Michael Fraser, of King's High School, asks :-

How is a Tsunami formed?

Robert Hall, a hydrographic surveyor and officer-in-command of HMNZS Takapu, responded.

A Tsunami or Seismic wave (often called a 'Tidal Wave') is usually the result of a massive undersea earthquake which sets up shock-produced, grouped waves directly from the site of the disturbance. Tsunamis usually originate in the earthquake zone of the Pacific, commonly known as the Pacific Ring of fire and through their massive force can travel enormous distances.

When these waves form they are unconnected with the ocean tides. Tsunami waves travel extremely fast in the deep ocean waters of the world, reaching recorded speeds of over 700 km per hour and wave lengths from crest to crest of over 160 kilometres, depending on the size of the earthquake (producing a traveling time of about l5 minutes) with a height of only 1/2 to 1 metre. On reaching shallow water the speed and force that the waves are moving at, combined with the shoaling effect of the ocean floor, causes these waves to increase in height, often reaching destructive size. Tsunamis have been recorded with heights of 15 to 17 metres.

The first wave to strike an obstructing land mass is often preceded by a very rapid lowering of the ocean water level. This rapid lowering is a warning that a Tsunami is only a few minutes away. The Tsunami often consists of a series or group of waves, the second and third being higher than the first, the rest slowly decreasing in size over a period of time, which may be as little as a few hours or as long as a several days.