How We Locate Earthquakes
The P-Wave
Before we give an explanation of how we locate
and determine the size of an earthquake, we must first look
as some of the physics behind them. Every time there is
an earthquake, there is a release of two main kinds of waves
that get sent through the rock, giving us the ground motion
that an earthquake can cause. The first of these waves is
called the primary wave, or P-wave. This wave is similar
to a sound wave that moves through the air. When the rock
becomes compressed, it compresses the rocks next to it,
and then rocks next to that one, and so on and so on, giving
an area where the rock is compressed and an area where it
is stretched. One can use a slinky to observe the movement
of the P-wave through the rock.
| Slinky Diagrams by Prof. Lawrence
W. Braile |
The P-wave moves faster through the
rock than any other wave produced by an earthquake, hence
the name primary wave. It is also the wave that causes the
initial felt shock of an earthquake, which some report they
can hear before they feel it. This noise comes from two
sorces. One sorce of the sound is the wave causing
buildings and objects to vibrate. The other source of the
sound comes from the wave itself. As it reaces the surface,
the compressed rock begins to compress the air, making sound
waves. These sounds are often described as the sound of an
incoming train.
The S-Wave
The next wave to hit is the secondary wave,
or S-wave. This wave creates an actual wave that you might
think of as in a wave pool, and it travels slower than the
P-wave. This wave is what you might feel when after the
initial shock has hit. It gives way to a very undulating
ground, which is why it may be very difficult to walk when
an earthquake occurs.
With this information, we can now tell how
far away one of our seismometer stations is from the earthquake.
With carefully calculated equations, we can measure the
how much longer it took for the S-wave to arrive compared
to the P-wave and can deduce the approximate distance from
the station to the quake. We can also use this information
to assess an approximate magnitude of the earthquake as
well.
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