What causes an earthquake?

Earth's outer shell is broken into giant pieces called tectonic plates. They move very slowly, pushing together, pulling apart or sliding past each other. Where plates meet, the rocks can get stuck. Pressure builds up for years, like bending a ruler further and further. Suddenly the rocks snap and slip along a crack called a fault. The stored energy rushes out as seismic waves, and the ground shakes.

The place underground where the slip starts is the focus. The spot on the surface directly above it is the epicentre, where shaking is usually strongest. Scientists record quakes with a seismometer and describe their size with a magnitude number. Each step up the scale releases about 32 times more energy, so a magnitude 7 is far, far stronger than a magnitude 5.

Most earthquakes are too small to feel. Most of the big ones happen near plate edges, especially around the Pacific Ocean's Ring of Fire, which is also home to many volcanoes. An earthquake under the sea can lift the water and send out a tsunami, a series of very large waves.

Nobody can predict the exact day an earthquake will strike. Instead, scientists map which areas are at risk, engineers design buildings that bend instead of breaking, and many countries practise safety drills. The slow, unstoppable plate movement behind earthquakes is the same force that builds mountains.