What does the behavior of S-waves reveal about Earth's outer core?

• A. S-waves do not propagate through liquids, so their absence in the outer core indicates it is liquid.
• B. S-waves travel freely through liquids like the outer core.
• C. S-waves travel faster than P-waves in the outer core.
• D. S-waves are unaffected by phase changes in Earth's interior.

Answer: (A)

S-waves are shear waves that need a solid to propagate. They involve twisting motion that a solid can resist, but liquids cannot—fluids can’t sustain shear stress. Because of this, S-waves do not travel through the liquid part of Earth known as the outer core. Seismologists observe S-waves disappearing or being blocked when their paths cross that region, even though P-waves (which compress and expand and can move through both solids and liquids) continue on and are refracted at the core boundary. That pattern—S-waves not propagating through the outer core while P-waves do—tells us the outer core is liquid.