Chris de Freitas

Chris de Freitas

It is widely thought tsunamis are rare, many countries believe they are immune to them, and popular wisdom holds earthquakes responsible for the killer waves. None of these beliefs is entirely true.

A tsunami is a surge of water, or a series of surges generated by an impulsive, shock-displacement of ocean water that can occur anywhere.

Like earthquakes, volcanoes can cause these surges, and often do. One of the most destructive tsunamis in recent history occurred when the island volcano of Krakatoa erupted in 1883.

Submarine landslides, which can involve thousands of cubic kilometres of material, can also generate a tsunami.

Tsunamis can have their origins in space. Australian geographer Professor Ted Bryant points out that a meteorite striking the ocean can have a devastating effect. He maintains that on February 22, 1491, a meteorite strike caused tsunamis more than 130m high along the Australian coast.

Many countries believe they are immune from tsunamis but almost all coasts are at risk, says Bryant.

There was a tsunami in India in 1941. The 1755 Portuguese earthquake is reported to have caused a 15m tsunami that destroyed part of Lisbon and the nearby coasts of Spain and Morocco.

Tsunamis have been common around the Japanese islands for the past 200 years. Other large tsunamis occurred in Alaska in 1946, 1957, 1958 and 1964.

Bryant has found signs of tsunami waves more than 100m high on such unlikely places as coastal southeast Australia and the Scottish coastline north of Edinburgh.

Geographers Drs Roy Walters and James Goff have classified tsunamis by the distance from their source to the area of impact; that is, local and remote tsunami.

Locally generated tsunamis have short warning times - 15 to 30 minutes - while remote tsunamis have warning times ranging up to several hours.

The destructive potential of a tsunami is not simply a function of the size of the underwater disturbance, the so-called "source characteristic".

The gradient and shape of the seashore, coastal topography and shoreline configuration are, in many instances, as important as strength of the initial water displacement.

These "coastal response characteristics" and the source characteristics, determine the impact potential.

In 1958, a landslide into Lituya Bay, Alaska, created tsunami waves reportedly more than 400m high along a wilderness coastal area, stripping the forest to bare rock to an incredible height of more than 500m above sea level. Presumably this mammoth wave resulted from the distinct configuration of the coast, in particular the shoreline topography, which channelled the water along a narrow bay.