For earthquakes, there is a lapsed time between the arrival of the
p-wave and the s-wave, because the waves travel at different speeds. The length
of this period of time determine the
distance to the epicenter.
Earthquakes create seismic waves. P waves are faster than S
waves. As the P and S waves move out from an earthquake, the P waves get ahead
of the S waves. The distance of a seismic station from an earthquake can be
determined from the time difference between the time of arrival of the first P
wave and the first S wave.
