SEISMIC SHADOWING

'Seismic shadowing' is a global effect of an earthquake. The seismic waves generated by an earthquake pass through the body of the Earth, but between 104° and 140° from the focus of an earthquake, little or no seismic waves can be detected. This is because primary waves (P-waves) are refracted by the Earth's core and secondary waves (S-waves) are stopped by the core.
When an earthquake occurs, seismic waves radiate out spherically from the earthquake's focus. When the waves reach the Earth's surface, a seismograph can detect the movement of the rock. The velocity of the waves is dependant on the density of the rock that the waves pass through. The density changes abruptly at the boundary between the Earth's solid mantle and liquid core, causing the P-wave wavefront to change direction and the S-waves to stop entirely.
P-waves are refracted by the Earth's core, but can still travel through the core to the other side of the Earth. The P-wave shadow zone extends from 104° away from the focus, to 140°.
S-waves are unable to pass through the Earth's core because it is a liquid. Therefore, the S-wave shadow zone covers the entire section of the Earth beyond 104°.