Why is there an S wave shadow zone?

Seismologists study the propagation of seismic waves to understand the structure of the Earth's interior. When an earthquake occurs, it generates two main types of seismic waves: P waves (primary waves) and S waves (secondary waves). P waves are compressional waves that can travel through solids, liquids, and gases, while S waves are shear waves that can only travel through solids. The shadow zone is a phenomenon that occurs due to the interaction of these waves with the Earth's core, which is liquid. When S waves encounter the liquid outer core of the Earth, they cannot propagate through it because shear waves require a solid medium to move. This results in a zone on the Earth's surface where S waves are stopped entirely. Meanwhile, P waves can travel through the liquid core, but they are bent or refracted as they do so. The bending of P waves is due to the change in speed as they move from one medium to another with different densities. The shadow zone is typically observed at angular distances of 104 to 140 degrees from the point of the earthquake's origin. Within this range, the P waves have been refracted enough that they no longer directly reach the area, and since S waves cannot pass through the liquid core at all, this creates a region where the seismic wave activity is significantly reduced.

The shadow zone is the area of the earth from angular distances of 104 to 140 degrees from a given earthquake that does not receive any direct P waves. The shadow zone results from S waves being stopped entirely by the liquid core and P waves being bent (refracted) by the liquid core.