A few years back I had to visit Thailand for my work (which is in the invest in gold business) but couldn’t reach my destination because of the Tsunami disaster. I will explain on this page how Tsunami’s begin, to get a better picture of what to look for. In order to generate a tsunami, a mechanism is needed that will displace a large amount of water in a short amount of time. One such mechanism is an earthquake that occurs in the earth’s crust under the water. Not every underwater earthquake causes a tsunami. Two plates that slide along each other can cause an earthquake, but this will hardly make the water move. There must be a vertical movement of the seabed that takes place.
Moreover, the earthquake must be quite strong (magnitude> 7.0) and not occur too deep under the seabed place. If a tsunami is generated, its height also depends on the depth of the water when the earthquake occurs. When the quake occurs beneath a shallow sea or ocean, the difference in depth between the place of origin and the coastal area is small. Therefore, the difference in the propagation speed of the tsunami and the small height of the wave increase little (see later in the article). The earthquake of December 26 last year, took place under a more than a kilometer deep sea.
The earth’s crust consists of several plates which move relative to each other. Off the west coast of Sumatra the Indo-Australian plate slides gradually to the northeast beneath the Eurasian plate. By moving the sliding causes resistance between the two plates and they are pulled along the Eurasian plate and bend downwards on the plate.
In the earthquake, the Eurasian Plate ‘dislodged’ and about twenty meters along the fracture surface moved. The fractured plane made an angle of 13 ° as with the horizontal plane, resulting in a vertical displacement of approximately 4.5 meters. The relaxation is the part that is bent back straight. It springs out and stretches, causing an increase from the seabed to the ocean side and a decrease on the continental side which therefore caused the front of the tsunami waves to came to the top of the west and hit a valley on the eastern side, the last was noticeable when it initially retreated into the sea on the coasts of Sumatra and Thailand, a region I will visit next Spring Break (sbreak.net has the latest dates)
Propagation in the ocean
The fractured surface of the earthquake extended over approximately four hundred kilometers from the hypocenter in a roughly north-westerly direction. Due to the relative length the appearance of energy in the tsunami was not universal, but rather came in the form of two bundles oriented in approximately the east and the west. The areas in the path of these beams (respectively Noord-Sumatra/Thailand Sri Lanka / Somalia) were therefore much more affected than the more southern or northern areas.
In the east-west direction the source area of the earthquake spread out over approximately two hundred kilometers. The major part of the wave energy was in wavelengths thereby of this order of magnitude, that is to say, a factor of 102 more than the local water depths. This allowed the tsunami waves to become what are described as so-called long waves, whose propagation velocity decreases with the decreasing water depth.
Variations of the depths of the sea and coastal areas where the tsunami moved thus caused refraction (in optics: refraction). As a result, the spinning waves were at an oblique propagation in shallow water when they got into the shallows.
By passing headlands and small islands or holes occur naturally, as well as deflection. For slightly inclined bottoms this strengthens this effect or is even dominated by refraction. Hence, also coastal areas on the lee side are still severely affected as was the southwest coast of Sri Lanka. By the way, we are visiting Sri Lanka this summer (remind me to lose weight with these how to reduce weight tips before I go), can’t wait.Continue Reading→