WJEC GCSE Physics
WJEC GCSE Physics 1.7 Seismic Waves Exam Questions
Free WJEC GCSE Physics revision for 1.7 Seismic Waves: what you need to know, common mistakes and exam-style questions with answers. Covers P, S and surface waves, lag time, locating an epicentre, wave paths through the Earth and shadow zones.
Practise 1.7 Seismic Waves with past papers
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About WJEC GCSE Physics 1.7 Seismic Waves
Seismic waves in Unit 1 shows how earthquake waves reveal the structure of the Earth. You need to compare P, S and surface waves by type, speed and what they can travel through. You must interpret seismometer traces, use the lag time between P and S wave arrivals to find the distance to an earthquake, and locate the epicentre using readings from three stations. The paths of P and S waves through the Earth and the S wave shadow zone provide the evidence for a liquid outer core. This topic draws on the wave terms in 1.5, and examiners regularly set lag time calculations and epicentre construction questions.
What you need to know
- P waves Primary waves are longitudinal, travel fastest and can pass through solids and liquids, so they arrive first at seismometers.
- S waves Secondary waves are transverse, slower than P waves and can only travel through solids.
- Surface waves Surface waves are the slowest, travel along the surface of the Earth and cause most damage.
- Lag time The time difference between P and S wave arrivals increases with distance from the epicentre. Using v = d / t for each wave, the distance can be calculated from the lag time.
- Locating an epicentre Calculate the distance from at least three stations, draw a circle of that radius (to scale) around each, and find where the circles intersect.
- Earth's structure Waves curve as they travel because the density of the Earth changes. S waves are not detected in a large shadow zone opposite the epicentre, showing that the outer core is liquid. P waves refract at the core boundaries, giving a smaller P wave shadow zone.
Common mistakes in 1.7 Seismic Waves
- Calling P waves transverse. P waves are longitudinal and S waves are transverse.
- Saying S waves cannot travel through the core. They cannot travel through the liquid outer core; this is the key evidence for it being liquid.
- Using only two stations to locate an epicentre. Two circles cross at two points, so a third station is needed for a single location.
- Mixing up which wave arrives first. P waves are faster, so they arrive first; S waves follow.
Try three 1.7 Seismic Waves exam-style questions
1. P waves travel at 8 km/s and S waves at 4 km/s. A station is 400 km from the epicentre. Calculate the lag time. (3 marks)
Show the mark scheme answer
P time = 400 / 8 = 50 s; S time = 400 / 4 = 100 s; lag time = 50 s.
2. Explain how seismic waves give evidence that the outer core is liquid. (3 marks)
Show the mark scheme answer
S waves are transverse and cannot travel through liquids; S waves are not detected in a shadow zone on the opposite side of the Earth; so part of the core (outer core) must be liquid.
3. Give two differences between P waves and S waves. (2 marks)
Show the mark scheme answer
P longitudinal, S transverse; P faster than S (accept P travels through solids and liquids, S only through solids).
1.7 Seismic Waves FAQs
What does WJEC GCSE Physics 1.7 Seismic Waves cover?
It covers the properties of P, S and surface waves, seismometer traces, lag time calculations, locating an earthquake epicentre, wave paths through the Earth and the S wave shadow zone.
Which unit is Seismic Waves examined in?
It is examined in Unit 1: Electricity, Energy and Waves, a 1 hour 45 minute written paper worth 45% of the GCSE, sat at foundation or higher tier. The Unit 3 practical assessment draws on the same practical skills.
How do I do lag time calculations?
Calculate the travel time for each wave with t = d / v, then subtract. If you are given the lag time and need the distance, set up both times in terms of d and solve.
