As a geophysicist with expertise in seismology, I can explain why S waves do not travel through the outer core of the Earth. S waves, also known as secondary waves or shear waves, are a type of elastic wave that can only propagate through solids. They are characterized by their motion, which is transverse to the direction of wave propagation. This means that they cause particles to move back and forth in a direction perpendicular to the direction in which the wave is traveling. The outer core of the Earth is composed primarily of molten iron and nickel, which is in a liquid state. Since shear waves require a solid medium to transmit their energy by shearing the material, they cannot travel through a liquid medium. When S waves reach the boundary between the mantle and the outer core, they are unable to continue into the liquid outer core because there is no solid material to support their transverse motion. On the other hand, P waves, or primary waves, are longitudinal waves that can travel through solids, liquids, and gases. They cause particles to move back and forth in the same direction as the wave is moving. Because of this, P waves can pass through both the mantle and the core, although they are slowed and refracted at the boundary between the two. The inability of S waves to pass through the outer core is a key piece of evidence that has led scientists to conclude that the outer core is in a liquid state. This is a fundamental observation in the study of Earth's internal structure and the behavior of seismic waves. 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 >>