Laboratory Manual for Introductory Geology 4e

There are two kinds of body waves, distinguished by how particles move as the wave
passes through rocks: P-waves (primary waves) and S-waves (secondary waves). In
P-waves (FIG. 11.2a), particles in rock vibrate back and forth (red arrow) in the direction
that the wave is traveling (green arrow). A wave that moves this way is called a longitudinal
wave. You can demonstrate a P-wave by stretching a Slinky on a table and pushing
on one end while keeping the other end in place. As the “P-wave” passes through the
Slinky, the coils move as shown in Figure 11.2a: instead of remaining equal distances
apart, they bunch together in some places and move farther apart in others.
FIGURE 11.2 Different types of earthquake waves.
Body waves
P-waves
Compressions
Vibration direction
Dilations
Undisturbed rock
Wave propagation
(a) P-waves can be generated by pushing and pulling on the end of a spring. P-waves are
compressional body waves, so the vibration direction is parallel to the direction of wave
movement.
S-waves
Vibration
direction
Amplitude
Undisturbed rock
Wavelength
(b) S-waves can be produced by moving the end of a rope up and down. S-waves
are shear body waves. As the waves pass through rock, the vibration direction is
perpendicular to the direction of the wave movement.
Wave propagation
Surface waves
L-waves
Ground surface
R-waves
Surface waves die
out with depth.
Particles underground follow a
circular path as the wave passes.
Ground
surface
Ground
surface
Wave propagation
Wave propagation
(c) When an L-wave passes, the ground surface moves back and forth. R-waves make the ground surface
move with a rolling motion.
11.2 CAUSES OF EARTHQUAKES: SEISMIC WAVES
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