The P waves undergoes compression and dilation during motion in its direction of propagation. This waves reaches the station first because the P waves travels at a speed, that is much faster in comparison to the S waves. The S waves are the secondary waves and are commonly known as the shear waves.
The P waves have the ability to travel in both solid as well as in liquid, whereas the S waves can propagate only in solid region because they are absorbed at the core-mantle boundary.
Thus, the P waves will arrive the station first and the difference between the arrival of P and S waves are plotted in the graph that helps in depicting the epicenter and the amount of energy released during an earthquake.
