Building travel time graphs gives a clue to inner earth structure. Notice how the difference in arrival time between P-waves and S-waves at individual stations reflects the relative velocities of these waves. P-waves are transmitted faster (have higher seismic velocities) than S-waves. Notice that the arrival of P-waves (and S-waves) does not plot on a straight line with distance from the earthquake source. This behaviour can be explained if further traveled waves pass through material with faster seismic velocities than do the waves that travel short distances. In effect this means that seismic velocity increases with depth in the Earth (or at least its mantle). We can plot these up in increasing distance around the earth.

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