Compare and contrast primary (P) and secondary (S) waves. How are they used to determine the nature of the land?
P (or primary) waves travel faster than S (secondary) waves through rocks.
P waves are longitudinal or compression wavess that oscillate in the direction of their motion (like a worm.) S waves are transverse waves that oscillate perpendicular to their direction of travel (like a snake.) This means they travel at different speeds through different rocks. We know the speeds they travel in rocks from lab experiments and so can judge from their arrival times (and often the difference in arrival times or S minus P time) what types of rock they have travelled through to move from source to detector.