The correct option is It proves that the outer core is liquid..
Explanation
Seismic waves behave differently as they travel through the Earth's interior based on the physical properties (density and state) of the layers they encounter. The "shadow zone" refers to specific areas on the Earth's surface where seismographs do not detect direct earthquake waves due to refraction or absorption by the Earth's internal layers.Analysis of Options:
- It proves that the outer core is liquid. is Correct: S-waves (Secondary or Shear waves) are transverse waves that can only propagate through solid materials because liquids and gases do not support shear stress. The S-wave shadow zone, which extends beyond 103° from the earthquake epicenter, exists because S-waves are completely stopped by the outer core. This blockage provides direct evidence that the outer core is in a liquid state.
- It proves that the inner core is solid. is Incorrect: While the inner core is solid, this is inferred primarily from the behavior of P-waves (specifically their acceleration and reflection at the inner core boundary), not the S-wave shadow zone. S-waves do not reach the inner core to provide this data directly.
- It indicates that the mantle is liquid. is Incorrect: S-waves travel effectively through the mantle, which indicates that the mantle is solid. If the mantle were liquid, S-waves would not propagate through it at all.
- It shows that S-waves travel faster than P-waves. is Incorrect: While it is a fact that P-waves travel faster than S-waves, the existence of the shadow zone is related to the state of matter (solid vs. liquid) of the core, not the relative speeds of the waves.
Key Takeaway:
The S-wave shadow zone is the primary geophysical evidence that the Earth's outer core is liquid, as shear waves cannot transmit through liquid media.