Seismologists, who study earthquakes and the propagation of elastic waves through the Earth, understand that different types of seismic waves travel through the Earth's layers differently. S waves, also known as shear waves, are a secondary type of seismic wave that follows the faster P waves. Unlike P waves, which can travel through solids, liquids, and gases, S waves can only travel through solids. The mantle is the layer of the Earth between the crust and the core, and it is solid. Therefore, S waves can indeed travel through the mantle. However, when S waves encounter a boundary where the material changes from solid to liquid, they cannot pass through. This is the case at the boundary between the mantle and the outer core, which is believed to be liquid. The inability of S waves to travel through the liquid outer core is one of the pieces of evidence that has led geophysicists to conclude that the outer core is not solid. In summary, S waves can travel through the solid mantle but are absorbed at the mantle/core boundary because they cannot travel through the liquid outer core. read more >>

P-waves pass through both mantle and core, but are slowed and refracted at the mantle / core boundary at a depth of 2900 km. S-waves passing from the mantle to the core are absorbed because shear waves cannot be transmitted through liquids. This is evidence that the outer core does not behave like a solid substance.read more >>