M A S T E R P A G E
Unlike body waves, which travel through the Earth,
surface waves travel around it. The two main types of
surface waves are called Rayleigh waves and Love
waves. These surface waves travel more slowly than S
and P waves, and attenuate more quickly.
Within Rayleigh waves, Earth particles move in
elliptical paths whose plane is vertical and set in the
direction of energy transfer. When an Earth particle is
at the top of the ellipse, it moves toward the energy
source (seemingly backwards), then around,
downward, and forward, away from the source. It then
moves around and upwards back to its original
position. This produces a ripple effect at the Earth’s
surface that is similar to ripples on a pond. The orbits,
or paths, of these particles become smaller and finally
die out at a certain depth within the Earth.
Love waves move particles in a back and forth
horizontal motion as the energy moves forward. If you
could see a Love wave inside the Earth, you would
notice a zigzag horizontal motion.
Putting Them Together
Since these four types of waves shake a location on
Earth’s surface in various ways and directions, a
seismic station needs at least three seismographs to
glean a reasonably good image of ground shaking at
that location. One seismograph is built to measure
vertical motions, and two others, aligned perpendicular
to each other, measure horizontal motions. Although P,
S, and Rayleigh waves may be recorded on all three
seismographs, P waves are best recorded on the
vertical component and S waves on the horizontal.
Love waves are only recorded on the horizontal
components, but Rayleigh waves are recorded on all
three.
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