Chris de Freitas: No coast safe from wave of destruction

Chris de Freitas

Opinion

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.

Some earthquakes generate tsunamis disproportionately large for the surface movement, or so called "surface wave", created.

For example, on September 1, 1992, an earthquake with the magnitude of 6.9 generated a tsunami with waves up to 15m high that struck 26 towns along 250km of Nicaragua's Pacific coast. The waves swept as far as 1km inland at one point. The tsunami left more than 110 people dead and 490 injured.

Experiences of highly destructive tsunamis in our general region are not as uncommon as many people might think.

According to physical geographers Dr Willem de Lange and Professor Terry Healy, of the University of Waikato, there have been 11 tsunamis in ocean waters next to the Auckland metropolitan region since 1840. Most are thought to have been less than 2m high.

However, 150 years is not a long time and more extreme events are likely to have happened in the past. Local sources (earthquakes and volcanic eruptions) are thought to produce the most damaging tsunamis, but none have occurred during recorded history.

The Auckland Regional Council believes there is about a 50 per cent chance that within the next 50 years Auckland will be hit by a tsunami originating from a large earthquake off the west coast of South America. Estimates are that wave heights of around 4m could occur in the outer Hauraki Gulf.

Major tsunamis occur in the Pacific Ocean region only about once a decade. The Moro Gulf, Philippines, tsunami in 1976 was followed by another highly destructive tsunami in New Guinea in July, 1998. An earthquake off northwest New Guinea has been blamed for this tsunami, which killed around 2000 people near Aitape.

But Bryant and others argue the 7.1 magnitude earthquake was too small to be responsible for the 15m wave that swept 500m inland at Aitape. They believe that a submarine landslide was the likely source.

The consequences of the Aitape event were, fortunately, quite localised. This is not always the case.

The earthquake that caused the catastrophic Boxing Day tsunamis was hardly felt in Indonesia, and not at all in Sri Lanka, yet the water displacement caused by the driving of the Indian plate beneath the Burma plate created waves that killed people on the east African coast almost 5000km away.

Following the great Chilean earthquake of 1960, tsunamis travelled almost 10,000km to Hawaii, where waves of more than 10m killed 60 people and destroyed many buildings along the coast of Hilo.

There is no doubt that tsunamis are an underrated hazard.

The biggest question in natural hazards research is not will events like these happen again, but when?

* Chris de Freitas is an associate professor in physical geography at the University of Auckland.