P-wave shadow zone
Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Class 11, Ch 3 "Interior of the Earth"
Meaning
The P-wave shadow zone is a band around the earth, between about 105° and 145° from the epicentre, where no direct P-waves arrive. P-waves can pass through liquid, but they bend sharply when they enter the liquid outer core at about 2,900 km depth. This bending leaves a gap on the surface, and the P-waves come back to the surface beyond 145°. Every earthquake has its own shadow zone, so it moves when the epicentre moves. The bending of P-waves showed scientists how big the core is.
Example
For the Bhuj earthquake of 2001 in Gujarat, the far side of the earth lay in the south-east Pacific. Stations there, more than 145° away, recorded P-waves. Stations in the ring between 105° and 145° recorded no direct P-waves.
Don't confuse with
- S-wave shadow zone: this covers everything beyond 105°, because S-waves cannot pass through liquid at all. The P-wave shadow zone is only the band from 105° to 145°.