Seismologists study the propagation of seismic waves to understand the Earth's interior structure. When it comes to the behavior of S waves, or secondary waves, in the Earth's outer core, there is a fundamental reason why they cannot travel through it. S waves are shear waves, which means they move through a material by causing particles to move parallel to the direction of the wave's travel. This type of wave can only propagate through solids, where the particles are closely packed and can exert force on each other. In contrast, the outer core of the Earth is in a liquid state, composed primarily of molten iron and nickel. In a liquid, particles are not fixed in place and can move past one another freely, which means that shear waves cannot be sustained because there is no resistance to shear stress in the liquid. When S waves encounter the boundary between the solid mantle and the liquid outer core, they are absorbed and cannot pass through. This is because the liquid outer core lacks the rigidity necessary to transmit the shearing motion of S waves. The inability of S waves to travel through the outer core provides strong evidence that the outer core is indeed a liquid, which is a key aspect of our understanding of the Earth's internal dynamics. 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 >>