On 26 December, the most powerful earthquake for 40 years — measuring 9.0 on the Richter scale — struck off the coast of Indonesia. It unleashed a massive wave called a tsunami that travelled across the Indian Ocean, killing more than 100,000 people when it reached land.
In this article, Annie Schleicher of the US Public Broadcasting Service (PBS), draws on information from scientists to explain what causes tsunamis, how they cause the damage they do, and how they can be detected.
Last week's earthquake caused two plates on the Earth's crust to collide. This forced millions of gallons of water to rise, creating a massive wave, which then travelled for thousands for kilometres with little loss of energy.
Tsunamis travel at the speed of a commercial jet in all directions from the epicentre of the earthquake. As they reach land, friction forces the waves to slow down. This causes them to 'stack up', reaching up to ten metres in height. The force of the waves is enough to flatten trees and buildings and can carry them miles in land.
The earthquakes that cause tsunamis are hard to predict. But ocean sensors can be used to determine the size and direction of tsunami waves. The Pacific Ocean has a system of these sensors but the Indian Ocean — where tsunamis are relatively rare — has no such system in place.