Geology Reference
In-Depth Information
and expanding until it reaches the other end. P waves can move through solids
and liquids.
S waves
are
shear waves.
These waves move more slowly and move rocks in an up-
and-down motion, very much like holding one end of a piece of rope and wiggling
it up and down. Because they move more slowly, S waves arrive after P waves at a
seismograph,
or earthquake measuring station. S waves can travel only through
solid material.
Measuring magnitude
When people first started “measuring” earthquakes a little more than 100 years ago,
they used a scale based on eyewitness accounts of destructive damage to buildings.
This is called the
Mercalli intensity scale
after its developer, Giuseppe Mercalli. Obvi-
ously, a scale like this is not very useful for measuring earthquakes in areas that do not
have buildings. Since then, a Modified Mercalli Intensity Scale has been developed using
modern buildings in California as its standard. However, it is useful for measuring earth-
quakes only in the United States and parts of Canada because many parts of the world
have very different styles and standards for buildings.
Nowadays seismologists use a more accurate method of measuring earthquakes. A box
called a
seismometer
is buried in the solid rock layer beneath the soil. As the waves pass
through, shaking the rocks, the seismometer box also moves, and a weighted pen sus-
pended inside (which doesn't move with the earthquake waves due to its weight and the
fact that it is suspended) records the shaking movements as a series of squiggly lines.
Figure 10-5 shows what a seismometer looks like.
A seismometer sends a signal to a machine in a lab or monitoring station called a
seis-
mograph,
which records the squiggly pattern of wave energy. The printout of this pat-
tern of wave energy is called a
seismogram.
Figure 10-6 shows a seismogram of both S
and P waves recorded by a seismometer.
Figure 10-5:
A seis-
mometer.
