Which statement best reflects the behavior of seismic waves with respect to Earth’s outer core?

• A. S-waves cannot travel through liquids; P-waves are compressional and faster
• B. P-waves cannot travel through liquids
• C. S-waves travel through liquids
• D. P-waves are slower than S-waves and can travel through liquids

Answer: (D)

When seismic waves meet a liquid layer like Earth’s outer core, the difference in how they move through liquids versus solids is what matters. Liquids can’t support shear forces, so shear waves (S-waves) cannot propagate through a liquid. In contrast, compressional waves (P-waves) move by pushing and pulling particles in the direction of travel and can travel through both solids and liquids, though their speed changes with the material they move through. As a result, P-waves can pass through the outer core while S-waves do not, which is why the outer core creates a shadow for S-waves and how we infer its existence. Note that, in general, P-waves are faster than S-waves in materials that support both types, so saying P-waves are slower is not typically correct.