Laboratory Manual for Introductory Geology 4e

S-waves (FIG. 11.2b) are seismic waves in which the particle movement is
perpendicular to the direction in which the wave is traveling. A wave that moves this way
is called a transverse wave. You can demonstrate an S-wave by having two people hold
the ends of a rope and asking one to whip the rope in an up-and-down motion. As
the “S-wave” passes, the rope wriggles like a snake.
P-waves travel faster through rock than S-waves and therefore reach the surface
first. When P- or S-waves reach the surface, some of their energy is converted
to two types of surface waves: the L-wave (Love wave), a shear wave similar to
an S-wave in which particles vibrate horizontally, parallel to the ground surface,
and the R-wave (Rayleigh wave), a unique wave type in which particles move in a
circular pattern opposite the direction in which the wave is traveling (FIG. 11.2c).
Particle motion in L-waves is horizontal—you can model this with a rope as for the
S-wave, but whip it horizontally rather than vertically. Particle motion in R-waves is
circular, like the wheel of a bicycle as the bike moves. This analogy describes the
rotational motion of the ground as the R-wave passes through, but isn’t perfect
because the rotation is actually in the opposite direction from that in which a
bicycle wheel rotates.
Seismic waves are detected with instruments called seismometers. These instruments
are anchored in bedrock to measure the amount of ground movement
associated with each type of wave. Seismic recording stations use separate seismometers
to measure vertical and horizontal ground motion (FIG. 11.3). As the ground
vibrates, the rotating drum moves with it, but the inertia of a weight keeps the pen
in position, causing the pen to trace ground movement on the drum. Modern
seismometers replace pen and paper with digital recorders. The printed or digital
record of ground motion is a seismogram.
Exercise 11.1 will help you to understand how seismic waves affect buildings and
cause damage.
FIGURE 11.3 Vertical and horizontal seismometers.
Motion
direction
Motion
direction
Spring
Wire
Pivot
Pivot
Weight
Pen
Rotating
drum
Weight
Bolt
Pen
Rotating
drum
Ground
(a) Vertical seismometer: The pivot lets the pen record only
vertical motion.
(b) Horizontal seismometer: The pivot allows the pen to
record only horizontal motion.
278 CHAPTER 11 EARTHQUAKES AND SEISMOLOGY