Geology Reference
In-Depth Information
graph
) that records the earthquake wave movements on a printout called a
seis-
mogram.
Scientists watch the seismographs as they print the seismograms to see
when the P waves and S waves arrive. Here's why:
P waves travel quickly through solid materials and slow down, slightly changing
direction, as they move through liquid materials. By recording where each P wave
starts and how long it takes to reach the other side of the planet, scientists have
recognized that it must move through regions of solid and liquid materials within
the earth.
S waves travel through solid materials but cannot travel through liquid at all. When
scientists record the path that S waves take through the earth, they find that some
S waves never reach the other side — they simply disappear, suggesting that they
have hit a section of liquid material.
Figure 4-2 illustrates how the earthquake waves would travel if the interior of the earth
was made of one continuous, solid type of material. And Figure 4-3 illustrates how P
waves and S waves actually travel through earth, illustrating the different physical prop-
erties of its interior.
Figure 4-2:
The
path of wave travel
if earth's interior
were a continuous
solid.
Figure 4-3:
The re-
corded path of P
waves and S
waves.
