Mantle-Core Boundary Wave Behavior

Seismic waves, like S-waves (Shear waves), change their behavior when they encounter boundaries between different layers within the Earth. The boundary between the Earth's mantle and the outer core is significant because the mantle is primarily solid, while the outer core is liquid.

S-Wave Interaction at Boundary

S-waves possess shear motion and cannot travel through liquids. When an S-wave travelling through the solid mantle reaches the liquid outer core boundary:

• A portion of the S-wave energy is reflected back into the mantle.
• The portion of the energy that is transmitted into the core cannot continue as an S-wave.

Wave Conversion and Refraction

Since S-waves cannot propagate in the liquid outer core, the transmitted energy is converted into P-waves (Pressure waves), which can travel through liquids. This process is a form of refraction where the wave type changes.

The direction of this refracted P-wave depends on the angle the S-wave strikes the boundary and the properties of the core. Based on seismic observations and theoretical models, this refracted wave travels into the core.

Therefore, the energy of an S-wave incident on the mantle-core boundary is partly reflected back into the mantle and partly refracted (as a P-wave) into the liquid outer core, directed away from the Earth's centre.