Reflection and Refraction of Seismic Waves

If composition (or physical properties) changes abruptly at some interface, then seismic wave will both reflect off the interface and refract (or bend) and they pass through the interface. Two cases of wave refraction and reflection can be recognized.

Refraction :

• The amplitude of the reflection depends heavily on the angle that the incidence wave creates across the boundary with the boundary and the contrast in material properties.
• If the seismic wave velocity in the rock above an interface is less than the seismic wave velocity in the rock below the interface, the waves will be refracted or bent upward relative to their original path (similar to the Snell’s law)
• If the seismic wave velocity decreases when passing into the rock below the interface, the waves will be refracted down relative to their original path.
• If the seismic wave velocities gradually increases with path in the Earth, the waves will continuously be refracted along curved paths that curve back towards the Earth’s surface.

Types of seismic waves :

Based on the depth from where a seismic wave is refracted/reflected they can have various names as follows –

P, S	Direct compressional or shear waves
SP	S-wave converted into P upon reflection at the earth’s surface
PcP, ScS	P or S wave reflected at the core mantle boundary
PcS, ScP	P or S wave converted respectively into S or P upon reflection at the core mantle boundary
PKiKP	P wave reflected at the boundary of inner core
PKKP	P wave reflected from the inside of the core-mantle boundary

P & S Wave Shadow zones :

One of the important discontinuity is at a depth of 2900km where the velocity of P-waves suddenly decreases. This boundary is the boundary between the mantle and the core and was discovered because of a zone on the opposite side of the Earth from an earthquake focus receives no direct P-waves as they are refracted inward as a result of the sudden decrease in velocity at the boundary. This zone is called a P-wave shadow zone.

The Discovery was followed by the discovery of S-wave shadow zone. The S wave shadow zone occurs because no S-wave reach the area on the opposite side of the Earth from focus. Since no direct S-wave arrives in this zone, it implies that no S-waves pass through the core. This further implies that the velocity of the S-waves in the core is zero. In liquids μ =0, so S-wave velocity is also equal to zero. From this it is deduced that the core or some part of the core is in the liquid state.

Since no S-waves are transmitted through liquids, the S-wave shadow zone is best explained by a liquid core.