What is a tsunami?
The official definition of a tsunami is: “a wave train, or series of waves, generated in a body of water by an impulsive disturbance that vertically displaces the water column”.
What causes a tsunami?
In general, anything that is capable of moving large water masses can cause a tsunami. Various sources as earthquakes, landslides, volcanic eruptions, explosions, and even the impact of cosmic bodies, such as meteorites, can generate tsunamis.
Shallow water waves
A tsunami is what we call a shallow water wave. A wave is a shallow water wave if the wavelength of the wave is much longer than the depth of the water the wave is traveling through. This is typically the case for tsunamis which have wavelengths exceeding 100 km while traveling through 5-7 km deep water. Because the rate at which a wave loses its energy is inversely related to its wave length, tsunamis not only propagate at high speeds, they can also travel great, transoceanic distances with limited energy loss (the rate at which a wave loses its energy is inversely related to its wave length).
This explains why the coast of Africa was still hit by a tsunami that originated over 5000 km away. Shallow-water waves move at a speed that is equal to the square root of the product of the acceleration of gravity (9.8 m/s/s) and the water depth - in a 6 km deep ocean a tsunami propagates with 875 km/hr, as fast as an airplane!
Difference between tsunami in the deep ocean and a tsunami approaching the coast
A tsunami behaves fundamentally different in the open ocean and close to the coast in shallow waters (Figure 2). A tsunami in the deep ocean has very long wavelengths and very low amplitude. Approaching the shore the tsunami will slow down in speed and amplitudes will increase dramatically. This is due to the fact that the tsunami’s energy . ux, which is dependent on both its wave speed and wave height, remains nearly constant. Consequently, as the tsunami’s speed diminishes as it travels into shallower water, its height grows. Because of this shoaling effect, a tsunami, imperceptible at sea, may grow to 10-50 meters near the coast.
The probability of tsunami hazard is assumed same as earthquake because it will happen when earthquake occur. But probability for landslide is higher than others because not only earthquake and tsunami that is causing landslide but also precipitation too (hard rain). In this case, we assumed that high rainfall also happen when rainy season. As we know, Indonesia has two seasons in one year those are dry season and rainy season. It means the probability of landslide hazard is 1 time per year (in rainy season) so the value is 1 (one).
Tsunami effects:
There are several speci. c characteristics related to tsunamis that make it clearly distinguishable from other types of waves and that are the cause for the devastating force a tsunami might have:
* Tsunamis can appear as a falling or rising tide, waves or bore
* Tsunamis can last for several hours.
* A tsunami consists of several wave trains following each other.
* A pattern of high water levels is alternated with low water levels
Created by: ratih_nabila as Individual Assignment Double Degree Environmental Remote Sensing GMU-CU, 2009.
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