S-waves are shear waves that move by displacing atoms perpendicular to the wave’s path. This requires “shear strength,” which only solids possess. Since the outer core is a liquid (molten iron and nickel), it has no shear strength, causing S-waves to stop completely at the mantle-core boundary. ANSWER: (C) They cannot travel through liquid media
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This “disappearance” is one of the most famous observations in geophysics. When an earthquake occurs, a massive “S-wave Shadow Zone” is created on the opposite side of the Earth (beyond 103°). Because P-waves can pass through liquids (by compression) but S-waves cannot, this confirmed that a major part of the Earth’s interior—the outer core—is liquid. This finding is fundamental to our understanding of the Earth’s magnetic field, which is generated by the movement of this liquid iron.