GEOS-240: The Definition and Characteristics of a Tsunami
A tsunami is a series of three to four seismic waves that form as gravity equalizes a significant amount of oceanic water that was displaced by a natural disturbance, such as a seaquake or marine or continental landslide. Though a tsunami's peak wave may occur at any point in the series (Bryant, 2001), all waves within a tsunami wave series have common characteristics. Tsunami waves are characterized as long waves due to their extensive wavelength that can span up to 240 kilometers (150 miles) (Travers, 1996). They are also considered to be shallow water waves because the measurement of a tsunami wave's wavelength will always exceed the depth of the ocean at any given point (Joseph, 2011).
This image shows how wave shoaling decreases wave length as it increases wave height. Additionally, it notes the varying speed of a tsunami wave in relation to water depth. (Joseph, 2011)
Initially, tsunami waves are so small and travel so fast that they go undetected by ships at sea. In deep waters, a tsunami wave will measure up to a mere 0.4 meters but travel at a speed of 600 – 900 km hr-1 (Bryant, 2001), or that of a jet plane. However, as a tsunami wave approaches land its speed will decrease due to the accumulation of friction created between the moving oceanic water and the increasingly shallow ocean depth. At the submerged edge of a landmass, known as the continental shelf, tsunami wave speeds decrease to 100 – 300 km hr-1 (Bryant, 2001). At shore, tsunami waves slow to about 36 km hr-1 (Bryant, 2001). However, because the overall energy of the wave must be conserved, a decrease in speed causes water molecules to vertically accumulate. This process, known as wave shoaling , amounts to wave heights of up to 20 meters (65 feet) (Travers, 1996). Thus, a tsunami wave's height is also influenced by seafloor bathymatry. The interconnected relationship between a tsunami wave's speed, wavelength, and wave height relative to ocean depth is expressed in the equation below:
c = √gH, where H = (D + η) OR c = √g(D + η)
where c is the wave's velocity, g is the wave's speed due to gravity, H is the sum of the depth of the ocean below sea level (D) and the wave's height (η).(Joseph, 2011)
Unlike wind-waves that move in a circular direction, tsunami waves move forward in a linear direction due to the compaction of energy from wave shoaling that causes the tsunami wave to adopt the properties of a solitary wave, which do not disperse easily (Joseph, 2011).
Furthermore, bathymetry (Joseph, 2011) may reflect and refract waves as they propagate outwards from the origin of the natural disturbance. When these waves collide with one another they combine to form a larger wave with a height equal to the sum of the two waves' individual amplitudes. This is known as constructive wave interference and increases tsunami wave's height.
Finally, tsunami waves are spaced 10 – 500 kilometers apart (Bryant, 2001). Therefore, a tsunami's wave period, or the time it takes for one wave to surpass a given point, ranges between 100 – 2, 000 seconds (1.6 – 33 minutes) (Bryant, 2001). Though it may take hours for a tsunami wave series to reach land, it can be present at a location for up to three hours once it has arrived as tsunami wave succeed each other after about 15 minutes (Travers, 1996). The aggregation of a tsunami's force, wave height, and longevity is a deadly combination. In future blogs, the origin of tsunamis and the specific devastation that they cause will be discussed.
References:
Bryant, E., 2001, Tsunami: The Underground Hazard, United Kingdom, Cambridge University Press, p. 26 – 28
Joseph, A., 2011, Tsunamis: Detection, Monitoring, and Early-Warning Technologies, Oxford, United Kingdom, Elsevier Inc., p. 73, 78, 82
Travers, B., 1996, The Gale Encyclopedia of Science, Detroit, Michigan, Gale Research, p. 3751 – 3753
http://blogs.denison.edu/geosciences/2012/03/27/geos-240-the-definition-and-characteristics-of-a-tsunami/
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