Tsunami are very low frequency, long wavelength, ocean waves. The largest and most destructive are initiated by changes in elevation of large areas of sea-bed, due to Earthquakes of 8.0 and above on the Richter scale. Vertical amplitude in deep water is small, (only cms), but there is lateral movement from seabed to surface. The period can vary from 5 minutes to an hour and the wavelength can be 100 km or more.

Tsunami are shallow-water waves. Velocity is depth dependent; ~800 km/h in very deep ocean; reducing in shallow water to ~50 km/h. As a wave comes ashore a breaking wall of water many meters high rises at the leading edge followed by a flood of deep rushing water. The flood does the damage.

The waves that devastated the shores of the Indian Ocean on the 26/12/2004 were generated by sea-bed movement (~1-3 m vertically in three stages 2-3 minutes apart) along a 1200 km section of plate boundary to the west and north of Sumatra. The Peninsula Hotel in Bangkok swayed for 8 minutes. A woman from the Seychelles tells of watching a bay fill and empty many times over several hours.

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