Geoscience Reference
In-Depth Information
(b)
(c)
T
(a)
I
M
E
Figure 4.1.
Successive stages in the deformation of (a) a block of material by (b)
P-waves and (c) SV-waves. The sequences progress in time from top to bottom and
the seismic wave is travelling through the block from left to right. An arrow marks
the crest of the wave at each stage. (b) For P-waves, both the volume and the shape
of the marked region change as the wave passes. (c) For S-waves, the volume
remains unchanged and the region undergoes rotation only. The particle motion and
deformation for SH-waves (horizontally polarized S-waves) are the same as for
SV-waves (vertically polarized S-waves) but occur entirely in the horizontal plane
instead of the vertical plane; imagine (c) rotated through 90
◦
. (From Phillips (1968).)
4.1.2 Body waves
Body waves are seismic waves that travel through the body of the Earth. The
propagation of body waves is similar to that of light: body waves are reflected
and transmitted at interfaces where the seismic velocity and/or density change,
and they obey Snell's law (Section 4.3.2). There are two types of body waves.
1. P-waves (P stands for primary, pressure or push-pull). These waves involve compres-
sion and rarefaction of the material as the wave passes through it but not rotation.
P-waves are most correctly called dilatational or irrotational waves. They are the ana-
logue in a solid of sound waves in air.
2. S-waves (S stands for secondary, shear or shake). These waves involve shearing and
rotation of the material as the wave passes through it but no change in volume. S-waves
are most correctly called rotational or equivoluminal waves.
Figure 4.1 shows the deformation undergone by a block of material when P- and
S-waves pass through it. The P-wave particle motion is longitudinal, meaning
that the particles making up the medium through which the P-wave is passing
