Stations A through D will respond to an
earthquake with compressive p-waves,
shearing s waves, and rolling surface
waves. The signals are recorded on the
seismograms by color code as they arrive
at each station. In reality, only Station A
would respond to the earthquake
whereas the distant stations can only
detect seismic waves using sensitive
instruments. But here we jostle them in
cartoon fashion to emphasize compressive
and shearing wave motions that appear
on seismograms. We can use the travel time
distance between the fast P waves and
the slower S waves to figure out how far
the earthquake was from each station
using a simple travel time curve that
seismologists developed to locate earthquakes.
Let's look at Station B. The
seismogram shows that the P wave
arrived 9 minutes and the S wave arrived 
15 minutes after the earthquake occurred;
a difference of 6 minutes. If we replace
the seismograms with a Travel-time Graph
and take a small bar 6 minutes long, we
find that it fits in only one location:
between the red P and blue S wave curves,
a location that indicates that Station B
is about 2,700 miles from the epicenter
 
