School › Earthquakes › Ages 10–11
Stuck plates and seismic waves
How moving plates build up energy on faults, what the epicentre and magnitude are, how often earthquakes happen, and an earthquake in our own region. With a stick-slip model you can test.
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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.[1]
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.[3]
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.[1]
The point underground where the rupture starts is the hypocentre; the point on the surface directly above it is the epicentre. A big earthquake is always followed by smaller aftershocks, sometimes for weeks or years.[1]
The size of an earthquake is its magnitude. Each step up the scale (from 5 to 6, say) means about 32 times more energy.[2]
Small earthquakes are very common, big ones rare. On average each year there are about 800 earthquakes of magnitude 5, about 120 of magnitude 6, about 18 of magnitude 7 and about 1 of magnitude 8 or more.[4]
Earthquakes also happen near us. On 13 April 1992 an earthquake of magnitude 5.8 (ML) shook the region around Roermond in the Netherlands, about 19 km deep. The Royal Observatory of Belgium lists it in its catalogue.[9]
Nobody can predict exactly when an earthquake will happen. Scientists can only work out the chance of one in an area over a number of years. That is why practising Drop, Cover, Hold on matters.[7][8]
Try it: Stick-slip model of a fault
You need
- A heavy book or a wooden block
- A strong rubber band
- A ruler (30 cm)
- Sandpaper and smooth paper, taped to the table
- Sticky tape
Safety
Pull slowly and keep faces away from the stretched rubber band in case it snaps; wear safety glasses. Keep the book on the table.
What to do
- Tape the sandpaper flat on the table. Tie or tape the rubber band to the book.
- Put the book on the sandpaper. Put the ruler beside the rubber band.
- Pull the free end of the rubber band very slowly along the ruler. Watch the book.
- Measure how far the rubber band stretches before the book suddenly jumps. Do it 3 times.
- Repeat on smooth paper. Plan a fair test: same book, same band, same slow pulling.
- Compare: on which surface does the band stretch more before the jump? Which jump is bigger?
What happens, and why?
The rough surface makes the book stick while you keep pulling, so energy is stored in the stretched band. Then the book suddenly jumps. Faults work in a similar way: rough plate edges stick, energy builds up, and the sudden slip releases it as an earthquake.[1]
Play: Plates apart, together or sliding?
Which kind of plate boundary is it?
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Quiz
Other ages: Ages 6–7 · Ages 15–16