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This cartoon shows schematically how a subduction earthquake generates a tsunami. The green material is the lithosphere (which is brittle) and the purple-red material is the asthenosphere (which, while still solid, is deformable). The lithosphere is divided into the overriding (continental) plate on the right and the downgoing (oceanic) plate on the left. The overriding plate is locked against the downgoing plate, so as the downgoing plate subducts, the overriding plate is progressively deformed. The front edge of the overriding plate is flexed downwards while the coastline is lifted. After a few hundred years, so much strain energy has been stored in the system (think of a spring that is wound up too tight) that something has to give. The result is an earthquake. The earthquake relieves the accumulated strain in just a matter of minutes, during which the overriding plate relaxes back to its original unstrained condition. The toe of the overriding plate kicks up and the shoreline drops.

When a major undersea earthquake occurs near the coast, a destructive tsunami can result that will hit near-by coasts in minutes and also travel across entire oceans causing damage 1000’s of kilometers away and up to 24 hours later. To alert far-away coasts, internationally coordinated tsunami early warning systems, such as the Pacific Tsunami Warning and Mitigation System (PTWS), have been established to quickly provide tsunami threat information to countries.