Tsunami can be generated when there is a large displacement of the ocean or sea floor such as in an earthquake.  Large earthquakes can cause many metres of slip on a fault surface and this large displacements of the sea floor.  Tsunami travel across the ocean at speeds comparable to jumbo jets so depending where the earthquake occurred, communities can have only minutes warning if the earthquake occurred off the coast, or several hours if located on the other side of an ocean.  The height of the tsunami is related to the depth of the water body it is travelling through.  Away from the coast across the ocean a tsunami may only be detectable with instrumentation.  However, as it approaches the coast, as the water depth shallows, the tsunami slows down which results in the wave height increasing.  In the worst cases this can be many metres high.  The height of the tsunami and how far in land in penetrates will depend on many factors including the earthquake size as this is dependent on the amount of slip, the displacement of the sea floor, the depth and shape of the coastline, any offshore barriers and physical protective measures, as well as on land obstacles – natural and people-made.  Large area of low-lying land can be particularly vulnerable to having large areas destroyed by tsunami.

Some countries such as Japan have well-developed tsunami warning systems with both the technical and social elements tried and tested.  Where there is a known history of tsunami residents tend to be better informed and ready to respond to warnings.  However, communities who lack such experience may be less ready to take effective action.  Those living by the coast and on low-lying land are among the most vulnerable.

Following guidance from advisories and warnings will be critical for saving lives.  Tsunami can have several waves (not just one as sometimes depicted in movies).