Why do some earthquakes send a giant wave across an entire ocean?

A tsunami is a series of waves created when a huge volume of water is suddenly pushed out of place. Eruptions and landslides can do it, but an earthquake does it by making the sea floor itself jump. When a fault under the ocean breaks and one block of the sea floor is thrown upwards, it lifts all the water above it. That water then falls back and spreads outwards as waves.

Thrust faults at convergent plate boundaries are especially good at it, because their movement has a strong vertical part, and that is what shoves the water. Yet not every big quake makes one. In 2005 a magnitude 8.7 earthquake struck almost the same area where the deadly 2004 one had been, and there was no major tsunami.

Out in the deep ocean, a tsunami is almost invisible. It travels at well over 800 kilometres an hour but has a very long wavelength, so a ship might feel only a gentle swell. Near the coast, the water gets shallow, the wave slows below 80 km/h and piles up, and its height grows enormously.

In 2004 the Indian Ocean tsunami killed an estimated 227,898 people, and the lack of any warning system in the Indian Ocean meant almost everyone was taken by surprise. A ten-year-old tourist in Thailand named Tilly Smith had just studied tsunamis at school. When she saw the sea draw back and frothing bubbles, she and her parents warned others, and the beach was evacuated safely.