Seismic Waves: P Waves, S Waves and Locating an Earthquake — Answers
Answer sheet with model answers and success criteria for the Year 9 earth, rocks & weather worksheet.
Success TutoringWorksheetsAnswer sheetYear 9 · Science · Earth, Rocks & WeatherSeismic Waves: P Waves, S Waves and Locating an Earthquake — Answers
Part A: Types of seismic waves
An earthquake happens when stress builds up in rocks, usually at plate boundaries, until they suddenly break and move. Energy spreads out from the focus as seismic waves, which are recorded by instruments called seismometers.
P waves are longitudinal (they push and pull the rock) and are the fastest. S waves are transverse (they shake the rock side to side) and cannot travel through liquids. Surface waves are the slowest but have the largest movements.
- P wave (primary)
- S wave (secondary)
- Surface wave
- Slowest; travels along the surface and causes the most damage
- Transverse; slower; travels through solids only
- Longitudinal; fastest; travels through solids and liquids
Answer: P wave (primary) → Longitudinal; fastest; travels through solids and liquids; S wave (secondary) → Transverse; slower; travels through solids only; Surface wave → Slowest; travels along the surface and causes the most damage
Answer: The focus is the point underground where the rock breaks and the earthquake starts. The epicentre is the point on Earth's surface directly above the focus.
Answer: False
P waves are faster, so they always arrive first. That is why they are called primary waves.
- AP waves
- BS waves
- CSurface waves
- DAll cause equal damage
Answer: Surface waves
Surface waves arrive last but have the largest amplitude, producing the strongest shaking at the surface.
Part B: Locating the epicentre
Use these simplified speeds through the crust: P waves 8 km/s and S waves 4 km/s. time = distance ÷ speed.
Because P waves travel twice as fast, every 8 km of distance adds 1 s to the lag between the arrival of the P and S waves: distance (km) = 8 × lag time (s).
Answer: 150 s
- time = distance ÷ speed
- 1200 ÷ 8 = 150 s
Answer: 300 s; lag 150 s
- 1200 ÷ 4 = 300 s
- Lag = 300 − 150 = 150 s
Answer: 400 km
- distance = 8 × lag time
- 8 × 50 = 400 km
- Check: P takes 400 ÷ 8 = 50 s, S takes 400 ÷ 4 = 100 s, lag = 50 s ✓
Answer: 240 km
- 8 × 30 = 240 km
Answer: The P waves gain more and more of a lead over the slower S waves the further they both travel, so the gap between their arrival times grows.
Answer: One station gives only the distance, so the epicentre could be anywhere on a circle of that radius. Two circles cross at two points. A third circle crosses only one of those points, which is the epicentre (triangulation).
Part C: Evidence and magnitude
Answer: The outer core must be liquid, because S waves cannot travel through liquids, so they are stopped by the outer core (creating an S-wave shadow zone).
- AEarth is the same all the way through
- BEarth is made of layers with different properties
- CEarth's centre is hollow
- DSeismic waves only travel in straight lines
Answer: Earth is made of layers with different properties
Waves refract (bend) when they move into material with a different density or state, which shows Earth has a crust, mantle, outer core and inner core.
Answer: 100 times
- 6 − 4 = 2 steps
- 10 × 10 = 100 times
Answer: About 1000 times (32 × 32 = 1024).
Part D: Apply
Model answer: P waves take 240 ÷ 8 = 30 s and S waves take 240 ÷ 4 = 60 s, so there is a 30 s gap (and the damaging surface waves arrive later still). An early-warning system uses seismometers to detect the fast but less damaging P waves, works out the location and size of the earthquake, and immediately sends alerts to phones, schools and businesses before the stronger shaking arrives. In those seconds people can drop, cover and hold on; trains can be slowed; gas supplies can be shut off automatically; surgeons can stop operations; and lifts can stop at the nearest floor. If the earthquake is under the sea, it can also trigger tsunami warnings so people can move to higher ground.
Success criteria:
- Correct P and S travel times
- Correct lag of 30 s
- Explains that the system detects P waves first and sends alerts
- Gives at least two actions that reduce harm
Marking tips
- Give credit for correct method even when the final answer slips — the working shows where the thinking went right.
- For open-ended tasks, use the success criteria as a checklist rather than looking for one "right" answer.
- Celebrate what went well first, then pick one thing to work on next.
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