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Tsunamis in the Atlantic Ocean

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Christiana Hanaman
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« on: January 01, 2012, 12:29:38 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).


http://www.maine.gov/doc/nrimc/mgs/explore/hazards/tsunami/jan05.htm
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Christiana Hanaman
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« Reply #1 on: January 01, 2012, 12:30:56 am »

http://www.noaanews.noaa.gov/video/tsunami-indonesia2004.mov
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Christiana Hanaman
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« Reply #2 on: January 01, 2012, 12:31:39 am »



Figure 1. Diagram of the characteristics of an ocean wave. The wave height (H) is the distance from the crest of the wave to its trough. Wavelength (L) is the distance between subsequent crests (in this case, A and B), while period (T) is the time it takes for subsequent crests to pass the same point. The majority of wind-driven waves have periods of less than 20 seconds and wavelengths of several hundred meters. Tsunamis can have periods upwards of an hour and wavelengths of several kilometers.
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Christiana Hanaman
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« Reply #3 on: January 01, 2012, 12:32:16 am »

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):


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Christiana Hanaman
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« Reply #4 on: January 01, 2012, 12:33:59 am »

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.
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Christiana Hanaman
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« Reply #5 on: January 01, 2012, 12:34:41 am »



Figure 2. A) An earthquake results from plate shifting at a subduction zone (oceanic plate being subducted under continental plate). Displaced water forms a tsunami. B) Tsunami separates into two distinct waves - local (to right) and distant (to deep water). C) As local wave encounters continental slope, it gains amplitude (height) and slows down. The trough of the wave, if it encounters the coast first, will cause a drop in water level (drawdown). Note the distant wave traveled much farther from the point of origin since it is moving faster in deep water (adapted from USGS). Note: Wave heights and slopes are exaggerated in comparison to water depths.

Last updated on October 6, 2005
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Christiana Hanaman
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« Reply #6 on: January 01, 2012, 12:35:07 am »

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.
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Christiana Hanaman
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« Reply #7 on: January 01, 2012, 12:36:11 am »



Figure 3. Image showing the characteristics of a tsunami as it passes through the mid-ocean and approaches a coastline. Note the high speed (>400 mph in 4000 m depth), large wavelength (213 km), and low amplitude (usually a meter or less) of the wave as it passes through deep water. Because of their large wavelength and deep water (thus low amplitude), most tsunamis pass in the mid-ocean unnoticed. However, as the wave encounters shallower water, its velocity slows, wavelength decreases, and its amplitude increases (image from GlobalSecurity.org).
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Christiana Hanaman
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« Reply #8 on: January 01, 2012, 12:42:57 am »



Figure 4. When the tsunami encounters the coastline, it breaks, surging water forward. Note the runup distance and height in relation to the normal sea level (adapted from USGS).
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Christiana Hanaman
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« Reply #9 on: January 01, 2012, 12:43:11 am »

Figure 3 summarizes how the characteristics of a tsunami change as the wave propagates through deep water towards the coastline.
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Christiana Hanaman
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« Reply #10 on: January 01, 2012, 12:43:25 am »

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.
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« Reply #11 on: January 01, 2012, 12:44:05 am »

How big can tsunami get?

In general, most tsunamis range from several centimeters to tens of meters. Depending on the distance from the point of origin and various other factors, a tsunami may only appear as brief, discrete elevations of the water level, noted only by tide gauges. On the other extreme is the mega-tsunami, which can reach heights of several hundred meters. The largest recorded tsunami reached an astounding 516 meters (1,720 feet) of runup! This tsunami struck Lituyah Bay, Alaska in July, 1958, the result of an 8.3 magnitude earthquake at the headland of the bay that sent the gigantic wave roaring into the small enclosed bay, tearing up the sides of the bay. An excellent summary and analysis of this event, with pictures, is provided at the Dr. George Pararas-Carayannis website. On the Hawaiian island of Lanai geologists have located three stacked sedimentary deposits from the runup of a giant wave train that may have originated from a nearby submarine landslide about 105,000 years ago (Moore and Moore, 1988). The Lanai deposits, called the Hulopoe Gravel, are found up to an elevation of 375 m (1230 feet) but are more widespread below 100 m (330 feet) inland of the south shore from Kaluakoi to Hulope Bays. This may be the highest known wave runup due to a submarine landslide.
What was the most devastating recorded tsunami?

In terms of human loss, the most recent December 26, 2004 tsunami in Indonesia, which killed over 150,000 people (at the time of this writing), is by far the worst recorded tsunami. A huge tsunami on the order of 30 meters resulted from the explosion of the volcano Krakatoa (in Indonesia) in August 1883, killing over 36,000; while a 1755 earthquake off the coast of Portugal triggered a tsunami that killed over 60,000 in Portugal, Africa, and Spain combined. Although it is under scientific debate, some scientists believe that the most devastating tsunami in terms of global size, may have occurred as a result of a meteor that is believed to have struck the earth near the Yucutan Peninsula some 65 million years ago.
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« Reply #12 on: January 01, 2012, 12:44:23 am »

Tsunamis in the Atlantic Ocean and Caribbean Sea

Most tsunamis occur in the Pacific Ocean since it is the hotbed of continental and oceanic plate activity and volcanism. Figure 5 shows the locations of plate margins, recorded earthquakes, and active volcanoes around the world. Note the dominance of both seismically active and volcanic regions in the Pacific Ocean. By contrast, the Atlantic Ocean is home to much less seismic and volcanic activity than the Pacific, which is why the Atlantic has fewer tsunamis. In the Atlantic Ocean, the Mid-Atlantic Ridge is a spreading center and the east coast of the United States is a passive margin; that is, no plates are colliding or sliding against each other as in the Pacific Ocean. The majority of the Atlantic Ocean's active areas in terms of both seismic and volcanic activity is concentrated near the Caribbean Islands, and at the Scotia island arc chain (South Sandwich Islands) near Antarctica. In the Caribbean, just north of Puerto Rico lies the Puerto Rico Trench, the deepest point in the Atlantic Ocean. This is where the North American Plate (moving west) meets the Caribbean Plate (moving east), resulting in relatively active subduction zones and volcanic island-arc systems (Figure 6). The Antilles subduction zone is just southeast of this. Similarly, the South Sandwich Islands in the southern Atlantic also mark an active subduction zone. Here, the Atlantic Plate is being subducted below the Antarctic Plate, resulting in the formation of the volcanic South Sandwich Islands. Since these are well south in the southern Atlantic, they will not be discussed further.
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« Reply #13 on: January 01, 2012, 12:44:54 am »



Figure 5. Map of the world showing active plate boundaries (blue lines), recorded seismic activity (yellow dots), and active volcanoes (red triangles). Note the general lack of strong seismic volcanic and seismic activity in the northern Atlantic Ocean. The mid-Atlantic ridge is a passive boundary, a spreading center, which is spreading at a rate of about 25 mm/yr. The only active subduction zones are at the Puerto Rico Trench (Caribbean Plate and North American Plate), and at the South Sandwich Islands (image adapted from San Diego State University and NASA).
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« Reply #14 on: January 01, 2012, 12:45:27 am »



Figure 6. Detailed 3-D interpolated image showing the interaction of the Caribbean and North American Plates and the location of the Puerto Rico Trench. The U.S. Virgin Islands and Lesser Antilles are located to the southeast of Puerto Rico (Image courtesy of the U.S. Geological Survey).
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