Epicentre Triangulation

Students drag the three stations or the red earthquake across a map where one square equals 10 km. Each ring shows the distance a station works out from the delay between P and S waves. Sliders set P speed, S speed and timing noise, a toggle hides or shows the quake, and New random quake sets a fresh puzzle. Students place the green pin at the crossing point and read the Guess error. It shows why one station gives only a circle and three stations pinpoint an epicentre.

• Press New random quake, hide the quake, and drag the green pin to where the three rings meet.
• Read the S-P gaps and Distances readouts, then check the Guess error after you place the pin.
• Drag one station closer to the quake and watch its ring shrink.
• Raise Timing noise and see how the rings blur and the meeting point shifts.
• Change P speed and S speed and see how the distances change for the same gap.
• Turn Show quake on to check where the real epicentre was.

1. 1Explain that P waves travel faster than S waves, creating a delay at each station.
2. 2Calculate distance to a quake from the S-P time gap and the wave speeds.
3. 3Use three distance circles to locate an epicentre.
4. 4Describe why one or two stations are not enough to fix a location.

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