As the distance to the epicenter increase, the time delay between the P and S wave arrivals increases as well. The greater the distance from the epicenter the more progressively the S wave will lag behind the P wave. P waves (primary waves) are longitudinal waves that travel through the earth.
When placed in solid/ dense material, they can travel almost twice as fast as S waves and can travel through any medium. S waves (secondary waves) are transverse and can travel only through solids because liquids and solids cannot support shear stresses.
P waves travel about 60% (a little more than half) faster than S waves in a given material. S waves arrive second in a seismic station because of their slower speed. P waves were able to travel between degrees of 0-98° from the epicentre of the original earthquake. They were detected by just under half of the monitoring stations. Monitoring stations were able to detect all S waves from the original earthquake except for the monitoring station at Kipapa, Hawaii (104°).
Seismologists noticed that records from an earthquake made around the world changed radically once the event was more than a certain distance away, about 105 degrees in terms of the angle between the earthquake and the seismograph as measured at the center of the earth. After 105 degrees the direct P- and S- waves disappeared almost completely, but slow surface waves and waves taking other paths would arrive from over the horizon. The area beyond 105 degrees of distance forms a shadow zone. At larger distances, some P waves that travel through the liquid core would arrive, but still no S waves.
The Earth has to have a molten, fluid core to explain the lack of S waves in the shadow zone, and the bending of P waves to form their shadow zone. So in the simplest form- a shadow zone is where waves of certain types don’t reach the surface relative to another place on the surface (about 105 degrees from center). The refraction properties at the boundaries between each layer create this “shadow zone,” from which geologists can infer that there is a liquid layer of the core (the outer layer).
The Earth has to have a molten, fluid core to explain the lack of S waves in the shadow zone, and the bending of P waves to form their shadow zone.