Earthquake. Cutaway artwork of an earthquake. An earthquake is caused by sudden movement of rocks in the Earth's crust. The focus of the earthquake is deep underground and the epicentre is the point on the ground above the focus. Primary and secondary body waves travel outward from the focus, causing vibrations and fracturing in the rock. Where they reach the surface, they cause surface waves that travel outwards from the earthquake's epicentre. An earthquake's intensity is shown by concentric circles of the same intensity, known as isoseismal lines. If an earthquake occurs in a built-up area it can cause extensive structural damage and loss of life.
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