion that is back
and forth, in the same linear direction as energy
transfer. These waves carry energy through the
Earth, usually at the rate of 3.5–7.2 km/sec in the
crust and 7.8–8.5 km/sec in the mantle.
Secondary (S) waves are transverse; the particle
motion they cause is perpendicular to the direction
of energy transfer. Their usual speed is 2.0–4.2
km/sec in the crust and 4.5–4.9 km/sec in the mantle.
Longitudinal (P) waves can be transmitted through
solids, liquids, and gases, while transverse (S) waves
(with the exception of electromagnetic waves) can
only be transmitted by solids. Waves can be
reflected and refracted (bent) when they move from
material of one density to that of another density.
Wave energy can also be changed to other forms. As
they move through the Earth, the waves decrease in
strength, or attenuate. Waves attenuate more slowly
in solid rocks than in the basins full of sediment so
common in the West. Because of this, an earthquake
in the crust of the eastern United States is felt over a
wider area than a quake the same size in the rocks of
the western states.
Surface Waves
Seismograph stations detect surface waves from
many, but not all, quakes. Whether a station detects
them or not depends on the strength of the quake’s
energy release, the depth of the quake, and the
station’s distance from the focus.
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