Teacher sheet · Ages 15–16 · about 45–50 minutes
Seismic waves, magnitude and locating earthquakes
Lesson page: echoesof.science/school/earthquakes/ages-15-16/ · Pupil worksheet: echoesof.science/school/earthquakes/ages-15-16/print/
Learning aims
- Describe P- and S-waves and how a seismometer works.
- Calculate earthquake distance from S−P times and locate an epicentre by triangulation.
- Use the logarithmic magnitude scale for amplitude and energy.
- Evaluate uncertainty in wave speeds and discuss the limits of prediction.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: show a real seismogram. Why are there two clear arrivals? |
| 15 min | Read the text; derive the S−P formula together. |
| 15 min | Locate the epicentre (data investigation). |
| 5 min | Play P-wave, S-wave or both? |
| 5 min | Quiz and discussion: why can't we predict earthquakes? |
Materials for the experiment
- Graph paper (scale 1 cm = 10 km)
- A pair of compasses and a ruler
- Calculator
- Station positions (x, y in km): A (145, 174), B (36, 140), C (196, 39)
- S−P times: A 5.0 s, B 10.0 s, C 15.0 s
Safety: No practical hazards: this is a data investigation. Take care with the compass point.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Magnitude 8 is twice as strong as magnitude 4. | The scale is logarithmic: each step is 10 times the amplitude and about 32 times the energy. |
| Magnitude tells you how strongly your town shook. | That is intensity; it varies with distance, while magnitude is one value for the earthquake. |
| Earthquakes are increasing because more are reported. | More small ones are detected because networks have grown; larger ones have stayed stable. |
Worksheet answers
- Flat trace, then small P-wave arrival, larger S-wave arrival, then the largest surface waves; S−P time marked between the first two arrivals.
- tS − tP = d/vS − d/vP = d(1/vS − 1/vP); d = 12 ÷ 0.119 ≈ 101 km (about 100 km).
- Magnitude is one value for the size of the earthquake; intensity is the strength of shaking at a particular place and depends mainly on distance from the rupture.
- 120 ÷ 18 ≈ 7: earthquakes one magnitude step smaller are several times more common, so small earthquakes are much more frequent than large ones.
Quiz answers
- With vP = 6.0 km/s and vS = 3.5 km/s, the S−P time is 20 s. How far away is the earthquake? About 168 km. d = 20 ÷ (1/3.5 − 1/6.0) ≈ 20 × 8.4 ≈ 168 km.[5]
- How much larger is the seismogram amplitude of a magnitude 7 than a magnitude 5 earthquake? 100 times. Ten times per step: 10 × 10 = 100.[2]
- About how much more energy does a magnitude 7 release than a magnitude 5? About 1,000 times. 32 × 32 ≈ 1,000.[2]
- Why can S-waves not pass through liquids? They are transverse waves, which liquids cannot carry. S-waves cannot travel through air or liquids.[5]
- Why does a seismometer's suspended mass let it measure ground motion? The mass tends to stay still while the ground moves. The relative motion of ground and mass is recorded.[6]
- Why are more small earthquakes recorded now than a century ago? Monitoring networks have expanded. There is no evidence earthquakes are becoming more frequent.[4]
Going further
Download a seismogram from an online seismic network for a recent large earthquake recorded at two or three stations. Measure the S−P times, estimate the distances and compare your epicentre with the official one. Discuss which assumptions cause the difference.
Sources used in this lesson
- [1] The Science of Earthquakes (USGS Earthquake Hazards Program): reliability High
- [2] Earthquake Magnitude, Energy Release, and Shaking Intensity (USGS Earthquake Hazards Program): reliability High
- [3] This Dynamic Earth: Understanding plate motions (US Geological Survey (USGS)): reliability High
- [4] Are earthquakes becoming more frequent? (Earthquake FAQ) (British Geological Survey (BGS)): reliability High
- [5] Seismic waves (Earthquake guide) (British Geological Survey (BGS)): reliability High
- [6] How we measure earthquakes (Earthquake guide) (British Geological Survey (BGS)): reliability High
- [7] Can you predict earthquakes? (US Geological Survey (USGS)): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/earthquakes/sources/
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