Earthquakes produce different types of
seismic waves that can be identified on seismograms.
But did you know that each
seismograph station produces three different seismograms
for each earthquake.  Why is that?
Doesn't one tell you how big the earthquake was?
Earth's movement is three dimensional,
and one component only reflects one direction of movement.
If a station had only one component
you would only know about movement one direction.
different instruments on the seismograph measure
three types of motion:  Up down or vertical motions.
North-south motion and east/west motion.
Let's take a closeup, exaggerated look at how a house would be
affected by each direction of motion.
When the house is shaken in the
east-west direction this shows up on the
lower component.  Nothing is recorded on
the north-south or vertical components.
When the house shifts north-south,  this
shows up on the second line.
When the house is shaken in an up-down or
vertical motion, only the top seismogram
records the motion.
During an actual earthquake the house can move in all direction.
Why is that earthquakes produce
 
different types of waves like
body waves and surface waves. And these
waves travel out from an earthquake in
all directions. Body waves, like P&S waves,
travel a curving path through the earth.
As a result they arrive at seismic
stations from below. P waves are
compressive waves they cause temporary
microscopic deformation of the earth in
the direction the wave is traveling thus
energy is primarily recorded on the
vertical component of the seismogram
with less energy recorded horizontally.
Swaves move more slowly and are more
destructive since they advance with a
shearing or back-and-forth motion.
Because S waves arrive later than P
waves they are usually seen on a
seismogram as a second pulse. S waves
cause deformation perpendicular to the
direction of wave travel which is
primarily horizontal or back-and-forth
motion with less energy recorded on the
vertical or up and down component.
The slower surface wave such as love and
Rayleigh waves move in a more complex
undulating pattern and thus show up
on all three components.
In conclusion, it is the combination of signals
that gives eismologists information about
the magnitude, distance, and type of
earthquake that occurred.
