Knowledge Organiser: Seismic Waves
Key Terms
- P-waves: longitudinal, travel through solids + liquids
- S-waves: transverse, solids only
- Shadow zone: region where waves don't arrive
- Seismograph: instrument detecting seismic waves
Earth's Layers
- Crust (thin, solid)
- Mantle (solid, thick)
- Outer core (LIQUID — S-wave evidence)
- Inner core (solid — P-wave evidence)
Key Evidence
- S-wave shadow zone → outer core is liquid
- P-wave shadow zone → core has different density
- P-waves still detected → inner core is solid
Exam Tips
- S = Stopped by liquid (transverse can't pass through liquid)
- P = Passes through everything (longitudinal)
- Always explain WHY waves can't pass through liquid
- Outer core = liquid; inner core = solid
Key Equations
- v = f × λ (wave speed = frequency × wavelength)
- P-waves: faster, longitudinal, travel through solids and liquids
- S-waves: slower, transverse, travel through solids only
- Wave speed increases with density/rigidity of medium
Common Mistakes
- Confusing P-wave and S-wave properties: P-waves are longitudinal (compressions) and travel through everything; S-waves are transverse and cannot travel through liquids — use "S = Stopped by liquid" as a memory aid
- Not explaining why S-waves can't pass through liquid: Transverse waves require a medium that can undergo shear stress — liquids cannot support transverse vibrations, so S-waves are absorbed at the outer core
- Confusing outer and inner core states: The outer core is liquid (S-waves are blocked here); the inner core is solid (S-waves could pass through but don't reach it directly)
- Saying seismic waves prove Earth's structure directly: Seismic wave evidence is indirect — we infer the structure from the pattern of which waves arrive where, not by drilling to the core
- Forgetting waves refract inside Earth: Seismic waves change direction (refract) as they travel through layers of different density — this creates shadow zones where no waves are detected
Memory Aid
- P-waves = Primary (arrive first) = Push/pull (longitudinal) = Pass through everything (solids and liquids)
- S-waves = Secondary (arrive second) = Shake sideways (transverse) = Stopped by liquid