Seismic Waves

    OCR
    GCSE

    Seismic waves are mechanical vibrations generated by earthquakes that propagate through the Earth's interior, serving as the primary diagnostic tool for determining planetary structure. Primary (P) waves are longitudinal compressions capable of traversing solids and liquids, whereas Secondary (S) waves are transverse oscillations restricted to solid media. The detection of S-wave shadow zones provides definitive evidence for a liquid outer core, while the refraction of P-waves indicates density gradients across the mantle and core.

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    Objectives
    3
    Exam Tips
    3
    Pitfalls
    4
    Key Terms
    4
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating P-waves are longitudinal and S-waves are transverse
    • Credit responses that identify S-waves cannot travel through the liquid outer core, creating a shadow zone
    • Award 1 mark for explaining that refraction occurs (curved paths) because wave speed changes with density/depth in the mantle
    • Candidates must link the detection of P-waves but not S-waves at specific angles to the presence of a liquid layer

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the wave types, but you need to explain *why* the paths curve in the mantle (refraction due to density changes)"
    • "Good recall of S-wave properties. Now, explicitly state how the S-wave shadow zone proves the outer core is liquid"
    • "Be careful with your terminology: use 'longitudinal' and 'transverse' rather than just describing the movement"
    • "To achieve higher marks, connect the change in wave speed at the core boundary to the change in material density"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating P-waves are longitudinal and S-waves are transverse
    • Credit responses that identify S-waves cannot travel through the liquid outer core, creating a shadow zone
    • Award 1 mark for explaining that refraction occurs (curved paths) because wave speed changes with density/depth in the mantle
    • Candidates must link the detection of P-waves but not S-waves at specific angles to the presence of a liquid layer

    Examiner Tips

    Expert advice for maximising your marks

    • 💡Use the mnemonic 'S-waves: Secondary, Shear, Solids only, Slower' to recall key properties instantly
    • 💡In Level of Response questions about Earth's structure, explicitly link the observation (e.g., S-wave shadow zone) to the deduction (liquid outer core)
    • 💡When asked to draw wave paths, ensure lines curve continuously in the mantle but refract sharply at the core-mantle boundary

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Confusing the propagation mediums: incorrectly stating S-waves travel through liquids or P-waves only through solids
    • Describing wave paths through the mantle as straight lines rather than curved due to gradual refraction
    • Failing to distinguish between the outer core (liquid) and inner core (solid) when explaining the specific geometry of shadow zones

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    P-waves (Longitudinal) vs S-waves (Transverse)
    Refraction due to density changes
    S-wave shadow zones and the liquid outer core
    Earth's internal structure (Crust, Mantle, Outer Core, Inner Core)

    Likely Command Words

    How questions on this topic are typically asked

    State
    Describe
    Explain
    Compare
    Deduce

    Practical Links

    Related required practicals

    • •{"code":"Modeling Waves","title":"Use of slinkies/springs to demonstrate longitudinal vs transverse motion","relevance":"Visualizing P-wave compressions vs S-wave shear motion"}

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