Geography K–10 · Years 7–8

Locating an earthquake from P and S wave arrival times

Landscapes and landforms (ACARA Year 8 sub-strand); NSW Stage 4 focus area Landscapes and Landforms (2015 syllabus) and Landscapes and landforms (2024 syllabus); geomorphological 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 the distance to the earthquake, and three distances fix the epicentre.

What you need

Three seismogram traces with time axes for one earthquake chosen from the Wilber 3 event lists (ds.iris.edu/wilber3), plotted for three stations that each lie within 150 km of the published epicentre. Check the station map before the event is chosen: Australia's national seismograph network is spread hundreds of kilometres between stations, so many Australian earthquakes have no three stations that close. Where three cannot be found for an Australian event, choose an event in a region whose network is dense enough and write on the record sheet that the event is not Australian and why it was chosen. For an Australian event the published epicentre and magnitude come from Earthquakes@GA; Geoscience Australia's classroom resource "Analysis of earthquakes using real map and data sets" shows how to use the GA portals.
A map at a stated scale showing the three station positions and the published epicentre
Pair of compasses, ruler, calculator
Wave speeds for crust: P 5–7 km/s, S 3–4 km/s (British Geological Survey); the class uses 6.0 and 3.5 km/s and states them. The same BGS table gives P above 8 km/s and S 4.5 km/s in the mantle, which is why only stations within 150 km are used

How to do it

On each seismogram mark the P arrival (first sharp onset) and the S arrival (the later, larger onset). Read the S − P time gap in seconds.
Compute the distance to each station: distance = gap ÷ (1/v_S − 1/v_P). With 3.5 and 6.0 km/s this is 8.4 km for every second of gap. These crustal speeds hold only while the first P and S arrivals travel through the crust: for an assumed crust 30 km thick over a mantle with P at 8.0 km/s, the mantle P wave arrives first beyond 2 × 30 × √((8.0 + 6.0) ÷ (8.0 − 6.0)) = 159 km, so the stations used are within 150 km (gaps up to about 18 s).
Set the compasses to each distance at the map scale and draw the circle around each station.
Mark the epicentre where the three circles meet (or the centre of the small triangle they make) and read its position.
Compare with the epicentre published by Geoscience Australia for the event and give the difference in km.
Using the published magnitude, compute how many times more energy a quake one unit larger would release.

What you should see

Three circles that meet at or near one point, and the distance in km between the class epicentre and the published one. For a gap of 10 s the distance is 84 km and for 15 s it is 126 km. A distant station misleads: a 30 s gap read with crustal speeds gives 252 km, while mantle speeds of 8.0 and 4.5 km/s give 30 ÷ (1/4.5 − 1/8.0) = 309 km, so circles drawn from distant stations fall short. Energy ratios: one magnitude unit is 10^1.5 = 31.6 times the energy (the USGS rounds this to about 32 times), two units 1000 times. It worked if the circles overlap in a small triangle rather than missing each other, which would mean a misread arrival or a station too far away for crustal speeds.

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.

The epicentre is where the shaking is felt most strongly.
Magnitude 6 is twice as strong as magnitude 3.

Safety card

Low riskLearners carry it out

Hazards

compass point

Controls

desk use

Note

No chemicals or heat are used. The activity is run under the school risk-assessment procedure; RiskAssess (riskassess.com.au) holds templates for school practicals and fieldwork. The NSW Department of Education 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.

Geography K–10 Syllabus (2015), NESA; current and taught in 2026, replaced from 2027. The outcome codes are not printed on the NESA landing page for this syllabus, which only links the document, so the url given is the syllabus document itself (geography-k-10-syllabus-2015.docx); the code and its outcome text were read in that document on 23 September 2026.GE4-2 GE4-7
Geography 7–10 Syllabus (2024), NESA; implementation from 2027, not yet taught; code read on this page on 22 September 2026GE4-PRI-01 GE4-TAP-01
Australian Curriculum v9AC9HG8K05AC9HG8S02AC9HG8S03

Sources

The pages the author read to write this activity.

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