Tsunami

A tsunami is a series of ocean waves that sends surges of water, sometimes reaching heights of over 100 feet (30.5 meters), onto land. These walls of water can cause widespread destruction when they crash ashore.

What Causes a Tsunami?

These awe-inspiring waves are typically caused by large, undersea earthquakes at tectonic plate boundaries. When the ocean floor at a plate boundary rises or falls suddenly, it displaces the water above it and launches the rolling waves that will become a tsunami.

Most tsunamis–about 80 percent–happen within the Pacific Ocean’s “Ring of Fire,” a geologically active area where tectonic shifts make volcanoes and earthquakes common.

Tsunamis may also be caused by underwater landslides or volcanic eruptions. They may even be launched, as they frequently were in Earth’s ancient past, by the impact of a large meteorite plunging into an ocean.

Tsunamis race across the sea at up to 500 miles (805 kilometers) an hour—about as fast as a jet airplane. At that pace, they can cross the entire expanse of the Pacific Ocean in less than a day. And their long wavelengths mean they lose very little energy along the way.

In deep ocean, tsunami waves may appear only a foot or so high. But as they approach shoreline and enter shallower water they slow down and begin to grow in energy and height. The tops of the waves move faster than their bottoms do, which causes them to rise precipitously.

'Harbour wave'

The word "tsunami" comes from the Japanese words for "harbour" and "wave".

At their point of generation, tsunamis have a relatively small wave height, with peaks far apart.

As the waves approach the shore they are compressed by the shelving of the sea floor, reducing the distance between the peaks and vastly increasing the height.

To those on the shore, the first sign of something amiss can be the retreat of the sea, which is followed by the arrival of large waves.

"The sea was driven back, and its waters flowed away to such an extent that the deep seabed was laid bare and many kinds of sea creatures could be seen," wrote Roman historian Ammianus Marcellus, awed at a tsunami that struck the then-thriving port of Alexandria in 365 AD.

"Huge masses of water flowed back when least expected, and now overwhelmed and killed many thousands of people.... Some great ships were hurled by the fury of the waves onto the rooftops, and others were thrown up to two miles (three kilometres) from the shore."

Several factors determine the height and destructiveness of a tsunami.

As with earthquakes, several attempts have been made to set up scales of tsunami intensity or magnitude to allow comparison between different events.[55]

Intensity scales

The first scales used routinely to measure the intensity of tsunamis were the Sieberg-Ambraseys scale (1962), used in the Mediterranean Sea and the Imamura-Iida intensity scale (1963), used in the Pacific Ocean. The latter scale was modified by Soloviev (1972). Is the "tsunami height," averaged along the nearest coastline, with the tsunami height defined as the rise of the water level above the normal tidal level at the time of occurrence of the tsunami. This scale, known as the Soloviev-Imamura tsunami intensity scale, is used in the global tsunami catalogues compiled by the NGDC/NOAA[57] and the Novosibirsk Tsunami Laboratory as the main parameter for the size of the tsunami.

In 2013, following the intensively studied tsunamis in 2004 and 2011, a new 12-point scale was proposed, the Integrated Tsunami Intensity Scale (ITIS-2012), intended to match as closely as possible to the modified ESI2007 and EMS earthquake intensity scales.[58][59]

Magnitude scales

The first scale that genuinely calculated a magnitude for a tsunami, rather than an intensity at a particular location was the ML scale proposed by Murty & Loomis based on the potential energy.[55] Difficulties in calculating the potential energy of the tsunami mean that this scale is rarely used.