Tsunami Hawaii Characteristics, Events, Zones, Terminology

The phenomenon we call a tsunami (soo-NAH-mee) is a series of waves of extremely long wave length and long period, generated in a body of water by an impulsive disturbance that displaces the water such as an earthquake, landslide, or sub-marine volcanic eruption. The term tsunami was adopted for general use in 1963 by an international scientific conference. Tsunami is a Japanese word represented by two characters: "tsu" and "nami". The character "tsu" means harbor, while the character "nami" means wave. In the past, tsunamis were often referred to as "tidal waves" by many English speaking people. The term "tidal wave" is a misnomer. Tides are the result of gravitational influences of the moon, sun, and planets. Tsunamis are not caused by the tides, nor are they related to the tides, though a tsunami striking a coastal area is influenced by the tide level at the time of impact. The scientific community once referred to tsunamis as "seismic sea waves". "Seismic" implies an earthquake-related mechanism of generation, which is often, but not alway the case.

1957 Aleutian Tsunami. On the left, a major wave arrived at Laie Point on the Island of Oahu, Hawaii. On the right, the arrival of another major wave showed more extensive inundation. (Images: National Geophysical Data Center)

Tsunamis are primarily associated with the occurrence of earthquakes in oceanic and coastal regions. When an earthquake occurs, the energy generated travels outward in all directions from the source. This can be illustrated by throwing a pebble into a small, still pond. The pebble represents a meteorite or some other energy source, and the pond represents the ocean. The ripples that travel out in all directions from the focus, or the point where the pebble hit the water, represent the energy that creates a sea wave or tsunami. Waves become larger as they reach the shore, where the water is shallower.

Detecting tsunamis is a very difficult thing to do. When a wave begins in the deep ocean waters, it may only have a height of about twelve to twenty-three inches and look like nothing more than the gentle rise and fall of the sea surface.

When a tsunami finally reaches the shore, it may appear as a rapidly rising or falling tide, or a series of breaking waves. Reefs, bays, entrances to rivers, undersea features, and the slope of the beach all help to modify the tsunami as it approaches the shore. Tsunamis rarely become great, towering, breaking waves. Sometimes the tsunami may break far offshore. On occasion, a tsunami may form a bore, a step-like wave with a steep breaking front, which can happen if the tsunami moves from deep water into a shallow bay or river.

The first wave may not be the largest in the series of waves. One coastal area may see no damaging wave activity, while in another area, destructive waves can be large and violent. The flooding of an area can extend inland by 1,000 feet (305 meters) or more, covering large expanses of land with water and debris. Flooding tsunami waves tend to carry loose objects and people out to sea when they retreat.

The vertical height reached by a tsunami onshore above sea level is called a run-up height. In extreme cases, the water level can rise to more than 50 feet (15 meters) above sea level for tsunamis of distant origin, and over 100 feet (30 meters) for tsunamis generated nearby.