Seismic Waves & Earth's Interior
Distinguish between P-waves (compression, travel through solids and liquids) and S-waves (shear, cannot pass through liquids); explain why a seismic shadow zone exists on the far side of an earthquake; describe how seismologists use wave refraction and reflection to infer that Earth has a solid inner core, liquid outer core, mantle, and crust
1 · Read
Seismometers record earthquake waves. P-waves squeeze and stretch material along their path. S-waves shake it across their path. P-waves cross solids and liquids, so a P-wave arriving does not prove that every layer is solid. S-waves cross solids but cannot cross Earth's liquid outer core. At a nearby station that receives both types from one earthquake, P-waves arrive first because they travel faster.
The liquid outer core blocks direct S-waves, leaving a broad S-wave shadow on the far side of Earth. Direct waves reach a station without bouncing off a boundary or changing wave type. P-waves can cross the core and reach the opposite side. However, bending leaves a separate P-wave shadow where direct P-waves are missing. A shadow zone is for a particular wave type, not a place with no shaking.
P-waves change speed and can bend, or refract, where material properties change at a layer boundary. They slow on entering the liquid outer core and speed up on entering the solid inner core. That deeper speed change, checked with other wave evidence, helps identify the inner core. Speed alone is not a test for whether any material is solid. Waves can also reflect, or bounce back, at boundaries.
The outer core is liquid metal, mainly iron and nickel. The inner core is solid metal.
Scientists compare wave arrival times with paths through possible Earth models. A model with a liquid outer core explains the missing direct S-waves. A model with a solid inner core also fits the deeper wave changes. Many observations help test each model.
A wave can change type at a boundary. Some shaking in an S-wave shadow has crossed the outer core as a P-wave. This does not mean S-waves travel through liquid.
Wave paths and arrival times reveal Earth's layers. The liquid metal outer core blocks direct S-waves.
The rewritten reading was written for this dataset.
2 · Watch
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