Tsunamis

Background

A tsunami is a wave, or a series of waves, that occurs in an ocean or other large body of water. It is caused by some sudden activity on the seabed or at the surface. It has the potential to cause large-scale damage and devastation to people and the environment.

These catastrophic events are often incorrectly called tidal waves. This is misleading because they have nothing to do with tides, which are caused by gravitational influences of the sun, moon and planets.

The correct official term adopted by the international scientific community is tsunami. The word 'tsunami' (pronounced soo-nam-ee) is derived from the Japanese meaning 'harbour wave'.

What causes a tsunami?

Tsunamis are large sea waves triggered by a sudden, catastrophic movement of the seabed, the sea surface or a coastal area. Most tsunamis result from submarine earthquakes, volcanoes and landslides, but in extremely rare instances, they may also be caused by asteroid or meteorite impact.

Events such as these displace huge volumes of sea water, creating a rise or fall in the level of the ocean above the site of disturbance. This triggers the initial formation of the tsunami wave.

If the shape and slope of the seafloor is unfavourable the resultant wave that hits the coastline may reach massive proportions.

How tsunamis work

Tsunamis are very different from the wind-generated waves which constantly roll onto our beaches. Tsunamis may be likened to a large-scale version of the ripples caused by a stone dropping into a pond. Similarly, tsunamis events may also consist of numerous, separate wave fronts moving away from the site of disturbance.

However, unlike normal waves, tsunami waves may be up to a hundred kilometres apart, thousands of kilometres long and impact hours apart.

The speed of a tsunami is dependent on the depth of water through which it travels. In the deep ocean they can travel at speeds of 1000 kilometres per hour, losing very little of their energy. At this speed, the wave would be travelling as fast as some jet planes – indeed some scientists have estimated that if an earthquake happened off the coast of Los Angeles in the United States, a tsunami would reach Tokyo, Japan quicker than flying there.

However, at this early stage of their development the wave may only be a few centimetres high. This makes the wave invisible from the air and would be barely noticeable by the crew of a ship sailing through it. This makes tsunamis very difficult to detect initially.

The damage tsunamis cause results from the wave entering shallower water near the coast. From here the tsunamis starts to act like a normal wave – only with far more destructive power. Shallower water slows the velocity, or speed, of the wave. As these initial waves slow, the later waves catch up. This causes a tremendous volume of moving water to build up creating a high (sometimes up to 35 metres) turbulent wave as it reaches the coast.

The resulting tsunami batters the coast with tremendous destructive energy which is often catastrophic for both the human populations and their infrastructure and the plants and animals that inhabit these areas. In particular, tsunamis can cause significant loss of life in the 'run up or inundation zone'. This is the area, up to 20 kilometres inland, where the worst effects of the waves will be felt.

Tsunamis – a historical perspective

Although relatively rare events, tsunamis have played an important role in human history. They have occurred in all the world’s oceans.

Some scientists have speculated that the legend of Atlantis was related to a tsunami event around 1628 BC. Similarly, the eruption of Thera (now know as Santorini) in the Greek Aegean Sea is estimated to have caused a 30 metres high tsunami that may hastened the collapse of the Minoan civilisation on nearby Crete.

Probably the most catastrophic tsunami in Europe during historical times occurred on 1 November 1755. An earthquake on the sea floor west of Portugal triggered three tsunami waves estimated to have been between 5 and 13 metres high. These waves caused considerable destruction and killed 60,000 people.

The eruption of Krakatoa in the Sunda Straits off Indonesia between the 26 and 27 August 1883, set off a tsunami that claimed the lives of more than 36,000 people.

In the last half of the 20th century there were five major destructive tsunamis in Pacific Ocean alone. Since 1990 over 80 tsunamis have been recorded – again most in the Pacific Rim area.

Tsunamis and Britain

There are few reports of tsunamis in Britain's historical records. This is largely because Britain has no active volcanoes and experiences very limited earthquake activity. However, there is the potential risk, albeit small, of tsunamis triggered by local or relatively distant underwater landslides.

Scientists have shown that waves generated by the Portuguese tsunami of 1755 hit Mounts Bay in Cornwall – however the waves had lost most of their destructive power by the time they reached Cornwall and little damage was recorded.

Evidence from geological records does show that larger scale events have hit areas of Britain. The most destructive was caused by an underwater slump, possibly caused by an earthquake, some 7200 years ago. Scientists have estimated that this tsunami flooded the Moray Firth and much of the east coast of Scotland.

Despite, these events, tsunamis are still relatively rare occurrences, especially for Britain.

Although we cannot prevent these events, scientists are constantly monitoring the seabed to get a better understanding of them to provide advance warning and protection.