concept

S wave

The second seismic wave to arrive, travelling by shaking material sideways, perpendicular to its direction of travel — it deforms rock in shape without changing its volume. That forces it to lean entirely on the material's resistance to shearing, and there the consequence is sharp: a liquid has no shear strength, its shear modulus is exactly zero, and so the shear-wave speed is zero too. An S wave cannot cross a liquid at all. That apparently small fact is what proved Earth's outer core molten.