Water And Oceanography

Tsunami Relation to Earthquakes Earthquakes and Tsunami Tsunami Explained Seismic Tsunami



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There are very few images that evoke the indescribable raw emotion after witnessing the aftermath of a natural disaster. Many words like, shock, destruction, devastation, and suffering, are used, but to the victims there are no real words that can express the horror of being caught in the cross fire of Mother
Nature's wrath. Water is one such force that holds little remorse, and is capable of unleashing a tremendous force known as a Tsunami.The Japanese term tsunami, broken down translates to: "harbor" (tsu) wave (nami), but the words truly pale in comparison to the actual magnitude of these omnipotent disasters. So, what are they, and what is the cause of a tsunami?

-What is a Tsunami-

A tsunami is basically a wall or series of waves that are generated when a large body of water, such as an ocean or sea, is suddenly displaced or jarred. A massive shift or disturbance in the Earth's surface beneath the water, or a drastic change in volume can cause a tsunami, such as when a large chunk of land or ice plunges into a body of water. Often referred as tidal waves, these massive swells of water are capable of reaching towering heights of up to 1720 feet, spanning a wavelength of 200 kilometers. At first inception a tsunami can be quite massive, traveling at break neck speeds of up to 500 mph, but due dissipate drastically close to landfall due to a process called wave shoaling, which I will detail further on in the article.

-How Are Earthquakes Related to Tsunamis-

There are only a few conditions or events that can trigger a tsunami, including the most common culprit: an earthquake. These are seismic events, which are caused by a sudden release of energy stored beneath the Earth's crust. Earthquakes are inherent to conditions like severe shaking, tremors and even landmass shifting. A natural earthquake begins when there is sufficient elasticity stored to drive a fracture proliferation along a fault plane. This event is known also as a tectonic earthquake.

Now, when such an event's epicenter (point of origin), takes place beneath a large body of water, it can result in a minor or major tsunami. To fully understand the relationship between tsunamis and earthquakes we have to consider that all landmasses, beneath the water and above are interconnected in a worldwide system of plates that are always in motion. Usually this motion is slow, like an inch or two per year, which is considered to be normal. However, when a plate edge somehow becomes lodged or stuck, it can build up considerable energy over time. These edges to geologists and seismologists are known as fault lines, and are considered hypersensitive seismic pressure. In some cases the pressure build between the two opposing plates, which can take a very long time, before something finally breaks or gives, causing a massive shift or earthquake.

So,
let's get back to the underwater scenario. A good way to demonstrate this affect is to take a simple Styrofoam cup of water, and simply pluck it with your fingertip at the base. The result will be a slight to moderate disturbance in the tension of the water surface, which will change depending on how hard you hit it. Now take a major volume of water like the ocean or a sea, and think how hard a pluck must be in order to cause a massive wall water to form from something that begins thousands of feet below sea level.

This being said, the tsunami process of conception is obviously much more complex than a simple pluck. Factors like depth, distance, and magnitude will all contribute to the overall destructive power the tsunami will possess. Once a disturbance occurs underwater, the pressure of tectonic shifting forces a massive volume of water upwards, which is then transferred to the surface of the water. Since the increase volume has to somehow level out, it generates a large wave pushing forward. This transferred energy, also disrupts a natural siphoning effect that controls tide, which normally causes one part of the body of water to rise, while on the other side the tide should fall. One of the largest indicators of a pending tsunami is a highly unusual tide receding, which suddenly leaves a majority of a beach barren.

As the displaced water conforms to gravity and natural inertia, the wave begins to build both in speed and height respectively. Most waves start out slightly above average wave height, but as the momentum of the wave radiates away from the epicenter, it will converge with increased displaced volume. The result can drastically cause the wave to expand in height, velocity as well as overall volume. The only saving grace is that the epicenter is so far from a coastal region, that there is enough time for the wave to dissipate. The average speed of the tsunami from inception ranges from 200-500mph, however drastically is slowed down as it enters into more shallow water, but depending on the volume destruction may vary well be imminent.



Now, you might be thinking how could a few waves cause something as catastrophic, such as the December 26, 2004 Indian Ocean quake that took over 225, 000 lives in eleven different countries? The difficult part of conceptualizing a tsunami of this magnitude is realizing the overall sheer volume displacement involved. I once had a backyard pool break open, and everyone inside was washed up against my house with enough force to get a few cuts and bruises. Now imagine how much water would be displaced by a moment magnitude of 9.3. The amount of energy displacement was enough to generate enough seismic oscillation of the Earth's surface, to shift the surface up to 20-30 cm, which was detected everywhere across the planet surface.

So, what does this mean in relation to the tsunami? The amount of energy released beneath the surface of the Indian Ocean, were the equivalent to over 1502 times that of the Hiroshima bomb, displacing enough water to cause as much damage as five megatons of TNT, reaching in some locations as far as 2km inland. Tsunamis of this magnitude have an inconceivable amount of destructive power, toppling structures, washing out bridges, shattering a human body.

The saddest part about the 2004 tragedy was the lack of warning systems in place in the Indian Ocean to detect the tsunami. It took several hours, from the point of moment of magnitude, until the initial tsunami impact. During the time no warnings were issued of the pending threat, taking almost all the victims completely by surprise when it hit. Since, there has been a great deal of changes in warning systems, and seismic monitoring, which will hopefully prevent another earthquake to cause such a tragedy.

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