Tidal waves
Seismic sea waves
Tsunamis (the Japanese name) are often called tidal waves, but this term is a misnomer. Unlike regular ocean tides, tsunamis are not caused by the tidal action of the moon and sun; they are very large, long-period ocean waves caused either by land masses collapsing into the sea, vertical displacements along earthquake faults on the sea bottom, submarine landslides and volcanic eruptions, explosions and associated avalanches, landslides and lava flows where they occur beneath the sea or close to the coast. When these waves reach coastal areas, they can go far inland. They can travel across the Pacific to coastal areas thousands of miles away.

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.

The circum-Pacific belt is the earth's most active seismic feature. Every island and coastal settlement in the Pacific Ocean area is vulnerable to the onslaught of the great waves. Those of 1868 and 1877 devastated towns in northern Chile, and caused death and damage across the Pacific. A series of waves generated by the eruption and collapse of Krakatoa in 1883 killed more than 36,000 persons in the East Indies. Japan lost 27,000 lives to the wave of 1896, and 1,000 more to that of 1933. There have been hundreds more whose effects were less spectacular but which took many lives and did much damage. The 1946 waves which struck Hawaii are a case in point, as are the waves sent out by the 1960 Chilean earthquakes. The great Alaskan earthquake's sea waves, in 1964, caused damage as far away as California, Hawaii, Chile, and Japan.

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.

The isolation of settlements without radio reduces the ability of warning systems. Fear of after-shocks and further waves has the population in a state of panic as the incidence of an epidemic increases as thousands of bodies remain unburried in the open and rescuers turn their attention to the survivors.
(D) Detailed problems