Why does the S wave shadow zone exist?

Seismologists have long observed that certain areas on Earth's surface do not experience the arrival of S waves from distant earthquakes. This phenomenon is known as the S wave shadow zone. The existence of this zone is due to the way S waves (or secondary waves) propagate through the Earth. S waves are a type of shear wave that can only travel through solids. They cannot pass through liquids. The Earth's outer core is in a liquid state, and when S waves encounter this liquid outer core, they are significantly attenuated or completely absorbed because they cannot propagate through it. Here's the step-by-step explanation for the formation of the S wave shadow zone: 1. Seismic Source: An earthquake generates two main types of seismic waves: P waves (primary waves) and S waves. 2. P Wave Propagation: P waves can travel through solids, liquids, and gases, so they can pass through the Earth's outer core. 3. S Wave Propagation: S waves, however, can only travel through solids. When they reach the boundary between the solid mantle and the liquid outer core, they are unable to continue through the liquid and are either reflected back or converted into other types of waves. 4. Shadow Zone Formation: The area on the Earth's surface where S waves do not arrive from a particular earthquake is the S wave shadow zone. This zone is typically situated at angles greater than about 104 degrees from the epicenter of the earthquake. 5. Detection Limitations: Seismographs located within the shadow zone can barely detect the earthquake's S waves, if at all, because the waves have been significantly weakened or stopped by the liquid outer core. The existence of the S wave shadow zone provides valuable information about the Earth's interior structure, particularly the boundary between the mantle and the outer core.

A seismic shadow zone is an area of the Earth's surface where seismographs can only barely detect an earthquake after its seismic waves have passed through the Earth. When an earthquake occurs, seismic waves radiate out spherically from the earthquake's focus.