The velocity of these waves in the open sea is high but dependent upon the depth of the sea (150m, 500ft, 150m, 75 knots; 1,500 m, 5,000ft, 240 knots; 45,000m, 15,000ft, 420 knots). Although they may be several metres in height at the point of origin and may reach 30 metres, they quickly lose height and become exceedingly long (up to 500 miles). The waves may last for a period between 10 and 60 minutes. They do not break in the usual manner but strike the coastline in a manner which gives rise to extremely dangerous and turbulent conditions. At the mouths of large rivers, tidal bores may be formed which may travel many miles upstream as solitary waves. Tsunamis may cause more havoc than the earthquakes which created them.
Vanimo in Papua New Guinea, 1998; the official death toll from a tidal wave of up to 10 metres (33feet) tall stood at 1,600 people with a further 4,000 to 5,000 missing. This is the countries worst disaster ever.