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Earth and Environmental Science 11–12 · Year 12

Locating an earthquake from S minus P times at three stations

Module 6: Hazards

Practical, model not builtLow risk

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The idea

P waves outrun S waves, so the gap between their arrivals at a station gives its distance to the earthquake, and three distances fix the epicentre.

What you need

  • three seismograms of one earthquake recorded at stations whose positions are known (for example from the IRIS station monitor that Geoscience Australia’s classroom resources link to)
  • base map with the stations marked, drawing compass, ruler, calculator
  • travel-time curve or the assumed velocities P 6.0 km/s and S 3.5 km/s for the crust

How to do it

  1. On each seismogram mark the P arrival and the S arrival and read the S minus P interval in seconds.
  2. Convert each interval to distance with the travel-time curve, or with d = Δt × V_P V_S / (V_P − V_S).
  3. Set the compass to each distance at the map scale and draw a circle about each station.
  4. Mark the epicentre where the three circles meet; measure the size of the misfit triangle.
  5. Compare the located epicentre with the catalogue position and explain the misfit (velocity assumptions, reading error, focal depth).

What you should see

With V_P = 6.0 km/s and V_S = 3.5 km/s every second of S–P delay is 8.4 km: 10 s gives 84 km, 20 s gives 168 km, 40 s gives 336 km. The three circles cross in a small misfit triangle and the epicentre lies inside it; the learner tests whether the misfit shrinks when regional velocities from the travel-time curve replace the assumed values.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • One seismogram can locate an earthquake.
  • The epicentre is where the shaking was strongest.

Safety card

Low riskLearners carry it out

Hazards

  • sharp point of the drawing compass

Controls

  • carry and store compasses closed

Note

No hazardous chemicals or heat sources: record the activity in the school's RiskAssess risk assessment, following the NSW Department of Education Science safety and compliance page; the Chemical Safety in Schools package is not triggered.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

  • Earth and Environmental Science Stage 6 Syllabus (2017), NESA; current, replaced by the 11–12 Syllabus (2025) from 2028EES12-13EES11/12-1EES11/12-2EES11/12-3EES11/12-4
  • Earth and Environmental Science 11–12 Syllabus (2025), NESA; to be implemented from 2028, not yet taughtEES-12-02
  • Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

  1. www.nsw.gov.au/education-and-training/nesa/curriculum/science/earth-and-environmental-science-stage-6-2017
  2. www.ga.gov.au/education/classroom-resources/analysis-of-earthquakes-using-real-map-and-data-sets
  3. www.ga.gov.au/education/classroom-resources/earthquakes-teacher-notes-and-student-activities
  4. earthquakes.ga.gov.au
  5. www.ga.gov.au/education/classroom-resources/earthquake-monitoring-iris

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