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The Earth's outer shell is broken into plates that keep slowly moving, sliding past and bumping into each other. Most earthquakes happen at plate boundaries.

Plates move slowly: along the San Andreas fault in California the Pacific Plate has moved past the North American Plate at about 5 cm per year.

The edges of plates are rough, so they get stuck while the rest of the plate keeps moving. Energy builds up. When the fault suddenly slips, the energy is released as seismic waves that shake the ground.

Experiment: Stick-slip model of a fault

1. Tape the sandpaper flat on the table. Tie or tape the rubber band to the book.
2. Put the book on the sandpaper. Put the ruler beside the rubber band.
3. Pull the free end of the rubber band very slowly along the ruler. Watch the book.
4. Measure how far the rubber band stretches before the book suddenly jumps. Do it 3 times.
5. Repeat on smooth paper. Plan a fair test: same book, same band, same slow pulling.
6. Compare: on which surface does the band stretch more before the jump? Which jump is bigger?

Surface	Stretch before jump, try 1 (cm)	Try 2 (cm)	Try 3 (cm)
Sandpaper
Smooth paper

Questions

1. Draw a fault before and after an earthquake. Label the hypocentre and the epicentre.

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2. In your stick-slip model, on which surface did the rubber band stretch more before the jump? Why?

3. A magnitude 7 earthquake happens about 18 times a year. About how many magnitude 6 earthquakes happen each year?

4. Why do scientists say they cannot predict earthquakes?

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

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