An earthquake is the trembling or shaking of the ground. It is caused by the sudden movement of the rocks in the Earth’s crust. This usually happens when the edge of one tectonic plate slides beneath or alongside another. The two plates sometimes lock together for a while, before the pressure becomes too great and the rocks suddenly snap apart. Shock waves travel out in all directions, causing the ground to shake.

It is mostly in towns and cities where earthquakes cause large loss of life. The sudden violent shaking of the ground may result in the collapse of buildings and bridges, roads splitting apart, pipes bursting and cables breaking. Landslides, fire and flooding also cause great damage.

The place where the rocks snap is called the focus. The point on the Earth’s surface above the focus is called the epicentre. Shock waves (also called seismic waves) travel out in all directions from the focus. In a small earthquake, the ground will tremble only slightly. In a large one, it may shake violently for several minutes.

Tsunami

A tsunami (sometimes called a tidal wave) is caused by an earthquake on the ocean floor. The sudden slip sets off a series of fast-moving waves. When they reach the coast, the waves surge inland with great power.