Question

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?

Answer 1

Primary (P) wavesa 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) wavesa seismic wave that involves oscillation perpendicular to the direction of propogation. 
second to arrive and bigger amplitude than P waves. These waves are shear.

Answer 2

Subtract the P from the S wave