waves arrive just after the S-waves. If the earthquake has a shallow focus, the surface waves
will be the largest ones recorded.
Figure 7.40: These seismograms show the arrival of P-waves and S-waves. The surface
waves arrive just after the S-waves and are difficult to distinguish. Time is indicated on the
horizontal portion (or x-axis) of the graph. ( 25 )
If a seismogram has recorded P-waves and surface waves, but not S-waves, the seismograph
wasontheothersideoftheplanetfromtheearthquake. Scientistsknowthattheearth’souter
core is liquid because S-waves cannot travel through liquid. The liquid outer core creates an
S-wave shadow zone on the opposite side of the planet from the earthquake’s focus where no
S-waves reach. The amplitude (height) of the waves can be used to determine the magnitude
of the earthquake. How magnitude is calculated will be discussed in a later section.
Finding the Epicenter
A single seismogram can tell a seismologist how far away the earthquake was but it does not
provide the seismologist with enough information to locate the exact epicenter. For that,
the seismologist needs at least three seismograms. Determining distance to an earthquake
epicenter depends on the fact that different seismic waves travel at different speeds. P-waves
always arrive at a seismometer first, but the amount of time it takes for the S-waves to arrive
after the P-wave indicates distance to the epicenter. If the epicenter is near the seismometer,
the P-waves, S-waves and surface waves will all arrive in rapid succession. If the epicenter
is further away, the S-waves will lag further behind. In other words, the longer it is between
the arrival of the P-wave and S-wave from an earthquake, the farther the epicenter is from
the seismometer.
After many years of study, geologists know the speed at which the different types of waves