APPLIED RADIOLOGICAL ANATOMY

Detailed explanation-1: -Primary (P) and secondary (S) waves are two types of waves caused by earthquakes. They are defined based on when they arrive and are felt on the surface. P waves, or primary waves, arrive first while S waves, or secondary waves, arrive second. Both waves cause the ground to shake when an earthquake occurs.

Detailed explanation-2: -The P waves are easy to identify because they are always first to arrive and their amplitude is smaller than the S wave. S waves are indicated by an abrupt change in wave amplitude. In the seismogram below, we see that the S wave arrived at about 34 sec after the P wave arrived.