Secondary waves (S-waves), also known as shear waves, cannot pass through liquids. They require a medium with “shear strength” (the ability to resist changing shape) to propagate. Since liquids (like the Earth’s outer core) do not have shear strength, S-waves are blocked, creating a “shadow zone” that scientists use to map the core. ANSWER: (C) S
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Seismic waves are the key to understanding Earth’s internal states. P-waves are longitudinal and can travel through solids, liquids and gases because they compress the medium. However, S-waves move the medium perpendicular to the direction of travel. In a liquid, the atoms slide past each other rather than snapping back, so the shear force isn’t transmitted. When an earthquake occurs, seismographs on the opposite side of the planet do not record S-waves. This “S-wave shadow zone” starting at 103° from the epicenter is the definitive evidence that the Earth has a liquid outer core, fundamentally changing our understanding of planetary physics.