2005-01-21 / Columnists

Drawing On Science

by Stephen Yaeger

Earthquakes occur when there is a vertical or horizontal movement of the earth’s crust. Such movements are caused by breaks in the crust known as faults , which are found where the boundaries of crustal plates meet. In the case of a submarine earthquake the water directly above the disturbance is pushed up all along the fault, vertically, forming a bulge or dome of water on the surface. When the bulge collapses the water then takes a horizontal direction of movement. This horizontal movement is a tsunami , a series of waves or wave train , which move away from the earthquake site in all directions. A tsunami, which is caused by faulting is also known as a seismic sea wave .

Tsunami (tsu-NAH-mee) is a Japanese word meaning “harbor wave.” It is spelled the same way whether singular or plural, although many prefer to add an “s” when making the word plural. They are sometimes referred to as tidal waves , but they have nothing to do with the tides except where how much damage they cause is due to how high or low the tide is at the time of contact with the shore. In any case much of the damage done by tsunami is due to the strong currents of water and debris caught up in the currents.

Unlike common wind-generated waves tsunami are shallow-water waves. Wind-generated waves come into shore one after another. They have fairly short periods (=the time it takes for two wave crests to pass a given point) and wavelengths (=distance between two wave crests). Shallow-water waves have long periods and wavelengths. For example wind-generated waves may have a period of 10 seconds and a wavelength of 500 feet. A tsunami may have a period of one hour and a wavelength of 60 to 100 miles. Since a tsunami consists of shallow water waves with large wavelengths and energy, it can travel in excess of 500 mph across many miles of ocean water. In fact the deeper the water the faster the waves travel.

We imagine tsunami as huge, vertical breaking waves (much like the surf) coming up on land. In reality they may come on shore in one of three ways: rapidly rising and falling tides, a wall of rough water called bores, or a huge wall of water. How the waves come on shore depends on the angle of the continental shelf . But no matter how the tsunami comes to shore one of the first clues to its approach is water moving out to sea exposing the wet sand. Unfortunately, by the time one sees the beach emptied of water it may too late to avoid being hit by the tsunami. The wave that follows the first wave will cause greater damage. This wave pushes the first wave’s waters further inland and adds to the depth of the water. Tsunami will travel farther inland than surf because of run-up (=how high the water is on shore compared to actual sea level).

It was estimated that a mega- tsunami (=waves over 70 feet high), 495 feet high, covered the shoreline of Lituya Bay in Alaska in the late 1800’s or early 1900’s. At the time no one knew how such a wave could have been formed. In 1958 the riddle was solved. Another mega-tsunami occurred. This time the wave reached 1,740 feet in height. This was almost 500 feet higher than the Empire State Building! It was confirmed that a massive landslide caused the 1958 tsunami and most likely the earlier one too.

In 1981 a study was made of the Cumbre Viejo volcano on La Palma Island, which is part of the Canary Islands. Research indicated that a collapse of the western wall of the volcano into the sea would generate a tsunami dome with a height of 2,970 feet. The resulting waves would be traveling 400 mph and reach a height of close to 200 feet when they come up on the east coast of the US! Now, don’t start moving inland just yet, but if you decide to do so, move some twelve miles away from the beach—to avoid getting wet! However, the volcano is, at present, dormant and there will be plenty of warning if it does start coughing. It must also be remembered that the landslide must fall at once and at a high rate of speed to create such waves. Based on the composition and water content of the volcano walls, it is possible that the slide will be slow; if such is the case the tsunami, if any, will not be so threatening.


E-mail Steve Drawingonscien ce@aol.com.

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