Answer:
P and S waves travel at known velocities or speed through the Earth. S waves are slower or weaker than P waves by a known amount or determined amount. As the P and S waves travel out from an earthquake the P waves get progressively farther ahead of the S waves. Therefore, the farther a seismic recording station is from the earthquake epicenter the greater will be the difference in time of arrival between the P and S wave. The distance of a seismic station from an earthquake is easily determined from the S-P interval, the time difference between the time of arrival of the first P wave and the first S wave.
