Underwater earthquake and tsunami. Cutaway artwork showing how an underwater earthquake (left) can trigger a tsunami and devastate coastal areas (right). The earthquake is indicated by concentric circles radiating outwards from a point where a tectonic plate has slipped as it moves below an adjacent plate. The energy released forms shallow but powerful waves in the ocean. These rise in height in shallow waters, causing the characteristic outward and inward rush of water (blue arrows, far right). Warning systems (left) can prevent large numbers of people being killed, as happened in the Indian Ocean in December 2004.