How do you determine the difference in time between the arrival of the primary (P) wave and the arrival of the secondary (S) wave during an earthquake?

Primary (P) waves a type of seismic wave that involves alternating compression and expansion of the material through which it passes.

6 to 13 kilometers per second, these waves arrive before S waves. 1.7 times faster than S waves. These waves are compressional. Secondary (S) waves a seismic wave that involves oscillation perpendicular to the direction of propogation.

second to arrive and bigger amplitude than P waves. These waves are shear.