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Earthquakes happen when two blocks of rock suddenly slip past one another on a fault. Plates keep moving, the rough fault sticks, strain energy builds up and is released as seismic waves. Transform boundaries such as the San Andreas fault, where plates move past each other at about 5 cm per year, generally produce shallow earthquakes.

P-waves are longitudinal: rock is compressed and stretched along the direction of travel, like sound. S-waves are transverse and cannot travel through liquids or air. In the crust P-waves travel at around 6–7 km/s and S-waves at around 3.5–4 km/s.

A seismometer works because a suspended mass tends to stay still while the ground moves. On a seismogram the P-wave arrives first, then the S-wave, then the surface waves.

Experiment: Data investigation: locate the epicentre

1. Using vP = 6.0 km/s and vS = 3.5 km/s, show that d ≈ 8.4 km for every second of S−P time.
2. Calculate the distance from each station to the earthquake.
3. Plot the three stations on the graph paper and draw a circle of the right radius around each.
4. Mark the epicentre where the three circles meet and read off its coordinates.
5. Repeat the distance calculation with vP = 7 km/s and vS = 4 km/s. How much do the distances change?
6. Write a conclusion with the epicentre and the main sources of uncertainty.

Station	S−P time (s)	Distance at 6.0 / 3.5 km/s (km)	Distance at 7 / 4 km/s (km)
A
B
C

Questions

1. Sketch a seismogram showing the arrival of P-waves, S-waves and surface waves. Mark the S−P time.

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2. Derive d = Δt ÷ (1/vS − 1/vP) and use it to find the distance for an S−P time of 12 s (vP = 6.0 km/s, vS = 3.5 km/s).

3. Explain the difference between magnitude and intensity.

4. There are about 120 magnitude-6 and about 18 magnitude-7 earthquakes per year. Estimate the ratio and comment on what it shows.

Sources for every fact on this sheet: echoesof.science/school/earthquakes/sources/

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