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Can a Continent Sink?

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Adam Hawthorne
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« Reply #15 on: April 09, 2007, 12:15:49 am »

Tsunamis in the Atlantic Ocean

With the recent December 2004 tsunami that wreaked death and destruction in Indonesia, Sri Lanka, and beyond (view an animation of the tsunami by the National Oceanic and Atmospheric Administration (NOAA) - Apple QuickTime required), many might wonder about tsunamis occurring in the Atlantic Ocean and striking the east coast of the United States; most notably, Maine. Although most people don't put "tsunami" and "Atlantic Ocean" in the same sentence, history and geology tell us that the Atlantic Ocean does experience tsunami activity, albeit on a less catastrophic scale. However, although highly debated in the scientific realm, one of the world's "ticking time bombs" that may trigger a tsunami is located in the Atlantic Ocean!

This Geologic Site of the Month provides background information and characteristics of tsunamis, some of their history in the Atlantic Ocean, and several possible locations where tsunamis could be triggered and impact the east coast of the United States in the future.

Background
What is a tsunami?
A tsunami is a wave produced by a disturbance that displaces a large mass of water - usually a result of geologic activities such as earthquakes, volcanic eruptions, underwater landslides, or in rare geologic cases, meteor strikes. After such a disturbance, displaced water travels outward from its site of origin as a series of unusually large waves at great speeds (Komar, 1996). Tsunamis are often mistakenly referred to as tidal waves, though tides play no role in their formation.  The term tsunami originates from the Japanese words tsu (harbor) and nami (wave). The term was created by fishermen who returned from fishing and found everything devastated in the harbor though they didn't see or notice the wave in the open water (Wikipedia, 2004).





Figure 1 The shape and characteristics of a tsunami are similar to wind-driven waves (Figure 1) - it has a wave crest, trough, wavelength (distance between two wave crests, A and B), and period (time it takes for crests A and B to pass a known point). However, unlike regular wind-driven waves, which generally have wavelengths of up to several hundred meters and periods less than 20 seconds, tsunamis can have wavelengths of several kilometers and periods anywhere from several minutes to upwards of an hour. Their velocity (C), like wind waves, is a function of water depth (h):




where g is gravitational acceleration (9.81 m/s). The rate at which a tsunami loses its energy is inversely proportional to its wavelength. Thus, a tsunami travelling through water depths of 4,000 meters would be moving at approximately 200 m/s, or close to 450 miles per hour! At the same time, because of its very large wavelength, it is losing very little energy. In deep water, a tsunami can pass underneath a ship undetected. This is because its wavelength is on the order of several kilometers. However, when a tsunami reaches the continental shelf and begins to shoal, it will slow and increase in height.



Figure 2 shows the generation of a tsunami from an earthquake and how it travels across an ocean. In this situation, an earthquake results from a sudden shift in the subduction zone between continental and oceanic crusts (2a). This abrupt motion displaces the overlying water upwards and downwards, initiating a tsunami. 2b shows the initial tsunami split into a deep ocean (distant) and coastal (local) tsunami, headed in opposite directions. Because the distant tsunami is traveling through deeper water, it is moving much faster. Figure 2c shows amplification of the local tsunami as it passes over the continental slope - this is due to the tsunami encountering shallower water. As it continues towards land, should the trough of the tsunami reach a coastline first, the water level along the coastline appears to fall rapidly, as if the tide is ebbing. This process, called drawdown, is due to the tsunami shoaling, increasing in height, and drawing water seaward.





Figure 3 summarizes how the characteristics of a tsunami change as the wave propagates through deep water towards the coastline.



Figure 4 When a tsunami reaches a coastline, several key factors influence its destructive force. These are the height of the tsunami, its runup height, and its runup distance. Its height is simply the excess height of the tsunami wave (crest) over the normal ocean level as it passes a given point. Its runup distance is the distance from the normal tide line, or shoreline, at the time of the tsunami's arrival to its maximum extent inland. The runup height is the elevation of the point of maximum runup above the normal ocean surface at the time of the tsunami (Figure 4).

The effect a tsunami has on a coastline also depends on the the origin of the tsunami, distance from its point of origin, its size, and the slope and configuration of the bathymetry and coast that the tsunami is approaching. Because of the long period of tsunamis, they can bend around obstacles such as islands, bays, and gulfs. They typically arrive at a coast in the form of suddenly decreasing then rapidly increasing water levels, bore-like waves (similar to a tidal bore), several large breaking waves, or a combination. Tsunamis rarely arrive as a giant breaking wave, as is most commonly depicted, but generally arrive as forceful and rapid increases in water level that violently flood the coastline.

http://www.maine.gov/doc/nrimc/mgs/explore/hazards/tsunami/jan05.htm
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