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Characteristics of Tsunami WavesWave propagation Tsunami waves radiate outwards at right angles to the long axis of an uplifted ellipse, or the orientation of the fault system and trench. They do not radiate circularly from a point source. Hence, tsunami wave energy directed perpendicularly from the Aleutian Trench has resulted in damage to Hawaii, and tsunamis originating off the Chilean coast have affected both Hawaii and Japan. Wave speed and height The velocity and height of tsunami waves is controlled by the depth of water through which the tsunami is moving. The velocity of tsunami waves is calculated by:
The velocity of tsunami waves is greatest in the open ocean where water depths are greatest. As the waves approach the continental shelf, where water shallows, some of the energy of the advancing tsunami is lost due to friction against the shelf. This causes the waves to slow and pile up to gain in height. In shallow waters, wave speed is often reduced to < 65 km/hr (compared to 1000 km/hr in the deep ocean), and wave height may increase to 10 m or more.
The depth of water along a coastline is determined by the slope of the continental shelf. In areas where there is a broad and shallow continental shelf (such as that surrounding New Zealand), much of the energy of a tsunami is dissipated by friction with the shelf before the waves reach shore. Hence, such coastlines are not normally affected by tsunamis. In contrast, Japan and Hawaii are not protected by extensive continental shelves, and are frequently affected by large, damaging tsunamis. The final height of tsunami waves at a shoreline is dictated by a combination of factors, including coastal geography (shape of the shoreline and water depth), wave refraction, tides and wind. Coastal geography The shape of a bay or harbour and its water depth determine the degree of interaction between approaching tsunami waves and the sea floor. Narrow, shallow bays maximise the interaction between tsunami waves and the sea floor before the waves arrive at the shore. This results in a rapid decrease in wave energy (because of frictional losses), a consequent shortening of wavelength, and an often dramatic increase in wave height. Deeper and broader bays do not induce such high cresting tsunamis at the shore line. Wave refraction and reflection Wave refraction is the process by which the direction of waves approaching a shore line is altered by interaction with the sea floor. As a wave moves from deep to shallow water, the direction of the waves becomes increasingly influenced by the submarine contour, until eventually the lines of the wave crests parallel the contours. Most refraction causes the waves to bend toward the shore. In areas of complex coastlines with many islands (e.g. in Indonesia) tsunami waves can reflect off one coast and onto another shoreline, affecting a much greater number of people. Reflection and refraction are responsible for most of the energy loss from a tsunami wave train, and help to determine the final wave height at the shoreline. Seiche Large earthquakes and landslides may cause oscillations of waters in relatively confined harbours, lakes and reservoirs. Such oscillations are termed seiches, or standing waves. Seiches were common in Alaskan lakes after the 1964 Alaskan earthquake, which generated a particularly destructive tsunami. In situations where tsunamis have been generated (most commonly by tsunamigenic earthquakes), the final height of the tsunami waves at the shoreline may be affected by the seiche. If the motion of both wave types is synchronised, then the effect to the seiche is to increase tsunami wave height. Tides As with seiches, tides can influence the height of tsunami waves, and tsunami wave runup. The height of a tsunami will be augmented if it coincides with high tide. Wind Strong winds produced during cyclonic storms cause water to pile up, especially in coastal embayments, raising sea level. The effect of a storm surge (i.e. the rise or pile up of water during a storm) is to add to and increase the wave height of the approaching tsunami waves. Shoreline drawdown Observers of tsunamis often record a dramatic withdrawal of the sea shoreline (drawdown), immediately before the tsunami wave arrives. The withdrawal of water exposes large areas of shore that would not normally be exposed, even during low tide, and can cause severe erosion of the shore. The drawdown phenomenon is due to the trough of a tsunami wave arriving at the shore, displacing the sea surface below its mean level. Drawdown events have also been known to put people at greater risk to following tsunamis. On several occasions, people observing such a low tide on the west coast of the U.S. have rushed out to collect shellfish not normally exposed. When the tsunami arrived they were lucky to escape with their lives. The bore phenomenon Most tsunami waves surge onto the shore, giving the impression of a rapidly rising and falling tide. When a tsunami wave arrives at a distant coastline it causes a slow rise of water level. However, some tsunami waves develop abrupt, steep fronts and arrive at the shore as a vertical wall of water (appearing similar to the front of a wave just before it breaks in the surf). A wave developing these characteristics is called a bore. Eye-witnesses of tsunami waves at Hawaii describe that it is often the second or third waves in a tsunami wave train that develop bores. The third wave of the tsunami train that struck the coast of Hawaii in 1960 reportedly developed a bore of around 11 m in height. Wave runup Wave runup is defined as the height above ambient sea level (i.e. at the time when the tsunami strikes) to which the waves encroach inland of the shoreline (Fig. 3).
Tsunamis should not be confused with tidal waves or storm surges. While tsunamis are principally generated by submarine earthquakes, tidal waves (storm surges) are initiated by meteorological storms (cyclones).
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