The Dynamic Planet Pt. 3
Community Contributed by G.T. Larson
The March 11 earthquake and tsunami’s effects are still being assessed and analyzed by the scientific community. The earthquake itself did not affect Hawaii, but as we all know, many areas around the state including Molokai did suffer varying levels of damage from the tsunami that was generated. We have examined in part two how a tsunami can be generated; let us now take closer look at the tsunami itself.
The scientific term for a seismic sea wave is a tsunami. This is a Japanese word with a double root, tsu meaning port or harbor and nami meaning wave, usually translated, harbor wave. Ocean waves, whether of seismic or wind generated origin, have two parts: the crest, or highest point, and the trough or the low point. The amplitude (wave or crest height) and the wavelength (time or period between crests) determines the wave or surf height as it comes ashore. As a wave train of any origin reaches a coast line, it begins to slow down and stack up; this is referred to as wave shoaling. A typical wind generated swell that reaches Hawaii has a wave height of one to 15 feet or more, and a period of five to 16 or more seconds, resulting in breaking waves of one to 20-plus feet. Tsunamis have small amplitudes but very long wavelengths, often hundreds of miles between crests. This is one of the reasons they are almost imperceptible as they travel across the open ocean. Another parallel characteristic of tsunamis and wind generated waves is what is called drawback.
As a wave’s trough approaches a shoreline, there is a certain amount of withdrawal or receding of the ocean, called a drawback. Then the crest arrives and the wave breaks or comes ashore. In times past as a tsunami has approached Hawaii, such as Hilo in 1946, the ocean has receded hundreds of feet, exposing reefs and leaving many fish stranded. Through an unfortunate lack of understanding of what was occurring, many went out upon the exposed reef and began to gather the stranded fish. This massive draw back of the ocean was followed by an even more massive surge of water. There was no time to out run this wall of water and many drowned. Today, with the Pacific Tsunami Warning Center on Oahu and a much keener public awareness of the dangerous effects of tsunamis in Hawaii, the death tolls have thankfully been greatly reduced.
Over the years Hawaii has been hit by numerous tsunamis of distant origins, including at least 11 that have had nine foot or more run ups or surge heights. All of these having originated around the Pacific Rim, referred to as the Ring of Fire; an area of intense tectonic activity. This is not the only source of tsunami activity in the islands though. The largest tsunami waves to strike Hawaii were produced in Hawaii; we will examine this in our next installment. Aloha Ke Akua.